Controls of Microstructure and Chemical Reactivity on the

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Controls of Microstructure and Chemical Reactivity on the Replacement of Limestone by Fluorite Studied Using Spatially Resolved Small Angle X-ray and Neutron Scattering Juliane Weber, Michael C. Cheshire, Victoria Distefano, Kenneth C Littrell, Jan Ilavsky, Markus Bleuel, Jessica Bozell-Messerschmidt, Anton V. Ievlev, Andrew G. Stack, and Lawrence M. Anovitz ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00085 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Earth and Space Chemistry

Controls of Microstructure and Chemical Reactivity on the Replacement of Limestone by Fluorite Studied Using Spatially Resolved Small Angle X-ray and Neutron Scattering

Juliane Weber1*, Michael C. Cheshire1, Victoria H. Distefano1,2, Kenneth C. Littrell1, Jan Ilavsky3, Markus Bleuel4, Jessica K. Bozell-Messerschmidt5, Anton Ievlev6, Andrew G. Stack1, Lawrence A. Anovitz1 1Chemical

Science Division, Oak Ridge National Laboratory, Oak Ridge, USA. Current

Address: Lunar and Planetary Laboratory, University of Arizona, Tucson, USA. 2University

of Tennessee, Knoxville, USA.

3Argonne

National Laboratory, Chicago, USA.

4National

Institute for Standard and Technology (NIST), Gaithersburg, USA.

5University

6Center

of Nebraska, Omaha, USA.

for Nanophase Material Science, Oak Ridge National Laboratory, Oak Ridge,

USA.

This manuscript has been co-authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*Corresponding

author: [email protected], [email protected]

Abstract Fluid-mineral interactions can alter the pore structure and mineral composition of earth materials, sometimes leading to complete replacement of one mineral phase by another. A quantitative understanding of these processes is needed for the prediction of contaminant transport in nuclear waste management, oil and gas exploration, geothermal energy production and in many geological processes. Currently, a detailed understanding of how the original microstructure and chemical reactivity is affecting the replacement rate and porosity development is lacking, which would enable the prediction of contaminant transport.

Here, we present a systematic experimental study of limestone replacement in the model system calcite-fluorite varying both the texture and chemical reactivity of the parent rock. By combining X-ray ((U)SAXS) and neutron (ultra) small-angle scattering ((U)SANS) we quantified changes in the porosity as a function of depth within the sample and time. By

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shielding the samples with a set of annular Cd-masks during neutron scattering, we obtained

spatially-resolved

porosity

information.

Microstructural

changes

were

investigated using scanning electron microscopy (SEM), and composition changes were assessed using chemical imaging by Time-of-Flight Secondary Ion Mass Spectrometry (ToF—SIMS) and SEM-Energy-Dispersive X-Ray Spectroscopy (SEM-EDX).

The replacement of limestone via fluorite takes place via advantageous pathways enhancing the available reactive surface area, e.g. fractures, accessible, interconnected porosity, non-connected porosity grain boundaries and twin boundaries. Our results presented here emphasize the importance of a detailed structural and textural assessment of the starting material both in experimental studies and in modelling studies of natural processes to make accurate predictions about reaction rates.

Keywords: Small angle scattering, Small angle neutron scattering, Small angle X-ray scattering porosity analyses, dissolution-reprecipitation, replacement, multi-scale analyses

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1

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Introduction

Fluid flow in sedimentary rocks controls the migration and retention of water, gas, hydrocarbons and contaminants, as well as cement precipitation, alteration and replacement of the host mineralogy, and many other diagenetic processes. These properties, in turn, are controlled by the microstructure and evolution of pore space in geologic formations, in which the size, distribution and connectivity of the pores dictate fluid transport and surface wetting. Furthermore, accessible porosity, which includes grain boundaries, defines the reactive surface area of the mineral-water interface and, therefore, controls the rates of replacement reaction between solid mineral phases, which are among the most complex mineral reactions in nature.1–3

When fluids interact with rocks, the mineral phases of which the rock consists can be replaced by another mineral phase. These replacement reactions can proceed via: (1) solid-state

diffusion4,5

and

(2)

dissolution-reprecipitation1,6.

In

both

exchange

mechanisms, the parent mineral may be replaced in-situ by the product, often retaining its original morphology, forming pseudomorphs. Although, dissolution-reprecipitation, 4


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transport and reprecipitation at a different site are also common. In most cases, diffusion is too slow at ambient conditions to be a relevant exchange mechanism5, although it is a

well-known phenomenon at higher temperatures.7–12 Recent studies indicate that

dissolution-reprecipitation may be the governing mechanism for such replacement reactions at lower temperatures.1,6 These reactions proceed sequentially via growth of a

boundary fluid layer at the fluid-mineral interface. By dissolution of the surrounding phase, the fluid boundary layer can become supersaturated with respect to a new phase, i.e., the product phase, whose growth or precipitation can then occur. In a multi-grain rock, however, this process is further limited by transport to the reaction site. Reacted zones may, therefore, either form an outer “skin” on the rock or be more diffuse depending on the significance and rate of grain boundary transport.13–17 In recent years, it has been

recognized that such replacement reactions are ubiquitous in geologic systems and also impact a number of energy-related industrial activities, e.g. mining and nuclear waste storage.3,18,19

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One of the most important characteristics of replacement reactions is the change in the porosity associated with them. In early studies, it was assumed that the change during these replacement reactions is defined only by the difference in molar volume between the primary and the secondary phases. Since then, however, several studies have identified that changes in porosity are due to a combination of molar volume and solubility differences.20–24 In order to both predict how geological systems evolve and

examine the reactive history of a rock, therefore, the physical and chemical “fingerprints” of these replacement reactions must be explored using more realistic systems. This begins by investigating replacement reactions in rock samples, expanding on previously published results of laboratory experiments using single crystals.6,20,25–29 Few previous

studies on the effect of the microstructure on replacement reactions have been published,30–32 which have indicated the importance of the initial microstructure and its effect on the replacement reaction. Because of the existence of grain boundaries and other pre-existing pore structures, porosity changes in real rocks span a wide range of length scales from nanometer to macroscopic. This makes making complete porosity 6


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characterization challenging, and the overall nature of the transformation more complex.33

In theory, two factors control porosity generation: (1) the difference in the molar volumes of the parent and the product phases, and (2) their relative solubilities at the mineral-water interface, which in turn may be affected by changes in fluid properties under spatial confinement.2,20,34,35 Volume-reducing reactions, with high molar volume differences between the parent (higher molar volume) and the product (lower molar volume) will lead to material loss, and subsequent increased porosity [in percent] as:

Δ𝑉 = 100 ×

(

𝑛𝑝𝑉𝑚,𝑝 ― 𝑛𝑑𝑉𝑚,𝑑 𝑛𝑑𝑉𝑚,𝑑

)

(1)

Where np and nd are the number of moles precipitated and parent dissolved and Vm,p and Vm,d are the molar volumes of the precipitating and dissolving phases, respectively. As the solubility of parent and product phase control the number of moles precipitated and dissolved, this equation also accounts for the solubility differences between parent and product phase.20 Volume-increasing replacement reactions,

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however, lead to pore sealing, fractures and/or pressure-induced crystallization.6,36–38

More accurate estimations of the expected volume changes must also account for the solubility ratio of the two minerals, as the fluid volumes available for exchange are usually limited.20 However, it is not always clearly observed that volume-decreasing reactions

lead to porosity increase and vice-versa, as previous studies have shown porosity generation during replacement processes that involve both molar volume decreases as well as increases.36–39 As noted above, the latter may reflect the formation of multiscale

fracture networks generated by the pressure of crystallization.

A number of studies have considered porosity changes during dissolution reprecipitation reactions,1,24,36–38,40 and furthermore, several studies have investigated

the effect of pre-existing microstructure on replacement reactions.31,32,41,42 Naturallyreacted samples obtained during field studies on the other hand, contain significant microstructure43–46 but are often too complex to evaluate completely as the conditions

under which they formed are, at best, uncertain. We have, therefore, performed controlled 8


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laboratory replacement experiments on samples from three rock formations, two limestones and a dolostone, to study the relations between microstructure and replacement extent and speed.

In this study, we varied the microstructure of the starting material to investigate the effect of that microstructure on the replacement reaction. To do so, we replaced the calcite and/or dolomite in the starting material with fluorite by reaction with a saturated ammonium fluoride solution as:

CaCO3 + 2NH4F  CaF2 + 2NH3 + H2CO3

(2)

The expected volume change based on the molar volume of parent and product phase for this reaction, (Δ𝑉) is -33.51 vol. %. Thus, significant porosity should be formed. A previous study by Pedrosa et al.47 conducted replacement experiments of calcite by fluorite and included the product and parent phase solubilities in this equation, leading to an expected porosity change of (Δ𝑉) = 30 vol %.

The equation for the reaction of dolomite with saturated ammonium fluoride solution is:

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CaMg(CO3)2 + 4NH4F  CaF2 + MgF2 + 4NH3 + 2H2CO3

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(3)

The change in molar volume for this reaction (Δ𝑉 = -32.43 %) is very similar to that of calcite-fluorite replacement (Reaction 2).

Reaction (2) was previously utilized by Pedrosa et al.47 to study the replacement

of calcite single crystals by fluorite. In these experiments, replacement of 1 mm cubes was observed after several hours at temperatures between 60 and 140 °C.47 Despite the

large change in molar volume involved, within the accuracy and resolution of SEM image analyses, the original shapes and volumes of the calcite crystals were preserved. The product fluorite was needle-shaped and oriented perpendicular to the reaction interface, which created a high porosity within the replaced sample. A distinct densification of the older (outer) fluorite was observed, along with the more fractured younger (inner) replaced region. The origin of this difference is uncertain. In a second study, Carrera marble was replaced by fluorite experimentally demonstrating a log-linear (power law)

10


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correlation between surface area, porosity and amount of replaced calcite with reaction time.48

Because previous studies indicated complex changes in the porosity during the replacement reactions, we have used spatially resolved multi-scale characterization methods for our porosity analyses. To span a wide range of pore sizes, we combined small angle scattering with ultra-small angle scattering using both neutron and X-rays, yielding spatially-resolved information on pore geometries at scales from 100 nm to 30 µm. By varying the initial porosity and chemical reactivity of the pristine limestone, we aimed to determine how the microstructure of the starting material influences replacement rates, extent and porosity changes during reaction. The relative effects of chemical reactivity and initial microstructure, however, probably depend on the magnitude of each. A reduced chemical reactivity in highly permeable rocks will likely lead to a diffuse reaction zone, while the opposite is likely to form a well-differentiated rim.

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2

Materials and Methods

2.1.

Characterization of Starting Materials

To test how initial microstructure affects replacement, we varied the microstructure of the limestones in our experiments by selecting two limestone of different porosity (high porosity Texas Cream “TC” limestone and low porosity Carthage Marble “CM” limestone), and a low porosity dolostone (Wisconsin Dolomite, “WD”). The porosity of the WD dolostone is comparable to that of the CM limestone but its reactivity is expected to be lower as solubility of dolomite (logKsp = -17.0949) is significantly lower than that of calcite

(logKsp = -8.4849). Furthermore, growth experiments using single crystal calcite have

demonstrated the inhibiting effect of Mg2+ in solution on calcite growth50,51. Replacement

experiments with NH4F were conducted using three different rock types: Carthage Marble (“CM”), Texas Cream Limestone (“TC”) and Wisconsin Dolomite (“WD”). These rocks were chosen for their relative pure composition and availability. Powder X-ray diffraction patterns were modeled via Riedveld refinement to obtain mineralogical composition of the starting material. Carthage marble is 100 wt% calcite with trace quantities of quartz. 12


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Texas Cream limestone is 99 wt% calcite, 0.5 wt% sylvite and 0.5 wt% quartz. Wisconsin dolomite is 80 wt% dolomite, 18 wt% quartz, 2 wt% orthoclase and contains trace amounts of kaolinite. Porosity of the original material varies, with TC being the most porous material with approximately 25% porosity, WD with 3% and CM with 1.5% based fluid saturation porosity determination (see Supplementary Information, Table S1). SEMBSE images of the starting materials are given in Figure 1.

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Figure 1

Scanning Electron Microscopy Backscattered Electron (SEM-BSE) contrast

images of the starting materials. (a) High porosity TC limestone, (b) low porosity CM limestone and (c) low porosity WD dolostone. The low porosity WD dolostone contains dolomite (dol), orthoclase (or) and quartz (qtz) as identified by XRD and SEM-EDS mappings (see Supporting Information, Figure S1 for mappings.)

The average porosity of our starting material was analyzed by fluid saturation porosity determination and (U)SANS. Both porosity determinations are in good agreement for CM and WD limestone, with 1.5% (1.8% by (U)SANS) for CM and 3% (2.2% by (U)SANS) for WD. For the high porosity TC limestone, the discrepancy between the two methods is larger with 25% (8% by (U)SANS). Most likely, this is caused by a significantly higher amount of macropores for the TC in comparison to the other two rocks. This is suggested by the Pore Size Distribution (PSD) analyses of the (U)SANS data (see Supplementary Information for PSD graph, Figure S2). SEM characterization of TC also showed a high percentage of macropores, often in the complex shapes of calciferous fossils (Figure 9 a). As (U)SANS characterization only probes pores between 100 nm and 30 µm, (U)SANS systematically underestimates the total porosity of the TC material.

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2.2.

Replacement Experiments

Samples of all three rock types were cut into cylinders 1.5875 cm long by 1.5875 cm diameter and reacted in steel pressure vessels with Teflon liners and a Teflon seal.52 Teflon liner dimensions are 1.905 cm inner diameter and 2.54 cm height with a

fluid volume of 12 ml. Experiments were fluid saturated and sealed to avoid water loss during the reaction. Cores were weighed before and after the replacement experiments. After the rock was placed in the vessel, it was packed with extra NH4F salt to ensure that the fluid remained saturated during the replacement reaction, and saturated NH4F solution was then added. Due to the additional salt in the reaction vessel, the solid/liquid ratio was slightly different in each experiment. For all conducted experiments, the solid/liquid ratio was between 316 and 525 g/L. Vessels were then sealed, preheated in boiling water to minimize the time needed to heat the vessel to the reaction temperature, and then placed in an aluminum block in a convection oven at 140 °C. An overview of the experiments conducted is given in Table 1.

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After removal from the furnace, the vessels were quenched in running cold water, opened, and the rock core was left in DI water overnight and subsequently dried. For further analyses, polished thin sections 150 µm thick were prepared by Spectrum Petrographics of each sample mounted on quartz glass slides (cf. Anovitz et al., 200953).

Table 1 Overview of experiments conducted in this study.

Experiment name

Rock type

TC_Reference

TC

CM_Reference

CM

WD_Reference

WD

TCF2

TC

CMF2

CM

WDF2

WD

TCF4

TC

CMF4

CM

WDF4

WD

TCF8

TC

CMF8

CM

WDF8

WD

TCF22

TC

CMF22

CM

WDF22

WD

TCF45

TC

CMF45

CM

WDF45

WD 17


Duration [days] -

2

4

8

22

45


2.3.

Optical Microscopy Analyses

Thin-sections were imaged using an Olympus BX60 microscope outfitted with a Nikon D700 camera in reflected light to obtain initial examinations of the alteration in each experiment and identify suitable regions for SEM characterization.

2.4.

Scanning Electron Microscopy (SEM) Characterization

Low-magnification (~300x), two-dimensional (2D) SEM/BSE images of unreacted and reacted cores were obtained from the polished thin sections using a Hitachi S3400 environmental scanning electron microscope operated at 50 Pa vacuum pressure (to eliminate charging of insulating samples) using a 15 kV electron beam. The 300x fold magnification provided a pixel edge length of 661 nm for 640 x480 pixel image, yielding an image size of 423.0 x 317.3 µm. In addition, samples were also imaged using a Hitachi S4800 SEM instrument operated at 15 kV electron beam in high-vacuum mode for higherresolution imaging using different magnifications. For imaging on this instrument, the thin

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sections were coated with a ~30 nm of carbon using a Crossington 208 carbon coater (TedPella Inc., USA). Reaction rim thicknesses were measured using ImageJ.

2.5.

X-ray Scattering Analyses

Thin sections were analyzed via ultra-small (USAXS) and small angle X-ray scattering (SAXS) for pore distributions and wide-angle X-Ray scattering (WAXS) (to reveal Bragg peaks of calcite/dolomite and fluorite phases) at the Advanced Photon Source at Argonne National Laboratory on beamline 9ID with 21 keV X-rays.54 This

instrument is a Bonse-Hart USAXS instrument, equipped with SAXS and WAXS pinhole cameras. Data were corrected for empty beam scattering and reduced using instrumentprovided data correction routines.55 Data were desmeared. The design of the instrument

guarantees that the Q ranges of different segments overlap. Collected data were put on an absolute intensity scale to enable quantitative porosity characterization. The beam size was 0.5 mm for (U)SAXS measurements. For (U)SAXS measurements, a line scan of 13 – 14 measurement points with 1 mm between them was scanned along the diameter

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of the samples core. For SAXS and WAXS, beam size was 0.2 x 0.5 mm, with data collection time of about 4 minutes per data set.

2.6.

(Ultra) Small Angle Neutron Characterization

The analytical approach for the (U)SANS characterization is described in detail in our previous papers33,53,56–60 and is therefore only summarized here briefly. Previous

studies demonstrated that pore structures of geological materials in the size range from ~200 nm to ~20 µm can be characterized using small- and ultra-small angle neutron scattering ((U)SANS).33,53,59,61,62 USANS measurements were conducted at the BT5

USANS instrument at the NIST Center for Neutron Research (NCNR)63 and the BL-1A

TOF-USANS instrument at the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS) in Oak Ridge, USA.64 The wavelength of neutrons was 3.6 Å and the width of the Darwin plateau was 5.3 arcsec. For SANS measurements, there were three instrument configurations (sample-to-detector distance of 1 and 7 m with λ = 4.75 Å, respectively, and 19m with λ = 12 Å). The resultant scattering vector ranged from 6 x 10-5 to 3 x 10-3 Å-1, which corresponds to pore diameters of 100 nm and 5 µm. 20


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For the BT5 USANS instrument at NCNR, the wavelength of the neutrons was 2.38 Å and the width of the Darwin plateau was 5 x 10-5 Å-1. The resultant scattering vector ranged from 3 x 10-5 to 2 x 10-3 Å-1, which corresponds to pore diameters of 300 nm and 30 µm. In both cases USANS data were corrected for empty-beam scattering, background counts, sample transmission and scattering volume and reduced to an absolute scale (differential cross-section) to the intensity of the direct beam using IGOR Pro.65

SANS measurements were performed on the NGB 30 m instrument66 at the

National Institute for Standard and Technology (NIST) Center for Neutron Research (NCNR) and the GP-SANS67 at the High Flux Isotope Reactor (HFIR) at ORNL, TN. On

the NCNR instrument, four sample-to-detector distances (1, 4 and 13 m, and 13 m with lenses) were used with λ = 6 Å and 8.4 Å at 13 m. This ensured an extension of the Q range to lower values and a better overlap of the USANS data at low q. The same sampleto-detector distances and wavelengths were used on the HFIR instrument. Data reduction

21



and desmearing were performed using data reduction software provided by NIST/NCNR65 and SNS/HFIR.67

The physical density and chemical composition of a material determines its scattering length density. In turn, the scattering intensity from an interface is a function of the square of the difference in the scattering length densities of the two phases (an empty pore is considered a “phase” in this cases) across that interface and the number of such interfaces of a given size. For a rock, the average scattering length density therefore depends on the volume percentage of each mineral it contains. X-ray and neutron scattering densities were determined based on the XRD analyses and are given in Table 2. For the determination of sample porosity from scattering curves, a two-phase system was assumed with the majority of scattering occurring at the mineral/pore interface.9,62

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Table 2 Scattering length densities used in this study.

Starting

Wisconsin

Carthage Marble

Texas Cream (TC) Fluorite

Material

Dolomite (WD)

100% calcite (CaCO3)

99% calcite

(CaF2)

80% dolomite

(CaCO3),

(Ca,Mg(CO3)2),

0.5% sylvite (KCl)

18% quartz

0.5 wt%

(SiO2),

(SiO2)

quartz

2% orthoclase (KAlSiO4)

X-ray

23.70

22.97

22.93

scattering length density [1010cm-2]

23


26.26


Neutron

5.16

4.72

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4.71

4.72

scattering length density [1010cm-2]

A special set of cadmium masks were fabricated for these experiments to analyze radial changes in porosity. These are shown in Figure 2, and described in Table 3. Except for the innermost, mask 0, these consisted of rings cut in 0.5 mm thick Cd sheet, with the inner part supported by thin Cd bars. Cd of this thickness is sufficient to block neutrons of the energies used here, and the masks were attached directly to the front of the sample, so the only part of the sample from which scattering could occur was that exposed by the slits in each mask. The innermost edge of each ring was the same as the outermost edge of the next smallest, so all the available area was covered, and the widths of each ring was selected to be reasonably simple to machine while yielding nearly equal areas for 24


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each. Absolute intensities are corrected for actual sample areas, so the slight differences between the areas of the masks are immaterial. Each mask also had a series of scribed marks aligned to its center. During the analyses the largest mask was used first, and this was aligned to the outer edge of the sample. A fine pen was then used to mark the positions of the scribed lines onto the glass slide, so that subsequent, smaller, masks could be aligned to the same center on the sample. As analytical time was limited, the full mask suite was only measured for some of the experiments. Only mask 0 (core) and 4 (rim) were analyzed for the shorter runs.

Figure 2

Radial Cd masks designed for this experiment.

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Table 3 Cd mask geometries.

Mask # 0 1 2 3 4

Outer Diameter [cm] 0.71438 1.03187 1.27 1.42875 1.58750

2.7.

Inner Diameter [cm]

Slit width [cm]

Area [cm2]

0.71438 1.03187 1.27 1.42875

0.71438 0.15875 0.11906 0.07938 0.07938

0.15773 0.17145 0.16942 0.13259 0.14808

Blocked Area [cm2] 0 0.15773 0.32918 0.49860 0.63119

ToF-SIMS Characterization

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements were carried out using a TOF.SIMS.5.NSC instrument (ION.TOF GmbH, Germany). A bismuth (Bi3+) liquid metal ion gun with an energy of 30 keV, current of 0.5 nA and spot size ~120 nm was used as a primary source for chemical imaging. Scanning was performed over areas of 250 x 250 µm with a resolution of 512 x 512 pixels yielding a pixel size of 488 nm. Analysis of the secondary ions was performed using time-of-flight mass analyzer run in both positive and negative ion modes with a mass resolution Dm/m = 100 — 200. Charge compensation was carried out using a low energy electron flood gun. 26


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2.8.

Small Angle Scattering Data Analyses

Both X-ray and neutron scattering data were analyzed using the IRENA and NIKA macro packages for IgorPro.55 Pore size distributions were calculated in Irena using the

total non-negative least square (TNNLS) method, which implements the work of Merritt and Zhang.55,68–70 In this method, a model pore size distribution of spheroid particles

(aspect ratio 1) are fitted to the scattering data. The minimum diameter was set to 100 nm and the maximum diameter to about 30 µm. Uncertainties calculated in Irena were obtained by running multiple fits to the data, and varying the data by adding Gaussian noise.55 A detailed description of how to apply scattering to the characterization of

porosity and pore structures can be found in Anovitz and Cole.33

3

Results

3.1.

Effect of the Microstructure on the Replacement of Limestone by Fluorite

Partial to full replacement of the limestone was observed (see Supporting Information, Figures S3, S4 and S5), depending on the starting microstructure of the pristine rock and reaction time. In general, the rock cores maintained their original shape 27



during the replacement within the error of analyses (several µm in SEM characterization). On some of the limestone cores, however, it was observed that the outermost layer became very brittle and was subject to disintegration when the core was dried, indicating significant microstructural changes due to replacement.

Changes in weight before and after replacement were used to get an initial, rough estimate of the extent of replacement (see Supporting Information, Table S2, Figure S6). This would, of course, be affected by the sample loss mentioned above. The high porosity TC limestone, after an initial decrease in weight, showed a rather constant decrease in weight until the end of the experiment. In comparison, the low porosity CM limestone showed a rather steep decrease in weight followed by an increase (see Supporting Information for a more detailed analyses). Due to sample embrittlement and subsequent loss, there is a significant error associated with these weight determinations.

As noted above, (U)SANS analyses of the cross-sections of the rock cores were performed using annular Cd-masks to shield different radii of the samples, which allowed us to spatially resolve and quantify the porosity generation as a function of depth within 28


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each core. These measurement techniques can be used to determine the average porosity within the material volume exposed to the neutron beam by the Cd-shielding. (U)SANS data (Figure 3 a) indicate increased porosity in the replaced rim after two days of reaction with NH4F for both the low and high porosity limestones. The resulting rim porosities for the two limestones are similar: 14.7 % for the low porosity CM limestone and 14.4 % for the high porosity TC limestone, as compared to 1.8 and 8 % porosities for the unreacted materials, respectively.

In both cases, however, the total (U)SANS porosity (14.7 % for the low porosity CM limestone and 14.4 % for the high porosity TC limestone) is less than would be expected from total fluorite replacement of a solid calcite crystal (30% replacement expected when considering both molar volume and solubilities47). Possible reasons for this could be a preexisting porosity, incomplete replacement reactions or the generation of macropores that are larger than what (U)SANS measurements can detect. Furthermore, the TC limestones contained 0.5 wt% quartz and 0.5 wt% sylvite, which is a larger amount of impurities than average for this type of sample (99.97 wt% CaCO3 with 29



0.02 wt% Mg and 0.01 wt% Sr for the islandic spar and 99.7 wt% CaCO3 with 0.03 wt% Mg for Carrera marble) contained in calcite samples used in previous experiments by Pedrosa et al.47,48. The porosity as a function of distance from the center of the core in the two-day experiments also increases approximately linearly with depth for both CM and TC (Figure 3). However, the relative porosity changes of 711% for the low porosity CM limestone is more than an order of magnitude higher than for the high porosity TC limestone (73%). This indicates that the porosity of the product phases, fluorite, is similar in the two reactions and therefore independent from the porosity of the parent phase.

A distinctly different porosity structure appeared after 45 days of reaction depending on the microstructure of the starting limestone (Figure 3 b). (U)SANS characterization of the low porosity CM limestone indicates an altered zone with up to 20% porosity, surrounded by an outermost rim of only ~13 % porosity. The inner core is still unreacted and shows a low porosity of 1-2 %, which is consistent with our measurement of the original CM limestone (dotted line in Figure 3 b). In the case of the 30


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high porosity TC limestone, however, the measured porosity at all depths within the core is higher than that of the parent phase indicating porosity generation and reaction throughout the core.

Figure 3

(U)SANS characterization after 2 and 45 days. (a) (U)SANS analyses of average

porosity in distance from the center of the core for low porosity CM and high porosity TC limestone reacted for 2 days. Porosity increases at the rim to 14 % for both limestones. (b) (U)SANS analyses of the porosity in limestones reacted for 45 days showing a reduction of porosity in the outer rim of both high and low porosity limestone.

The decrease in porosity near the rim observed in the (U)SANS characterization of the CM limestone sample after 45 days indicates that the replacement reaction is

31



accompanied by a secondary textural re-equilibration that is the final step of the replacement reaction.29 That is, after the initial replacement, a secondary, textural reequilibration occurs in which porosity is decreased by recrystallization (Figure 4).

Figure 4

SEM characterization of CM limestone replaced by fluorite after (a) 8, (b) 22 and

(c) 45 days. Near the outer rim of the samples, a region of reduced porosity is visible (marked by 1). At the rim between limestone (Calc) and fluorite (Fl), acicular fluorite growth is visible (2).

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Additional (U)SANS analyses (Figure 5 a) of samples of the low porosity CM limestone reacted for 2, 8, 22 and 45 days support our hypothesis that the replacement reaction is initially accompanied by an overall increase in porosity relative to the unreacted material before a decrease of porosity in the outermost rim can be observed. These secondary replacement layers can be clearly seen in the near-rim areas of Figure 4. A similar secondary replacement layer can be observed, but was not commented on, in the work of Pedrosa et al., 2017.48 Porosity at the center of the rock core remains constant

indicating that the replacement does not proceed throughout the complete rock core. Similarly, we also observed a decrease in porosity of the outermost layer for the higher porosity TC limestone, with reaction time again indicating a secondary recrystallization similar to that observed for the low porosity CM limestone (Figure 5 b). In contrast to the CM limestone, however, the TC limestone was replaced completely before the secondary replacement began. Therefore, textural re-equilibration seems to be independent of the extent of replacement. An interpretation consistent with our (U)SANS results would be, assuming a constant supply of reactant, that the rate of replacement with depth is 33



primarily a function of the porosity/permeability, and reactive surface area of the limestone.

Figure 5

Porosity distribution determined by (U)SANS for (a) low porosity CM limestone and

(b) high porosity TC limestone reacted for 2, 8, 22 and 45 days.

(U)SANS analyses were augmented with (U)SAXS analysis, which provides porosity analyses in a similar pore size range but with higher spatial resolution as the beam footprint on the sample, and therefore the sample volume generating the signal, is smaller. In addition, simultaneous wide-angle X-ray scattering (WAXS) can provide phase identification for each point analyzed. Here, we conducted line-scans from one edge of

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the core to the other consisting of 13 - 14 individual measurement points with (U)SAXS and WAXS spectra collected at each point. WAXS data were used to identify the sampled mineral phase (see Supporting Information, Figures S7 - S13 and Tables S3 - S11). Samples from day 4, 8 and 22 were characterized using (U)SAXS and WAXS.

35



Figure 6

: Porosity changes in CM limestone with increasing reaction time (a) 4 days reaction time,

(b) 8 days reaction time and (c) 22 days reaction time. Diagrams show porosity [%], Mean and Median Diameter [µm] for each (U)SAXS measurement point collected during linescan traversing the middle of the core indicated in (a), (b) and (c). Color code for measurement points indicates phases identified by WAXS pattern (for individual WAXS pattern see Supporting Information, Figures S7 - S9 and Tables S3 - S5), with orange indicating calcite, red indicating fluorite and pale yellow indicating a calcite/fluorite mix. Note the recrystallized area near the rim after 22 days.

For the high porosity TC limestone (Figure 7), (U)SAXS analyses show a uniform porosity. After four days of reaction time, the high-porosity TC limestone was completely

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replaced by fluorite as indicated by the WAXS diffraction pattern (see Supplementary Information). Once the limestone is replaced by fluorite, the (U)SAXS data also show no changes in the microporosity (pore sizes of 200 nm – 20 µm), which remains constant at around 10 % with a uniform distribution throughout the cross-section of the core based on (U)SAXS analyses (Figure 7) – only about 2 % higher than the porosity of the starting material. The mean and median pore sizes are approximately constant throughout the core as well (for individual values see Supporting Information, Tables S6 - 8, Figure S14), although there is some suggestion that it decreases near the rim. (U)SANS analyses, however, indicated a higher porosity in the rim of ~20 % averaging over a larger area, which can possibly be explained by the fact that the (U)SANS averages over a larger area than the highly localized (U)SAXS analyses.

37



Figure 7

Porosity changes in TC limestone with increasing reaction time (a) 4 days reaction

time, (b) 8 days reaction time and (c) 22 days reaction time. Diagrams show porosity [%] on the left y-axis and on the right y-axis mean and median diameter [µm] for each (U)SAXS measurement point collected during linescan traversing the middle of the core. The orange color code indicates that the measurement points were identified in WAXS pattern as fluorite (see Supporting Information, Figures S12 and S13, Tables S6 - S8 for individual patterns). Again, note the recrystallization of the outer edge.

The pore size distributions (PSD) of the low-porosity CM limestone generated from the (U)SAXS cross sections (Figure 8) further strengthen our hypothesis that, for a low 38


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porosity starting material, replacement results in generation of new nano-scale porosity. The PSD of the pristine low-porosity CM limestone is significantly narrower than that of the newly precipitated fluorite (Figure 8). For material reacted for 4, 8 and 22 days, we can clearly see a shift towards higher total porosity, a wider pore-size distribution, and an increase in nanoporosity in comparison to the unreacted core. The PSD of the newly precipitated fluorite also indicates an increase in nanopores consistent with the observed decrease in the mean/median pore diameter (see Supporting Information, Figures S14 and S15). For the high porosity TC limestone, the initial PSD is broader and more macropores are visible. As the high porosity TC limestone was replaced faster, it is, however, possible that the nanoporosity coalesced and formed new macropores. PSD graphs for high-porosity TC limestone are given in the Supporting Information (Figure S16).

39



Figure 8

: Pore Size Distribution analyzed by (U)SAXS for low porosity CM limestone replaced with

fluorite for 4 days (a), 8 days (b) and 22 days (c). Sample regions probed by (U)SAXS, which are fluorite as indicated by WAXS, are depicted in red, calcite is green and a mixture of the two phases is blue. Sample locations are the same as in Figure 7.

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To chemically identify the replacement phase(s), we coupled the small angle scattering data with scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). For both high- and low-porosity limestone (TC and CM), a clear reaction front between a reacted rim and an unreacted pristine rock in the middle of the core was observed in samples reacted for two days (Figure 9). For the low-porosity limestone (CM), the reacted rim was visible for all reacted samples in good agreement with (U)SAXS measurements and, as noted above, a secondary recrystallized rim was visible (Figure 4). The overall reaction front observed via SEM in the low-porosity CM limestone looks similar after 8, 22 and 45 days. Higher resolution SEM images (Figure 9) show that the reaction at the interface proceeds faster along cracks and grain boundaries, which thereby provide a fast pathway for fluids into the low porosity rock. Our data demonstrates, therefore, that, at least for the low-porosity/permeability limestone replacement by fluorite proceeds in a manner similar to that previously reported for dolomitization,71,72 in which grain boundaries act as reactive pathways.

41



When comparing BSE-contrast images to EDX-mapping for low porosity CM limestone reacted for 45 days (see Figure 10), we can clearly see a higher concentration of silicon at the grain boundary region. The origin of this silicon is unknown, but it probably indicates that smaller silicon-containing phases, e.g. micas, clays or quartz grains, are being dissolved during the experiment, releasing silicon, which then moves along the grain boundaries and reprecipitates, possibly during quenching. Its presence is, however, further proof of the importance of grain-boundary transport in the replacement process.

The newly formed fluorite crystals in the low porosity CM limestone exhibit acicular growth, but the individual crystals appear to be randomly oriented (Figure 4). In the prior study of Pedrosa et al.,47 in the case of pure calcite replacement, formation of oriented,

sheet-acicular crystals was reported, similar to other reports of the replacement of calcite by strontianite.41 In our experiments, the fluorite needles are randomly oriented, which

could be caused by the space that is available for precipitation within the limestone. Possibly, this acicular orientation is caused by the replacement, which proceeds faster

42


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along the grain boundaries than within the grains (Figure 11 b and Figure S17 of Supporting Information).

43



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Figure 9

: BSE-SEM images of high porosity TC and low porosity CM limestone at day 0, 2 and 45

of replacement reaction. The high porosity TC limestone (a) shows a reaction rim after 2 days of replacement (c) and is completely replaced after 45 days (e). Higher porosity in the replaced region is visible. The low porosity CM limestone (b) shows a clear reaction front indicating enhanced replacement along the grain boundaries after 2 days (d) and after 45 days (f).

Figure 10

SEM and SEM-EDX characterization of the replacement for the low porosity CM limestone.

(a) BSE image of low porosity CM limestone replaced for 45 days (CMF45) (b) Si distribution (c) Ca distribution and (d) F distribution. Enhanced F incorporation along the grain boundaries is visible.

For the high-porosity TC limestone, a reaction rim is visible after 2 days of reaction, which separates the unreacted limestone parent phase from the product phase (Figure 12). In comparison with CM, the higher porosity and permeability of the TC limestone increases 45



the surface area available for replacement. Therefore, the transport through the open matrix pore structure is more important than transport along grain boundaries. Thus, the sample becomes rapidly replaced in a more wholesale manner, leaving only larger grain cores to be replaced with time. Interestingly, we also do not observe acicular growth of the fluorite crystals in the product phase of the high porosity limestone experiments. This suggests that due to the higher porosity in the TC limestone, the new fluorite phase had more space during crystallization allowing an idiomorphic crystal growth (Figure 11).

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Figure 11

Two types of interface between original phase and product phase imaged SEM-BSE

contrast images of low porosity limestone reacted for 2 days (CMF2). (a) Interface with a gap between parent and product phase (b) Acicular fluorite crystal growth along preferential pathways.

47



Figure 12

Chemical mapping of the interface between limestone and fluorite in TCF2 sample showing

uniform incorporation of fluorine into limestone. The higher porosity of fluorite-replaced regions is visible in (a) with rectangular mapping area (b) marked. Mappings for Si (c), F (d) and Ca (e) show incorporation of fluorine into the limestone.

Our experimental results clearly indicate a faster replacement reaction for the limestone with a higher starting porosity. This strengthens the hypothesis that porosity acts as a fast pathway for fluorite solution to reach the inside of the rock core. Furthermore, the higher porosity provides more surface area which accelerates the replacement reaction.

3.2.

Effect of the Reactivity on Replacement by Fluorite

To identify the effect of chemical reactivity of the replacement reaction, we also reacted Wisconsin dolostone (WD) with the saturated NH4F solution. The starting porosity (3%

48


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based on fluid saturation analysis) of the low porosity WD dolostone is similar to the low porosity limestone CM (1.5% porosity). Like the limestone reaction, there was a tendency for reacted material to spall off the exterior of the cores. Gravimetric characterization after reaction with NH4F indicates relative weight decrease of up to 10%, similar to that observed for the low porosity CM limestone (Supporting Information, Table S2 and Figure S6).

(U)SANS observations (Figure 13 a) again show an increase in porosity in the outermost layer of the dolostone after replacement similar as observed above for limestone replacement. After two days of reaction with NH4F, an average porosity of 18.5 % is observed in the outermost rim, which is higher than the observed increase for both TC and CM limestone. After 45 days, porosity determined by (U)SANS is increased for all measurements in comparison to the pristine material with 3.9, 2.9, 7.8 and 4.2 % porosity for 0.4 cm, 0.5 cm, 0.6 cm and 0.8 cm distance from the core, respectively. Similar to the pure limestones, the dolostone also exhibits a secondary, textural reequilibration step in which porosity is reduced in the outermost rim (Figure 13 b) 49



Figure 13

(U)SANS analyses for low porosity CM sample and low reactivity WD sample reacted for

2 days (a) and 45 days (b).

Figure 14 a-c shows the reacted rim on the dolostone in contact with NH4F for 4, 8 and 16 days. The rim of the reacted dolomite cross-sections is clearly visible to the naked eye but perhaps not as prominent as in the previously described limestone reactions. No secondary recrystallization rim is obvious to the naked eye. In addition, especially in the 8-day sample, the replacement reaction does not form a simple reaction front but moves inward via advantageous pathways. As the initial porosity and permeability of this rock are relatively low, however, it is not clear whether these are

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controlled by pre-existing pathways within the rock, or represent wormhole-like, positivefeedback controlled pathways formed due to the new porosity caused by the reaction.73,74

Figure 14: Porosity changes during replacement of dolomite (WD) by fluorite as a function of reaction time. (a) 4-days reaction time, (b) 8-days and (c) 22-days reaction time, respectively. Porosity measurements were made using (U)SAXS.

Porosity analyses by (U)SAXS (Figure 14 a-c) did not show consistent porosity changes in the rim within the detectable pore size range. However, (U)SANS analyses showed a clear increase of the porosity with replacement (Figure 13), but this does not, 51



necessarily, correlate well with the optically observable replacement rim. After two days of replacement reaction, porosity increases near-linearly from approximately 7.1% in the core to approximately 18.5% at the rim, all of which are significantly greater than the 2.2% porosity observed in the starting material. Thus, despite the presence of the white rim near the edge of the core, it is clear that the reaction has proceeded to some extent throughout the entire core. The differences between the (U)SANS and (U)SAXS may be due to the averaging effects of the (U)SANS masks relative to the patchy replacement observed optically, whereas the highly localized (U)SAXS analysis may singly have only observed areas of no porosity change. From the optical observations, it is evident that the replacement proceeds along grain boundaries or other advantageous pathways. Possibly, the reaction proceeded throughout the core along these pathways, which are too small to be visible by the naked eye. Collected WAXS diffraction patterns of the replaced dolostone are more complicated to interpret in comparison to the limestone replacement characterization as two products (fluorite and sellaite) are expected to form. In nature, incomplete replacement of dolomite by fluorite was reported.75 52


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As noted above, optically the WD samples show a somewhat patchy outer rim with an unreacted-looking inner core. The appearance of this rim is consistent with SEM-EDX observations and ToF-SIMS mapping (Figure 15), but the structural details of the reaction rim are quite distinct from the ones observed in the limestone replacement. The previously observed reaction rims were present in the form of a sharp boundary between the replaced and the original phase. Three different regions were identified in the dolomite samples reacted for 45 days (Figure 15). One is the original dolomite limestone as identified by chemical mapping. Then, there are two other sections of the reaction rim that are apparent (regions 2 and 3). At the interface between the parent and product phases, region 2, the material exhibits a higher BSE contrast compared to regions 1 and 3, indicating a different chemical composition. Its chemical composition is Ca, F, Mg in a homogeneous distribution with grains of high Si/K concentrations (see Supporting Information, Figure S18 for elemental maps). Mineralogically, this most likely corresponds to a solid solution of fluorite (CaF2) and sellaite (MgF2) due to the homogeneous distribution of both calcium, magnesium and fluorite in these regions. Furthermore, grains 53



(50-80 µm size) of a mineral phase rich in K, Al, Si and O (possibly orthoclase feldspars KAlSi3O8) were identified by SEM-EDS mapping (Supporting Information, Figures S18 and S19).

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55



Figure 15

SEM-BSE and ToF-SIMS imaging of dolomite samples reacted with NH4F for 45 days. (a)

BSE image showing three different regions consisting of original dolomite (1), partially replaced dolomite by fluoride (2) and brittle rim zone (3). Chemical mapping by ToF-SIMS (b + c) near the outside edge (edge visible near right lower corner) indicated that the replaced region contains fluoride and that replacement propagates faster along grain boundaries. Grains visible in the F-rich parts are Si-rich grains (see Supplementary information, Figure S20 for detailed elemental maps).

The outermost rim, region 3, which has approximately the same gray value as in BSE-SEM images as the pristine core, is highly fractured parallel to the core surface, and contains large pores. The chemo-mechanical fracturing visible at the edge of the rock core (Figure 15 a, region marked with 3) is the likely origin of the missing material visible in the 8 and 22-day experiments shown in Figure 14. As was the case with the CM samples, ToF-SIMS maps of the fluorine distribution in WD reacted for 45 days indicate that fluorine migrates faster via the grain boundaries. In the case of the dolomite replacement, additional pathways within the calcite crystals via twin boundaries were preferential replacement routes (Figure 16). It is evident in SEM-EDS mappings, that the fluoride is incorporated along grain boundaries, but also along twin boundaries, which are common in calcite crystals.76 56


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Figure 16

SEM-EDX mapping of the outer rim region (region 3 in Figure 15) of dolomite reacted with

NH4F for 45 days (WDF45). An outermost rim of homogeneous Ca-F-Mg-phases is present, whereas towards the middle the dolomite is replaced along grain boundaries and possibly twin boundaries within the grains.

3.3.

Comparison between the Effect of Chemical Reactivity and Microstructure on

Replacement Extent

To compare the effect of the microstructure and the chemical reactivity on the replacement extent and replacement rate, we used rim thicknesses determined via image analyses for CM and WD samples and calculated the replacement reaction rate based on reaction time. Based on WAXS patterns for CM limestone and on SEM-EDS mapping for WD dolostone, we can safely assume that the reaction rim visible in optical light is 57



replaced material. Therefore, we have used photographs of the thin section (see Supporting Information Figure S21) and determined the rim thickness by averaging four measurements at each of the sides of the rock core thin section (left, top, right and bottom of circular sample). For high porosity TC limestone, a complete reaction of the core was observed in WAXS, therefore the reaction length was used as the radius of the sample. It is important to note that this assumption will only provide a minimum replacement reaction rate. The individual measurement values are presented in Table 4.

Table 4

Measured reaction rim width (see Supporting Information, Figure S21 for images used for

image analyses).

Sample name CMF4 CMF8 CMF22 TCF4 TCF8 TCF22 WDF4 WDF8 WDF22

Reaction rim width right [cm] 0.224 0.338 0.320

0.092 0.100 0.146

Reaction rim width Reaction rim Reaction rim width - Reaction rim width - left [cm] width - top [cm] bottom [cm] average [cm] 0.261 0.206 0.202 0.223 0.356 0.360 0.349 0.351 0.364 0.348 0.334 0.342 1.588 complete replacement based on WAXS 1.588 1.588 0.144 0.127 0.073 0.109 0.089 0.164 0.077 0.108 0.110 0.118 0.124 0.125

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Based on the measured reaction rim widths, replacement reaction rates were calculated after the following scheme: First, the volume of the core was calculated using 𝑉𝑐𝑜𝑟𝑒 = 𝜋 𝑟𝑐𝑜𝑟𝑒2 × ℎ with 𝑟𝑐𝑜𝑟𝑒 = 0.79375 cm2. We set the core height h equal to one (h = 1 cm) as all of the cores have the same height.

To calculate the reaction rim volume (𝑉𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑟𝑖𝑚), we first calculated the inner, unreacted core volume (𝑉𝑢𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑐𝑜𝑟𝑒) using the equation (4):

𝑉𝑢𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑐𝑜𝑟𝑒 = 𝜋𝑟𝑢𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑐𝑜𝑟𝑒2 × ℎ

(4)

The radius for the unreacted core (𝑟𝑢𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑐𝑜𝑟𝑒) was calculated by subtracting the reacted rim width determined via image analyses (Table 4) from the core radius. The sample height was treated the same as for the core volume calculation, namely h = 1 cm. 𝑉𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑟𝑖𝑚 was calculated using equation (5):

𝑉𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑟𝑖𝑚 = 𝑉𝑐𝑜𝑟𝑒 ― 𝑉𝑢𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑐𝑜𝑟𝑒

59


(5)


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The replacement rate 𝑅𝑟𝑒𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 was then calculated by dividing the volume of the reacted rim (𝑉𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑟𝑖𝑚) by the reaction time (𝑡𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) converted to seconds (equation 6):

𝑅𝑟𝑒𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 𝑉𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑟𝑖𝑚 𝑡𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

Table 5 Detailed results of replacement rate calculation.

Reaction rim Vcore runreacted core Vunreacted core Vreacted rim Sample name width - average 3 3 3 [cm] [cm ] [cm ] [cm ] [cm] CMF4 0.22325 0.571 1.022 0.957 CMF8 0.35075 0.443 0.617 1.363 CMF22 0.3415 0.452 0.643 1.337 WDF4 0.109 0.685 1.473 0.506 WDF8 0.1075 1.97933 0.686 1.479 0.500 WDF22 0.1245 0.669 1.407 0.572 TCF4 1.5875 TCF8 1.5875 not applicable as whole core reacted TCF22 1.5875

treaction [days]

treaction [s] 4 8 22 4 8 22

Rreplacement 3

[cm /s]

345600 691200 1900800 345600 691200 1900800

2.77E-06 1.97E-06 7.03E-07 1.46E-06 7.23E-07 3.01E-07

4 345600

5.727E-06

When comparing the relationship between replacement rate and starting porosity for the different materials (Figure 17), it is evident that a 4.6 times higher porosity leads to a change in replacement rate of half an order in magnitude. The reduced chemical reactivity does not change the replacement rate significantly. From the calculated rates for low 60


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porosity CM limestone and low porosity/low reactivity dolostone, it is evident that the reaction rate is not the same for all rock cores. This is most likely due to local inhomogeneities or higher availability of advantageous pathways.

Figure 17

Replacement reaction rate [cm3/s] as a dependence of the starting porosity for low/high

porosity limestone and dolostone.

61



4. Conclusion In conclusion, this study presents the comparison of the effect of porosity changes on replacement reactions with changes in the chemical reactivity and their effects. It is evident that a higher starting porosity accelerates the replacement reaction, as expected due to higher reactive surface area and more accessible pathways for the fluid to migrate into the rock. In the case of low porosity limestone replacement, grain boundaries are pathways into the rock core. During the replacement reactions, these grain boundary regions are dissolved first, and material is then reprecipitated in the shape of acicular crystals due to space constraints. In a second, textural reequlibration step, porosity is closed, and the acicular crystals are reformed into a more energy-efficient morphology. Furthermore, we were able to identify that changes in the high porosity limestone were mainly in the macropore regime, whereas the lower porosity limestone exhibits mainly changes on the nano-scale. Similar to the acicular fluorite growth, we hypothesize here that space constraints result in the formation of smaller pore sizes in the less porous limestone. 62


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In the case of dolostone replacement, which we used as an example of lowered chemical reactivity, the reaction proceeded even slower and was more complex to interpret. Similar to the limestone, a reaction rim formed, however, this reaction rim was not completely matched to the porosity changes. By chemical composition mappings, we were able to identify two different regions of the reaction rim: a fluorine-rich region near the interface to the original rock, which consists most likely of a solid solution between fluorite (CaF2) and sellaite (MgF2). The outermost edge of the reacted rim exhibited higher fluorine concentrations along grain boundaries and twin boundaries. Due to the lower chemical reactivity, the replacement reaction was decelerated, offering an opportunity to capture these replacement pathways. Lastly, the preferential pathways available in a rock’s texture consist of fractures, connected pores, unconnected pores, grain boundaries and twin boundaries. Depending on the availability of these features, replacement reactions proceed along these advantageous pathways. Our results emphasize the importance of detailed microstructural and textural composition analyses when addressing replacement rates and reactions in experimental systems and in nature. 63



Page 64 of 101

5. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne

National

Laboratory

under

Contract

No.

DE-AC02-06CH11357.

We

acknowledge the support of the National Institute of Standards and Technology, Center for Neutron Research, U.S. Department of Commerce in providing the research neutron facilities used in this work. Access to both NBG30 SANS and BT5 USANS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. Certain commercial equipment, instruments, materials and software are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and 64


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Technology or the Department of Energy, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. A portion of this research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. We would like to thank M. Pearce and two anonymous reviewers for their constructive and detailed comments as well as editor S. Chakraborty for editorial comments.

65



AUTHOR INFORMATION Corresponding Author * Phone: ++1 865-576-7184; e-mail: [email protected], [email protected]

Supplementary Information for Controls of Microstructure and Chemical Reactivity on the Replacement of Limestone by Fluorite Studied Using Spatially Resolved Small Angle Xray and Neutron Scattering: Porosity determination in reference materials, Pore Size distribution analysis in reference materials, wide-angle scattering spectra, SEM investigation of needle-shaped fluorite, detailed (U)SAXS and (U)SANS results.

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Aquilano,

D.

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(10.4),(01.8),(01.2), and (00.1) Twin Laws of Calcite (CaCO3): Equilibrium Geometry of the Twin Boundary Interfaces and Twinning Energy. Cryst. Growth

Des. 2010, 10 (7), 3102–3109. Table of Contents Graphic

83



Scanning Electron Microscopy Backscattered Electron (SEM-BSE) contrast images of the starting materials. (a) High porosity TC limestone, (b) low porosity CM limestone and (c) low porosity WD dolostone. The low porosity WD dolostone contains dolomite (dol), orthoclase (or) and quartz (qtz) as identified by XRD and SEM-EDS mappings (see Supporting Information, Figure S1 for mappings.) 214x466mm (150 x 150 DPI)


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Radial Cd masks designed for this experiment. 144x147mm (96 x 96 DPI)



(U)SANS characterization after 2 and 45 days. (a) (U)SANS analyses of average porosity in distance from the center of the core for low porosity CM and high porosity TC limestone reacted for 2 days. Porosity increases at the rim to 14 % for both limestones. (b) (U)SANS analyses of the porosity in limestones reacted for 45 days showing a reduction of porosity in the outer rim of both high and low porosity limestone. 244x90mm (300 x 300 DPI)


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SEM characterization of CM limestone replaced by fluorite after (a) 8, (b) 22 and (c) 45 days. Near the outer rim of the samples, a region of reduced porosity is visible (marked by 1). At the rim between limestone (Calc) and fluorite (Fl), acicular fluorite growth is visible (2). 118x268mm (300 x 300 DPI)



Porosity distribution determined by (U)SANS for (a) low porosity CM limestone and (b) high porosity TC limestone reacted for 2, 8, 22 and 45 days. 258x89mm (300 x 300 DPI)


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Porosity changes in CM limestone with increasing reaction time (a) 4 days reaction time, (b) 8 days reaction time and (c) 22 days reaction time. Diagrams show porosity [%], Mean and Median Diameter [µm] for each (U)SAXS measurement point collected during linescan traversing the middle of the core indicated in (a), (b) and (c). Color code for measurement points indicates phases identified by WAXS pattern (for individual WAXS pattern see Supporting Information, Figure S8 and S9 and Table S3, S4 and S5), with orange indicating calcite, red indicating fluorite and pale yellow indicating a calcite/fluorite mix. Note the recrystallized area near the rim after 22 days. 168x116mm (300 x 300 DPI)



Porosity changes in TC limestone with increasing reaction time (a) 4 days reaction time, (b) 8 days reaction time and (c) 22 days reaction time. Diagrams show porosity [%] on the left y-axis and on the right y-axis mean and median diameter [µm] for each (U)SAXS measurement point collected during linescan traversing the middle of the core. The orange color code indicates that the measurement points were identified in WAXS pattern as fluorite (see Supporting Information, Figure S13 and S14, Table S6, S7 and S8 for individual patterns). Again, note the recrystallization of the outer edge. 175x115mm (300 x 300 DPI)


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Pore Size Distribution analyzed by (U)SAXS for low porosity CM limestone replaced with fluorite for 4 days (a), 8 days (b) and 22 days (c). Sample regions probed by (U)SAXS which are fluorite as indicated by WAXS are depicted in red, calcite is green and a mixture of the two phases is blue. Sample locations are the same as in Figure 8. 668x899mm (150 x 150 DPI)



BSE-SEM images of high porosity TC and low porosity CM limestone at day 0, 2 and 45 of replacement reaction. The high porosity TC limestone (a) shows a reaction rim after 2 days of replacement (c) and is completely replaced after 45 days (e). Higher porosity in the replaced region is visible. The low porosity CM limestone (b) shows a clear reaction front indicating enhanced replacement along the grain boundaries after 2 days (d) and after 45 days (f). 224x245mm (300 x 300 DPI)


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: SEM and SEM-EDX characterization of the replacement for the low porosity CM limestone. (a) BSE image of low porosity CM limestone replaced for 45 days (CMF45) (b) Si distribution (c) Ca distribution and (d) F distribution. Enhanced F incorporation along the grain boundaries is visible. 231x181mm (300 x 300 DPI)



Two types of interface between original phase and product phase imaged SEM-BSE contrast images of low porosity limestone reacted for 2 days (CMF2). (a) Interface with a gap between parent and product phase (b) Acicular fluorite crystal growth along preferential pathways. 102x181mm (300 x 300 DPI)


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Chemical mapping of the interface between limestone and fluorite in TCF2 sample showing uniform incorporation of fluorine into limestone. The higher porosity of fluorite-replaced regions is visible in (a) with rectangular mapping area (b) marked. Mappings for Si (c), F (d) and Ca (e) show incorporation of fluorine into the limestone. 221x95mm (300 x 300 DPI)



(U)SANS analyses for low porosity CM sample and low reactivity WD sample reacted for 2 days (a) and 45 days (b). 304x109mm (300 x 300 DPI)


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Porosity changes during replacement of dolomite (WD) by fluorite as a function of reaction time. (a) 4-days reaction time, (b) 8-days and (c) 22-days reaction time, respectively. According porosity measurements using (U)SAXS. 232x154mm (300 x 300 DPI)



SEM-BSE and ToF-SIMS imaging of dolomite samples reacted with NH4F for 45 days. (a) BSE image showing three different regions consisting of original dolomite (1), partially replaced dolomite by fluoride (2) and brittle rim zone (3). Chemical mapping by ToF-SIMS (b + c) near the outside edge (edge visible near right lower corner) indicated that the replaced region contains fluoride and that replacement propagates faster along grain boundaries. Grains visible in the F-rich parts are Si-rich grains (see Supplementary information, Figure S20 and S21 for detailed elemental maps). 139x177mm (300 x 300 DPI)


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SEM-EDX mapping of the outer rim region (region 3 in Figure 15) of dolomite reacted with NH4F for 45 days (WDF45). An outermost rim of homogeneous Ca-F-Mg-phases is present, whereas towards the middle the dolomite is replaced along grain boundaries and possibly twin boundaries within the grains. 201x158mm (300 x 300 DPI)



Replacement reaction rate [cm3/s] as a dependence of the starting porosity for low/high porosity limestone and dolostone. 232x197mm (150 x 150 DPI)


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TOC 254x146mm (300 x 300 DPI)


Controls of Microstructure and Chemical Reactivity on the

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