Limits on Calcite and Chalk Recrystallization - ACS Publications

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Limits on Calcite and Chalk Recrystallisation L. Z. Lakshtanov, D. V. Okhrimenko, O.N. Karaseva, and S. L. S. Stipp Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00537 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Limits on Calcite and Chalk Recrystallisation

L.Z. Lakshtanov1,2, D.V. Okhrimenko1, O.N. Karaseva2, S.L.S. Stipp1 1

Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark 2 Institute of Experimental Mineralogy RAS, 142432 Chernogolovka, Russia

Corresponding author *

E-mail: [email protected]

Abstract Recrystallization requires dissolution in pore fluids and precipitation on existing particle surfaces in a process known as Ostwald ripening, where the smallest particles feed growth on the larger ones. Recrystallization conditions are optimized in industry when commercial products require larger crystals and it is the dominant process in burial diagenesis, which turns sediments into rock. In chalk, the original coccolith elements are often still clearly distinguishable, which is a surprise considering the rapid rates of pure calcite recrystallisation. We studied the rates of calcite precipitation on the surface of the particles in various chalk samples, using the constant composition method over a wide range of supersaturations. Our results show that dependence of calcite precipitation rate on supersaturation does not obey the parabolic law, which is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation mediated growth. The rates of calcite precipitation on the chalk surfaces at the lowest supersaturation examined in the present work are 3 orders of magnitude lower than on pure calcite surfaces. This means that during chalk recrystallization, when supersaturation is extremely low, recrystallization can be significantly suppressed. The presence of organic compounds has an important influence on recrystallization kinetics so along with other possible causes of inhibition, the mechanism of calcite precipitation is a factor in the extremely low rates of the chalk recrystallization.

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Introduction Calcite kinetics makes it a popular model solid for fundamental studies of crystal growth and dissolution. It is a common material in commercial products so considerable is known about its properties and behaviour. In a pure system, it recrystallises rapidly, increasing its crystal diameter dramatically, on the scale of days to months.1 Chalk, which is sometimes used directly for making cement and as a filler for example for paper, is a rock that can be composed of nearly pure calcite but surprisingly, it behaves quite differently. Chalks of North Western Europe, which often serve as the reservoirs for oil and gas and aquifers for water, were formed by sedimentation of coccoliths, the mineralized parts of unicellular algae. In addition to calcite, chalk contains minor amounts of silica and clays.2 In many chalk samples, the original calcite coccoliths are very well preserved as intact or fragmented disks and individual elements, with sizes ranging from 0.5 to 10 µm, implying that recrystallization during diagenesis was minimal or negligible. This results in the high porosity of chalks, which vary between 20 and 35%,2 a consequence of the lack of pronounced recrystallization and cementation.3 Formation and growth of coccoliths is controlled in vesicles inside the algae cell, by acidic polysaccharides.4,5 Coccolithophores use these complex organic polymers to control size and shape4-6 as well as to form a protective coating on the coccolith element surfaces.7-8 There are abundant evidences that polysaccharides essentially impede calcite crystallization.9-15 It is widely agreed that the inhibition of crystallization by impurities such as biogenic exopolymers (polysaccharides and polyaminoacids) on precipitation rates is caused by their adsorption at the active growth sites on mineral surfaces. Adsorption leads to a reduction of the effective surface free energy and therefore diminishes the energy barrier for nucleation. An adsorbed species can render difficult attachment of the lattice ions during growth and functions as an obstacle for step advance (called “step-pinning”16). Pinning inhibits crystal growth by producing many kink sites along the length of the step.14,17,18 Impurities can also modify the mechanisms of calcite crystallization, depending on the calcite surface history. Inhibitor molecules, adsorbed on calcite, can induce surface nucleation mediated growth.18,19

Ancient polysaccharides extracted from chalk are still active. They have been shown to drastically affect calcite morphology, dissolution and growth when they are present in a supersaturated solution.20,21 It is reasonable to assume that polysaccharides, inherent on chalk particle surfaces, are responsible for extremely low recrystallization rates. Concentration of organic carbon, observed with X-ray photoelectron spectroscopy (XPS) as C-O bonds on the ACS Paragon Plus Environment

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chalk particle surfaces, decreased slightly after treatment of chalk samples with oxidizing solutions such as H2O2 and NaOCl.22 Lower C-O concentrations after treatment correlated with higher recrystallization rates. However, chalk from water and gas saturated zones in the North Sea Basin, can contain other organic impurities than polysaccharides.23 Some of these serve as biomarkers,24 complex individual organic compounds (biomolecules) generated by organisms. Examples include n-paraffins, long-chain (C15-C19) n-alkanes, isoprenoids and various terpanes. These molecules are mostly composed of carbon and are thus seen in XPS spectra as C-C and C-H bonds.23 Low polarity makes them easily prone to extraction with nonpolar solvents. In contrast, polysaccharides are extremely difficult to extract from chalk because of their strong interaction with calcite surfaces.25 The presence of hydrocarbons decreases the surface energy of the chalk samples,23 while presence of polysaccharides reduces the recrystallization rates of chalk, prevents Ostwald ripening22 and changes the mechanism of calcite precipitation.19 However, the influence of both types of organic compounds on chalk recrystallization is not yet well understood. Lakshtanov et al.26 observed decreased calcite recrystallization rate in the presence of the polysaccharide, alginate. However, this inhibition was not as strong as would be expected considering the step pinning model of inhibition.14,17 It was suggested that the adsorbed alginate chains, on the one hand, pin the advancing step on the calcite surface and on the other hand, they facilitate surface nucleation of calcite, decreasing the effective interface free energy.26

This work represents continues efforts to understand the inhibitory effects of organic compounds on calcite and at the same time, to elucidate the reasons for the extremely low recrystallization rate of chalk. With the constant composition technique, we investigated calcite precipitation on samples of natural chalks. We observed the relationship between precipitation rate and supersaturation to determine the active mechanism of calcite precipitation on particle surfaces from a number of different chalk samples. Data obtained have been correlated with the surface concentration of organic carbon as well as with ζ potential measurements from the same samples. We used solvent extraction to remove some of the nonpolar organic compounds and then determined the rate of calcite precipitation and ζ potential for some samples following hydrocarbon extraction.

Materials and methods

Chalk samples ACS Paragon Plus Environment


Chalk samples were of Maastrichtian age and provided by Maersk Oil and Gas A/S, except for Ålborg chalk, which was collected from the Ålborg Portland quarry near Ålborg, Denmark. The Maersk Oil chalks were fragments from core samples drilled from a variety of locations in water saturated (2-3, 2-4, and 7-1) and gas saturated (10-4) zones in the Danish North Sea fields. Geologic evidence indicated that none of the samples had ever been in contact with oil. All chalk fragments were somewhat friable. They were gently crushed in an agate mortar and sieved, the size fraction 99.8%) and methanol (CH3OH, 7 vol.%; Sigma Aldrich, HPLC grade, purity >99.9%). After 12 hours, the extraction procedure was stopped and the chalk sample was collected and dried. This sample is referred in the text as 10-4 treated. Previously it was shown23 that extract from similar ACS Paragon Plus Environment

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chalk samples contains high concentrations of petroleum biomarkers, including steranes, diasteranes, triaromatic steroids and tri-pentacyclic hopanes, but concentration of the components found in crude oil (3–4 ring polycyclic aromatic hydrocarbons) was significantly lower. Extraction of these compounds decreased the intensity of the C-C/C-H peak in the XPS C1s high resolution spectra23 and increased hydrophilic properties of the chalk surface (more water wet; higher surface energy). The peak corresponding to C-O bonds remained unchanged, which is an indication that polysaccharides on the chalk surface were not influenced by the CH2Cl2/CH3OH extraction procedure. Another chalk sample, 2-3 was also divided into two portions. One portion was investigated without treatment. The other portion was treated with H2O2, to partially remove the polysaccharides from the chalk surface. Belova et al.22 showed that treatment with an oxidizing agent, such as H2O2, leads to decreased intensity for the peak representing the C-O bonds in XPS spectra, consistent with removal of polysaccharides. This oxidised sample was used in the zeta potential experiments and referred as to Chalk 2-3 t. Table 1. Atomic percent of carbon derived from the XPS peak intensity ratios from the chalk samples. The results of fitting the C1s peak with four organic carbon contributions: adventitious carbon and hydrocarbons (C-C and C-H), alcohol groups (C-O(H)), carboxylic groups (O=C-O) and carbonate (CO3)22,23,27 Chalk 2-3 2-4 Ålborg 10-4 7-1

Zone WS WS quarry GS WS

CO3 57.34 47.60 65.00 28.60 55.90

C-O 27.53 36.29 13.70 12.90 14.20

CC or CH 15.13 16.10 17.70 52.60 23.80

O=C-O 5 5.1 3.7 5.9 6.1

C-O/CO3 0.48 0.76 0.21 0.45 0.25

CC or CH/CO3 0.26 0.34 0.27 1.84 0.42

Precipitation experiments To study the calcite precipitation reaction, we used the constant composition method described earlier.12,14,15,19,28,29 The experiments were run thermostatically at 25° C. The working solutions consisted of appropriate amounts of CaCl2 and NaHCO3 solutions to achieve the targeted supersaturation with respect to calcite. The total volume of the solution was 60 mL The ionic strength was maintained by 0.1 M NaCl solution in all experiments. We adjusted solution pH to 8.5 by adding 0.1 M HCl or NaOH solutions. To maintain constant CO2 partial pressure in the working solution, we kept the headspace in the vessel as small as possible and verified this by monitoring pH of the solution 1 hour before the start of ACS Paragon Plus Environment


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each experiment. Small portions of powdered chalk sample (0.05 – 0.15 g) were added to the working solution to initiate precipitation. When calcite begins to precipitate, pH of the solution drops. This is compensated by titrating CaCl2 and Na2CO3 / NaHCO3 solutions using a very low flow rate peristaltic pump (Ismatech). The concentration of the carbonate solution was double the concentration of carbonate in the working solution. This procedure keeps steady state conditions for calcite precipitation. Rates were determined from the recorded amounts of added titrant solutions. To verify constant composition, we randomly sampled and analyzed Ca2+ concentration using atomic absorption spectroscopy (AAS). The supersaturation was constant within ±5%. The amount of mass added to the system during the experiments (3.5E-6 to 2E-4 g) can be determined from the volume of the solutions consumed in the constant composition set up. The new material had about 0.1 % added to the original mass. We used X-ray diffraction (XRD) (a Bruker D8 Advance Da Vinci diffractometer with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range from 10° to 60°, stepping 0.01° and 1 s/step) to analyse the chalk samples before and after the experiments to confirm that no other phases were produced. Although vaterite, a metastable CaCO3, is a possible product, it transforms rapidly to calcite in solution. Recrystallisation is also possible on dry surfaces, in the water layer that adsorbs from air.The detection limit for XRD is 1 to 2% of an ordered solid and considering that the experiments lasted 3-10 minutes and it took about 15 minutes to take the sample, separate it from the solution, carry it to the XRD, mount it and start the analysis, and diffraction data were collected over 80 min, we did not expect to see evidence of vaterite and indeed, found none. Neither did we find any phases other than those initially present in the chalk. Supersaturation, S, can be defined as 1/2

 IAP  S=    K sp 

 aCa 2+ aCO32− =   K sp

1/2

  , 

(1)

where IAP represents the ion activity product, aion, ion activity and Ksp, the solubility product of calcite. The geochemical computer code PHREEQC30 was used to calculate supersaturation and solution speciation. The precipitation rate, R, was estimated as follows: R=

[Ca ]titr dV [Ca ]titr R′ , = mA dt mA

(2)

where R′ stanfs for the solution addition rate, [Ca]titr, Ca concentration in the titrant CaCl2 solution, m, the initial seed mass, A, the specific surface area and V, the volume of the titrant added. All of the experimental conditions, including solution addition rate, R0, and calcite ACS Paragon Plus Environment

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precipitation rate, R, the initial mass of seed, m, specific surface area, A, and supersaturation, S, are listed in Table 2.

Table 2. Experimental conditions for chalk seeded precipitation of calcite. chalk 10-4

m, g 0.2 0.2 0.2 0.2

A, m2g-1 1.8 1.8 1.8 1.8

R0, mL s-1 4.00E-06 1.60E-05 4.00E-05 1.30E-04

R, mol m-2s-1 1.11E-09 4.44E-09 1.11E-08 3.61E-08

S 2.4 3.0 3.7 4.5

2-4

0.2 0.15 0.15 0.15 0.15 0.05

1.9 1.9 1.9 1.9 1.9 1.9

2.00E-06 1.50E-06 1.30E-05 4.60E-05 2.00E-04 7.00E-05

5.26E-10 5.26E-10 4.56E-09 1.61E-08 7.02E-08 7.37E-08

2.4 2.4 3.0 3.7 4.5 4.5

7-1

0.1 0.15 0.06 0.045

2.3 2.3 2.3 2.3

2.00E-06 2.60E-05 2.60E-05 6.60E-05

8.70E-10 7.54E-09 1.88E-08 6.38E-08

2.4 3.0 3.7 4.5

2-3

0.15 0.15 0.09 0.05

1.7 1.7 1.7 1.7

2.00E-06 2.00E-05 3.00E-05 5.50E-05

7.84E-10 7.84E-09 1.96E-08 6.47E-08

2.4 3.0 3.7 4.5

Ålborg

0.1 0.1 0.05 0.05

3.5 3.5 3.5 3.5

8.00E-05 1.60E-04 1.20E-04 2.00E-04

2.29E-08 4.57E-08 6.86E-08 1.14E-07

2.4 3.0 3.7 4.5

0.09

2

1.00E-06

5.56E-10

2.4

0.1 0.1 0.1

2 2 2

8.00E-06 4.00E-05 1.50E-04

4.00E-09 2.00E-08 7.50E-08

3.0 3.7 4.5

10-4 treated

BET Surface Area

The BET surface area was determined using a Quantachrome Autosorb-1 Sorption Analyzer. The nitrogen adsorption isotherms were recorded in the relative pressure range, P/P0 = 0.1 –



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0.3. Prior to analyses, the samples were degassed by heating to 120°C in vacuum ( 2, we plotted ln (R/f(S)) as a function of 1/ln S. In line with Eqs 4 and 7, this should give a straight line if the exponential growth law is functional during calcite precipitation on the chalk particles. Fig. 4 shows that the experimental data for the calcite precipitation rate, represented as ln(R/f(S)) versus 1/ln S (f(S) comes from Eq. 7) can satisfactorily be approximated by straight lines for all chalk samples. Deduced from the exponential law, the straight lines designate the change of rate determining mechanism for calcite precipitation on chalk surfaces from parabolic law growth on pure calcite to 2D nucleation mediated growth. We obtained similar trends in the rates of calcite precipitation on calcite seeds in experiments with adsorbed biopolymers, alginate and polyaspartate.19 From the slope of the line, B2D/3, using Eq. 8, we can calculate the edge free energy, κ, and from κ, the interface free energy, γ, can be estimated from the relationship:19

γ = κV −1/3 ,

(9)

where V stands for the molecular volume.



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Fig. 4. ln(R/f(S)) (Eq. 7) plotted versus 1/lnS for calcite precipitation on the various chalk samples. A summary of the interface free energy, γ, determinations are shown in Table 3, for chalk samples before and after solvent extraction. The interfacial free energy, γv, obtained by the vapour adsorption method is presented for comparison. We calculated γv from the work of adhesion, WA, determined for water vapour and surface energy, γt, of chalk samples from the same cores, reported by Okhrimenko et al.23 In that study, surface energy was obtained by means of water and ethanol vapour adsorption. Chalk samples were also treated to extract the nonpolar organic matter with the Soxlet extractor using CH2Cl2/CH3OH and changes in surface energy as a result of treatment were analyzed. The interfacial surface energy was obtained by rearranging the equation for the work of adhesion:

γ v = γ t + γ l − WA

(10)

where γv represents interfacial free energy; γt, chalk surface energy; γl, water surface tension (72.8 mJ/m2) and WA stands for work of water vapour adhesion on the chalk surface. Table 3. Interfacial free energy chalk-water (γ obtained from the regression analysis of the function ln (R/f(S)) as a function of 1/lnS; γv obtained using the vapour adsorption method23). Chalk

γ, mJ m-2

γv, mJ m-2

original

treated

original

treated

2-3

99

-

-

-

2-4

106

-

-

-

7-1

96

-

153

176

10-4

84

106

55

80


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Ålborg

-

-

291

263

The values of the interfacial free energy obtained by vapour adsorption are the lowest for the chalk with the highest nonpolar organic content (10-4) and highest for the outcrop Ålborg chalk, which has the lowest level of the organic contamination according to XPS data. Since for Ålborg chalk, the slope of the corresponding line in Fig. 3 is very close to 2, we can conclude that calcite precipitation on the chalk particle surfaces from this outcrop sample does not proceed by the surface nucleation mediated growth. Therefore, the interfacial free energy cannot simply be determined using Eqs (7) and (8), as was possible for other chalks.

According to the vapour adsorption results, after partial removal of nonpolar organic matter from chalk samples, the interfacial free energy increases for all chalks from the North Sea drill cores but decreases for outcrop Ålborg chalk. The increase is explained by removal of the organic matter, making chalk surfaces less hydrophobic. The opposite behaviour for the Ålborg chalk probably lies in the significant differences in surface composition and the low reproducibility of the vapour adsorption experiments reported for this type of chalk.23 According to XPS data,23 Ålborg chalk surfaces have twice as much Si as the North Sea chalk samples, which is explained by the near shore origin, where sponge spicules are expected to be more dominant in the sediment assemblage. At the same time, the amount of adsorbed organic material is the smallest. This could be a result of the location from which the samples were taken. The Ålborg Portland quarry is in the top meters of chalk that has been exposed to freeze-thaw cycles during the Quaternary glaciation and to groundwater infiltration over the 10,000 years since. Organic acids from the overlying soil, the organic compounds resulting from the organisms living on the surface and the presence of bacteria in the groundwater could have had a controlling effect on the original composition of the Ålborg pore network surfaces. The work of wetting changed very little after the solvent treatment for extracting organic material. Taking into account the lower reproducibility of the results for the Ålborg chalk, the differences in surface energies before and after extraction for Ålborg chalk were considered insignificant and it was concluded that this sample was unaffected by the solvent extraction treatment.23

The data shown in Figs 3 and 4 demonstrate that although the core plugs came from quite different areas and depths, from water and gas zones, all North Sea chalks behaved almost identically. The interfacial free energy, γ, obtained from the precipitation kinetics compares well for all chalks and locates in the range between those theoretically estimated for calcite ACS Paragon Plus Environment


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(120 mJ m-2) and vaterite (90 mJ m-2).35,36 So it can be inferred that the analysis of calcite precipitation on the chalk surfaces in line with the 2D nucleation-mediated growth mechanism gives realistic physical parameters. We examined the chalk particles before and after the experiments to look for evidence of calcite growth using high resolution scanning electron microscopy, both in environmental mode (where water vapour in the vacuum chamber minimises charging) and on samples coated with gold. In a previous study, using the same approach, we could see evidence of new material on the surfaces of the synthetic calcite.19 Chalk particles however, are much more rough initially than synthetic calcite so we were not able to unambiguously distinguish between bumps that were new material and bumps that were there initially. The slope of the straight line in Fig. 4 for Chalk 10-4, the sample with highest content of the organic matter (Table 1), increased after extraction of nonpolar organic matter, coming very close to that for other chalks. Equally, the thermodynamic barrier for nucleation and the surface free energy for Chalk 10-4 increased after treatment as well. Thus, extraction of nonpolar organic substances changed the energetics of the surface but it did not modify the mechanism of calcite precipitation, which was still determined by surface nucleation mediated growth. The nonpolar organic matter, that had been subjected to the extraction procedure, probably only served to shield the chalk particle surfaces, which were in turn covered by other types of organic compounds, such as polysaccharide polymers.

Polysaccharides interact much more strongly with calcite and cannot be easily extracted but they can be oxidized to some extent by H2O2. Organic substances adsorbed on the calcite surface decrease the effective interface energy, γ’, diminishing the energy barrier to calcite nucleation. If we assume calcite nucleation on the adsorbed organic molecules (e.g. adsorbed biopolymer such as polysaccharide), then one should take into account three interfaces: calcite-water (γcw), adsorbed biopolymer-water (γpw), and calcite-adsorbed biopolymer (γcp). Thus the effective interface energy of a calcite nucleus, γ’, would be written as:

γ ′ = γ cw + hγ cp − hγ pw ,

(11)

where h is the nucleus shape factor (1/2 for a hemisphere).37

To minimize the change in free energy, the organic material adsorbed on the surfaces of chalk particles tends to abolish its interface with water and to create a new interface, an organic compound-calcite nucleus. This implies that the effective interface free energy, γ’, is lower than that of the calcite-water interface, γcw that in turn means that γcp < γpw. This is consistent ACS Paragon Plus Environment

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with the Gibbs adsorption equation, verifying that adsorption of the solute decreases the surface free energy:

Γ=−

1  dγ   , RT  dC 

(12)

where Γ represents adsorption density and C, solute concentration.

Electrokinetic measurements

The ζ potential for the chalk samples was determined to gain better understanding about the interactions that occur at the chalk surface. Streaming potential measurements were made in the solution saturated with the corresponding chalk, with addition of 0.001 M NaCl, at atmospheric CO2 conditions. In these experiments, we also included the chalk 2-3 samples that had been treated with H2O2 (2-3 t),22 with the intention of removing polysaccharide polymers from the chalk surface. ζ potential for pure calcite seed at pH > 8.5 is negative and becomes slightly positive at lower pH (Fig. 5).

Fig. 5. pH dependence of ζ potential for calcite and the chalk samples. Chalk sample 2-3 t had been treated with H2O2.22 Such behaviour is consistent with the surface complexation model of the calcite-water interface,38,39 which predicts a point of zero charge at pH = 8.2 on the basis of the electrokinetic measurements.40 Chalk samples have negative ζ potential, much more negative than for pure calcite (Fig. 5). There is considerable evidence that the ζ potential of natural biogenic calcite (e.g. chalk) clearly reflects the effect of adsorbed organic matter. Adsorbed organic molecules reduce the positive charge of calcite surface and reverse it to negative.19,41ACS Paragon Plus Environment


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This can be seen in Fig. 6, where the ζ potential for calcite and chalk samples is plotted

versus the relative surface concentration of C-O bonds. The presence of C-O bonding on the chalk particle surfaces is consistent with the complex polysaccharides that algae use to control coccolith morphology.22 Thus, ζ potential is a clear measure of the amount of polysaccharide on the chalk particle surfaces. In this sense, the behavior of Chalk 2-3 is indicative, where ζ potential gets less negative after the H2O2 treatment, coming more close to that of calcite (Fig. 6). ζ potential for the Ålborg chalk is also close to that of calcite, which correlates with its insignificant content of associated organic material (Table 1).

C-O/CO3

Fig. 6. ζ potential of calcite and chalk samples at pH 9.6 as a function of the relative surface concentration of O-H bonds. Chalk sample 2-3 t had been treated with H2O2.22 Comparison of the ζ potential for chalk samples with calcite implies inconsiderable contribution of electrostatics to the interaction energy. Because the pKa of carboxylate polysaccharides is usually < 5,44-46 the molecules are deprotonated, thus anionic in the pH range > pKa. As explained earlier19, since at pH > 8.5, both the calcite surface and the deprotonated polysaccharides are negatively charged, and they adsorb irrespective of their charge, the ζ potential determination indicates the specific, coordinative interaction between the functional groups of organic molecules and the calcite surface. Such specific adsorption is the main reason for strong inhibition of calcite crystallization by polysaccharides and certain other organic compounds.

Concluding remarks


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Our results show that the rate of calcite precipitation on the surface of chalk samples does not obey the parabolic law that is characteristic for calcite growth. Instead, the dependence of calcite precipitation rate on supersaturation is exponential, which indicates surface nucleation mediated growth.

The interfacial free energy, γ, obtained from precipitation kinetics compares well for all chalk core samples and locates between those for calcite and vaterite. The presence of nonpolar organic impurities, such as biomarkers, decreases the energy of the surface but does not change the mechanism of calcite precipitation. In contrast, the presence of polysaccharides on the chalk particle surfaces determines the mechanism for calcite precipitation on chalk. Analysis of calcite precipitation on chalk particle surfaces according to the two dimensional nucleation - mediated growth gives realistic physical parameters.

The rates of calcite precipitation on the chalk particle surfaces at the lowest supersaturation investigated in the present work are three orders of magnitude lower than for pure calcite. This means that during chalk recrystallization, which is one of the dominant processes in burial diagenesis, when supersaturation is extremely low, recrystallization can be significantly suppressed. This, along with other possible causes of inhibition, can be responsible for the extremely slow rates for chalk recrystallization. Our results imply that kinetic data from pure system laboratory experiments is less than ideal for predicting behaviour in nature and probably in most industry applications.

Acknowledgements We thank Keld West and Marcel Ceccato for help in the laboratories, Nico Bovet for the XPS data discussions and Kim Dalby for help with SEM imaging. The work began under the Nano-Chalk Venture, funded by the Danish Advanced Technology Foundation (HTF), Maersk Oil and Gas A/S and the University of Copenhagen. It formed the base for some of the work in the EC Marie Curie ITN called MINSC (Mineral Scaling) and was completed with a University of Copenhagen Bonus Grant to SLSS and by the Russian Foundation for Basic Research (16-05-00234) to ONK and LZL. The manuscript was improved by comments from Manuel Prieto and the anonymous reviewer.



References

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For table of contents use only

Limits on Calcite and Chalk Recrystallisation.

L.Z. Lakshtanov, D.V. Okhrimenko, O.N. Karaseva, S.L.S. Stipp.

C-O/CO3

Synopsis Our results show that the rate of calcite precipitation on the surface of chalk samples does not obey the parabolic law that is characteristic for calcite growth. Instead, the dependence of calcite precipitation rate on supersaturation is exponential, which indicates surface nucleation mediated growth. This means that during chalk diagenesis, when supersaturation is extremely low, recrystallization can be significantly suppressed. This, along with other possible causes of inhibition, can be responsible for the extremely slow rates for chalk recrystallization.


Limits on Calcite and Chalk Recrystallization - ACS Publications

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