Article pubs.acs.org/crystal
Precipitation of Ordered Dolomite via Simultaneous Dissolution of Calcite and Magnesite: New Experimental Insights into an Old Precipitation Enigma G. Montes-Hernandez,*,†,‡ N. Findling,‡ F. Renard,‡,§ and A.-L. Auzende∥ †
CNRS, Institut des Sciences de la Terre (ISTerre), F-38041 Grenoble, France Université Grenoble Alpes, ISTerre, F-38041 Grenoble, France § Physics of Geological Processes, University of Oslo, Box 1048 Blindern, 0316 Oslo, Norway ∥ Institut de Minéralogie et de Physique des Milieux Condensés, CNRS−Université Paris Diderot−UPMC, F-75252 Paris, France ‡
ABSTRACT: In the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200 °C). The temperature and high-carbonate alkalinity have significantly copromoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200 °C from XRD patterns and FESEM observations. Conversely, a higher proportion of ordered dolomite, i.e., clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100 °C and two kinetic steps were identified: (1) protodolomite formation after about 5 days of reaction, characterized by rounded submicrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2θ in XRD patterns; (2) protodolomite to dolomite transformation, probably produced by a coupled dissolution− recrystallization process. Herein, the activation energy was estimated to be 29 kJ/mol by using a conventional Arrhenius linear equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems. success at high temperature (>100 °C).5,10 In a similar way, more sophisticated experimental setups have been built “hydrothermal flow reactors” to investigate the kinetic behavior of dolomite precipitation, but, these systems have systematically used pre-existent dolomite crystals “or seed material”, this can indeed provide idealized or limited information on the overgrowth of dolomite (syntaxial and/or epitaxial growth).9 2. Calcite dolomitization is carried out by placing high-purity calcite or limestone material in contact with Mg-rich solution. This calcite replacement by ordered dolomite is particularly favorable also at high temperature (>100 °C).11−13 This reaction mechanism could explain the massive dolomite formation in sedimentary environments if such sediments are submitted to significant temperature variations and/or to significant changes of pore-fluid chemistry over geologic times (see, e.g., ref 14). 3. Bioassisted dolomitization occurs by using sulfate-reducing or aerobic heterotrophic bacteria, hypersaline or seawater solutions, and anoxic or oxic conditions in controlled laboratory
1. INTRODUCTION The formation and textural properties of dolomite (CaMg(CO3)2) have already been investigated in the past two centuries.1−4 However, various questions still remain unanswered concerning its formation mechanism and kinetics in natural systems as well as its synthesis in the laboratory. For example, the formation of ordered dolomite at ambient temperature is virtually impossible, possibly due to the high hydration nature of Mg2+ ions in solution at low temperature.4−6 Moreover, the scanty distribution of modern dolomite in nature contrasts strongly with its common abundance in ancient sedimentary rocks of marine origin, leading to the paradox commonly referred to as the “dolomite problem”.7−9 Experimental syntheses regarding the physicochemical conditions, reaction mechanisms, and kinetics at which dolomite can be formed could resolve this paradox. Typically, the dolomite precipitation in the laboratory has been investigated by reference to the natural setting. In this way, three main kinds of experimental configurations have been carried out: 1. The first is direct and homogeneous precipitation by mixing (fast or slowly) two predefined solutions, one containing a Mg/Ca ratio ≥ 1 and the other containing dissolved carbonate ions. This simple reaction pathway has only © 2014 American Chemical Society
Received: October 17, 2013 Revised: December 5, 2013 Published: January 13, 2014 671
dx.doi.org/10.1021/cg401548a | Cryst. Growth Des. 2014, 14, 671−677
Crystal Growth & Design
Article
Table 1. Summary of Experimental Conditions and Mineral Content in Solid Products Deduced from Rietveld Refinements of XRD Patternsa pH
product amount (%) from XRD
run no.
solid reactants
t (days)
T (°C)
solution
initial
final
calcite
magnesite
dolomite
1b 2b 3b 4b 5c 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3 CaCO3−MgCO3
90 90 90 90 90 90 90 90 90 90 5 13 21 42 60 5 13 21 42 60 5 20
50 75 100 150 200 50 75 100 150 200 150 150 150 150 150 200 200 200 200 200 100 100
HAS HAS HAS HAS HAS PW PW PW PW PW HAS HAS HAS HAS HAS HAS HAS HAS HAS HAS HAS HAS
8.9 8.9 8.9 8.9 8.9 ≈6.5 ≈6.5 ≈6.5 ≈6.5 ≈6.5 8.9 8.9 8.9 8.9 8.9 8.9 8.9 8.9 8.9 8.9 8.9 8.9
9.0 9.1 9.2 9.3 8.0d 10.3 10.2 10.0 9.6 9.0 9.1 9.2 9.2 9.3 9.4 9.2 9.1 9.2 9.1 9.2 9.1 9.0
38 47 43 26 11 61 47 49 27 54 44 56 39 28 28 34 29 39 16 14 46 47
57 49 43 40 4 37 52 50 72 39 45 25 42 38 34 37 40 30 26 21 47 45
0 0 9 30 49 0 0 0 0 6e 9 13 16 26 28 24 26 28 50 52 3 6
a HAS, high-carbonate alkaline solution; PW, high-purity water; CaCO3, calcite; MgCO3, magnesite. bFor runs 1−4, natrite mineral NaCO3 was also quantified, completing it 100% in solid. cFor run 5, a microleakage was suspected at the end of experiment; probably this has enhanced the precipitation of eitilite (26%). The pH was measured ex situ at room temperature (≈20 °C). dUnrealistic pH. eDisordered dolomite.
(Ca(OH)2). The specific procedure and fine calcite characterization have already been reported by Montes-Hernandez et al.20 Magnesite. Rhombohedral single crystals (
