Experimental study of the aragonite to calcite transition in aqueous solution

Abstract

The experimental replacement of aragonite by calcite was studied under hydrothermal conditions at temperatures between 160 and 200 °C using single inorganic aragonite crystals as a starting material. The initial saturation state and the total [Ca²⁺]:[CO₃²⁻] ratio of the experimental solutions was found to have a determining effect on the amount and abundance of calcite overgrowths as well as the extent of replacement observed within the crystals. The replacement process was accompanied by progressive formation of cracks and pores within the calcite, which led to extended fracturing of the initial aragonite. The overall shape and morphology of the parent aragonite crystal were preserved. The replaced regions were identified with scanning electron microscopy and Raman spectroscopy.

Experiments using carbonate solutions prepared with water enriched in ¹⁸O (97%) were also performed in order to trace the course of this replacement process. The incorporation of the heavier oxygen isotope in the carbonate molecule within the calcite replacements was monitored with Raman spectroscopy. The heterogeneous distribution of ¹⁸O in the reaction products required a separate study of the kinetics of isotopic equilibration within the fluid to obtain a better understanding of the ¹⁸O distribution in the calcite replacement. An activation energy of 109 kJ/mol was calculated for the exchange of oxygen isotopes between [C¹⁶O₃²⁻]_aq and [H₂¹⁸O] and the time for oxygen isotope exchange in the fluid at 200 °C was estimated at ∼0.9 s. Given the exchange rate, analyses of the run products imply that the oxygen isotope composition in the calcite product is partly inherited from the oxygen isotope composition of the aragonite parent during the replacement process and is dependent on access of the fluid to the reaction interface rather than equilibration time. The aragonite to calcite fluid-mediated transformation is described by a coupled dissolution–reprecipitation mechanism, where aragonite dissolution is coupled to the precipitation of calcite at an inwardly moving reaction interface.

Introduction

The relative stability fields in P, T space of the two most common polymorphs of CaCO₃, calcite and aragonite, are well known (Carlson, 1983, Tucker and Wright, 1990). Aragonite is the high pressure phase and forms stably in metamorphic rocks. Nevertheless, during exhumation it is only rarely preserved and transforms to calcite, the stable phase at ambient conditions. The preservation of aragonite in high pressure rocks has been used as an indicator of the P−T trajectories during uplift (Carlson and Rosenfeld, 1981), assuming that transformation rates are controlled by the rate of aragonite–calcite interface migration and that activation energies derived in experimental fluid-free systems can be extrapolated to lower temperatures and pressures where the aragonite–calcite boundary is crossed. This extrapolation also assumes that the same mechanism operates in nature.

Aragonite commonly forms under conditions where calcite is considered to be the stable phase. Many sea shells and corals are composed totally or in part of aragonite which can also form inorganically as ooliths in warm shallow seas (Saylor, 1928, Margolis and Rex, 1971, Palmer and Wilson, 2004). Aragonite deposits have been found in the phreatic zones of thermal and hydrothermal caves (Dublyansky, 1995) as well as in the vadose zones of many limestone caves around the world (Hill and Forti, 1997). The precipitation of aragonite under conditions where calcite is the stable phase has been attributed to many factors, including the degree of supersaturation, the presence of impurity ions (magnesium and strontium), pH, temperature, pressure and carbon dioxide content (Curl, 1962, Berner, 1975, Hill and Forti, 1997, Škapin and Sondi, 2010).

Whatever the origin of the aragonite, the thermodynamic expectation is that, under earth surface conditions, it should transform to calcite with time, but the factors which inhibit or promote the transformation are not well understood. Whereas the solid-state transition of aragonite to calcite is slow, even by geologic standards (Kunzler and Goodell, 1970), dissolution and precipitation reactions can be fast (Wary and Daniels, 1957, Fyfe and Bischoff, 1965, Busenberg and Plummer, 1986, Deleuze and Brantley, 1997, Zhou and Zheng, 2001) and the transformation in an aqueous solution takes place far too rapidly to allow the preservation of aragonite in metamorphic rocks during the time-scale of uplift (Brown et al., 1962, Carlson and Rosenfeld, 1981). It is estimated that to preserve aragonite in high pressure rocks, it must enter the calcite stability field at temperatures between 125 and 175 °C (Carlson and Rosenfeld, 1981) and that its preservation requires the absence of a free aqueous fluid.

The recognition of an aragonite precursor is generally based on textural evidence since the aragonite morphology and microstructure is often preserved even after the replacement is complete (Kendall and Tucker, 1973, Folk and Assereto, 1976). For example, Mazzullo (1980) investigated limestone samples from the Capitan Limestone deposit, Guadalupe Mountains, New Mexico (USA) and found fan-shaped crystals in a replacement calcite crystal with inclusions that maintained the original aragonite outline. Most studies of speleothems show that their formation took place by transformation of aragonite to calcite and they exhibit a mosaic of calcite crystals with aragonite relics preserved within them (Martín-García et al., 2009). Diagenesis of speleothems is very often controlled by the chemical composition of the aqueous solution, which is always supersaturated with respect to calcite, but typically fluctuates from undersaturated to supersaturated with respect to aragonite (Sánchez-Moral et al., 2006). However, inhibitors (e.g. PO₄³⁻, Mn²⁺) are known to play a role in the preservation of aragonite even in the presence of an aqueous solution (Berner, 1971, Dromgoole and Walter, 1990).

Calcite trace element and isotopic compositions are widely used as environmental proxies (e.g. Hellstrom and McCulloch, 2000, Huang et al., 2001, Mason et al., 2006, Banner et al., 2007, Buhl et al., 2007, Fairchild and Treble, 2009) and the extent to which these compositions are primary or altered by diagenetic processes, in cases where the original carbonate was aragonite, raises important issues. In a study of the progressive alteration of primary biogenic aragonite to diagenetic low-Mg calcite, Brand (1989) concluded that the mechanism involved diffusion-controlled dissolution and reprecipitation, but that the isotopic chemistry of the calcite was determined by the biogenic aragonite. In a detailed study of aragonite–calcite relationships in speleothems, Frisia et al., 2000, Frisia et al., 2002 found that replacement calcite inherited the precursor aragonite δ¹³C signal and the U content, as well as its textural properties.

The mechanism and kinetics of the aragonite to calcite transformation under various fluid compositions has been studied extensively over many years (e.g. Chaudron, 1954, Brown et al., 1962, Davis and Adams, 1965, Bischoff and Fyfe, 1968, Bischoff, 1969) with many different experimental methods, including dilatometric thermal analysis (Topor et al., 1981), Electron Paramagnetic Resonance on Mn²⁺ ions (Lech and Slezak, 1989), potentiometric studies (Königsberger et al., 1989), differential scanning calorimetry (Peric et al., 1992), and Ca isotope attenuation (Berndt and Seyfried, 1999). The role of organic additives (e.g. amino acids (Allen et al., 1970)) and the behaviour of magnesium and strontium during the transformation (e.g. Katz et al., 1972, Yoshioka et al., 1986) have also been investigated. Nevertheless, a number of significant issues remain unclear, specifically the effect of the molar volume change on the transformation mechanism, the conditions under which the transformation is pseudomorphic and the role of fluid composition on the replacement process.

The mechanism of phase equilibration in the presence of an aqueous solution has been the subject of renewed interest, especially the conditions under which dissolution and precipitation are spatially and temporally coupled at a reaction interface, resulting in a pseudomorphic replacement preserving the external morphology as well as aspects of the crystallographic orientations of the parent phase (Putnis, 2002, Putnis, 2009, Putnis and Putnis, 2007). Pseudomorphic replacement has been found to be a common feature in many experimental mineral–fluid reactions in response to changes in fluid chemical composition (see Putnis, 2009, for a review) although the origin of the coupling, which implies that the rates of dissolution and precipitation are equal, is still the subject of some controversy, especially whether the coupling is a result of fluid composition (Putnis, 2002, Putnis, 2009) or pressure solution (Maliva and Siever, 1988, Nahon and Merino, 1997, Merino and Banerjee, 2008). The role of fluid chemistry in mineral replacement has been emphasised by Xia et al. (2009) who showed that for the replacement of pentlandite, (Fe,Ni)₉S₈, by violarite, (NiFe)₃S₄, the solution pH determined whether the rate limiting step in the reaction was the dissolution (in which case a perfect pseudomorph was formed) or precipitation (which partially uncoupled the precipitation from the dissolution and resulted in a less perfect pseudomorph).

A previous study has shown how single aragonite crystals can be experimentally transformed into calcite by means of a fluid-mediated reaction at temperatures lower than those required for a solid-state transformation (Perdikouri et al., 2008). In this paper, we describe a series of experiments to further clarify various aspects of the replacement of aragonite by calcite in the presence of aqueous solutions and to identify how fluid composition affects the degree of pseudomorphism. We performed hydrothermal experiments in solutions of various compositions over a range of temperatures and reaction times. The reaction products were characterized by scanning electron microscopy (SEM), Raman spectroscopy and some reaction products were also analyzed by electron microprobe. Experiments were also carried out using solutions prepared with water enriched with ¹⁸O (97%) to monitor the extent of oxygen exchange between the fluid and the solids in the fluid-mediated transformation from aragonite to calcite and hence to provide further insights into the mechanism of the mineral–fluid reaction.