Calorimetric study of olivine solid solutions in the system Mg2SiO4-Fe2SiO4

Solution enthalpies of synthetic olivine solid solutions in the system Mg₂SiO₄-Fe₂SiO₄ have been measured in molten 2PbO·B₂O₃ at 979 K. The enthalpy data show that olivine solid solutions have a positive enthalpy of mixing and the deviation from ideality is approximated as symmetric with respect to composition, in contrast to the previous study. Applying the symmetric regular solution model to the present enthalpy data, the interaction parameter of ethalpy (W_H) is estimated to be 5.3±1.7 kJ/mol (one cation site basis). Using this W_h and the published data on excess free energy of mixing, the nonideal parameter of entropy (W_s) of olivine solid solutions is estimated as 0.6±1.5 J/mol·K.     

Composition and solubility of precipitated copper(II) arsenates

Equilibrium reactions involving Cu(II) and As(V) have been studied with respect to formation of complexes in aqueous solutions as well as formation of solid phases. Potentiometric titrations performed at 25 °C (I = 0.1 M Na(Cl)) and at different Cu to As ratios gave no evidence for the existence of Cu(II) arsenate complexes in solution below the pH of the precipitation boundaries (pH ≈ 4), irrespective of the Cu to As ratio and pH. Mixing of solutions of Cu(II) and As(V) at different proportions and adjusting pH to values ranging from 4 to 9 resulted in precipitation of five different solid phases. The elemental composition of the solids was determined using X-ray Photoelectron Spectroscopy, and Environmental Scanning Microscopy-Field Emission Gun equipped with an energy dispersive spectroscopy detector. The average Cu/As ratio was determined by dissolving the solids. Total soluble concentrations of the components Cu(II) and As(V), as well as the basicity of the solid phases were determined by analysis of aqueous solutions. Based upon these experimental data the stoichiometric composition of the solid phases and their stability were determined. The resulting equilibrium model includes the solid phases Cu₃(AsO₄)₂, Cu₃(AsO₄)(OH)₃, Cu₂(AsO₄)(OH), Cu₅Na(HAsO₄)(AsO₄)₃ and Cu₅Na₂AsO₄)₄, where Cu₅Na(HAsO₄)(AsO₄)₃ and Cu₅Na₂(AsO₄)₄ have not been reported previously. In 0.1 M Na(Cl), Na⁺ was found to be a significant component in two of the solid phases. The Cu₅Na₂(AsO₄)₄ was formed in weakly alkaline conditions with pNa < 2.5. Stability constants for all solid phases have been determined. Distribution diagrams as well as predominance area (pNa-pH) diagrams are presented to illustrate stability fields of the different solid phases.     

Etude des solutions solides des néphélines (Na,K)AlSiO4 et (Na,Rb)AlSiO4

Ion exchange equilibrium of nepheline solid solutions (Na, K)AlSiO₄ and (Na, Rb)AlSiO₄ with hydrothermal solutions has been studied at 600°C and 2000 bars. The behaviour of dilute solid solutions was specially investigated.Na-Rb ion exchange data can be represented satisfactorily by a model taking into account the existence of two different sites in the structure of nepheline. At 600°C Rb atoms substitute almost exclusively for Na atoms situated in the larger sites. On the other hand, this model only partially applies to Na-K ion exchange equilibrium.Finally, the importance of the ion exchange data concerning extremely dilute solutions to calculate activity-composition diagrams is emphasized with special reference to the nepheline solid solutions.     

Miscibility in the CaSO4·2H2O–CaSeO4·2H2O system: Implications for the crystallisation and dehydration behaviour

SeO₄²⁻ ions can substitute for sulphate in the gypsum structure. In this work crystals of different Ca(SO₄,SeO₄)·2H₂O solid solutions were precipitated by mixing a CaCl₂ solution with solutions containing different ratios of Na₂SO₄ and Na₂SeO₄. The compositions of the precipitates were analysed by EDS and the cell parameters were determined by X-ray powder diffraction. Moreover, a comparative study on dehydration behaviour of selenate rich and sulfate rich Ca(SO₄,SeO₄)·2H₂O solid solutions was carried out by thermogravimetry.The experimental results show that the Ca(SO₄,SeO₄)·2H₂O solid solution presents a symmetric miscibility gap for compositions ranging from X_CaSO4·2H2O = 0.23 to X_CaSO4·2H2O = 0.77. By considering a regular solution model a Guggenheim parameter a₀ = 2.238 was calculated. The solid phase activity coefficients obtained with this parameter were used to calculate a Lippmann diagram for the system Ca(SO₄,SeO₄)·2H₂O–H₂O.     

Simulation of the nucleation and growth of binary solid solutions in aqueous solutions

We present a model which, for the first time, accounts for nucleation, growth and/or resorption of particles of variable composition in aqueous solutions (AS). Devised for describing the precipitation of binary solid solutions, it yields the time evolution of all ion activities in the AS, together with the particle population characteristics: number, size and composition profile of particles as a function of time and of their time of nucleation. We apply this numerical approach to the prototypical case of (Ba,Sr)CO₃ solid solution precipitation. We demonstrate the great sensitivity of the composition profiles and particle sizes to the initial conditions under which the AS is prepared, and thus illustrate the possibility of engineering the particle characteristics into a chosen state. Finally, by comparing the precipitation of two solid solutions (Ba,Sr)CO₃ and (Ba,Sr)SO₄, we evidence the sensitivity of the particle composition profiles to the ratio of the end-member solubility products, which leads to the formation of core-shell particles in the case of (Ba,Sr)SO₄.     

Partitioning of Fe and Mn between ilmenite and olivine at 1100 °C: constraints on the thermodynamic mixing properties of (Fe, Mn)TiO3 ilmenite solid solutions

The partitioning of Fe²⁺ and Mn²⁺ between (Fe, Mn)TiO₃ and (Fe, Mn)₂SiO₄ solid solutions in the system FeO-MnO-TiO₂-SiO₂ has been experimentally investigated at 1100 ^∘C and pressures of 1 bar and 25 kbar, over a wide range of Fe/Mn ratios, using electron microprobe analysis of quenched run products. The ilmenite solid solution in this system is within analytical uncertainty a simple binary between FeTiO₃ and MnTiO₃, but the olivine solid solution appears to contain up to 2.5 wt% TiO₂. The Fe-Mn partitioning results constrain precisely the difference in the thermodynamic mixing properties of the two solid solutions. If the mixing properties of (Fe, Mn)₂SiO₄ solid solutions are assumed to be ideal, as experimentally determined by Schwerdtfeger and Muan (1966), then the ilmenite is a regular, symmetric solution with W ^ilm _Fe-Mn=1.8±0.1 kJ mol⁻¹. The quoted uncertainty does not include the contribution from the uncertainty in the mixing properties of the olivine solution, which is estimated to be ±1.8 kJ mol⁻¹, and which therefore dominates the uncertainty in the present results. Nevertheless, this result is in good agreement with the previous experimental study of O'Neill et al. (1989), who obtained W ^ilm _Fe-Mn=2.2±0.3 kJ mol⁻¹ from an independent method. The results provide another item of empirical evidence supporting the proposition that solid solutions between isostructural end-members, in which order-disorder effects are not important, generally have simple thermodynamic mixing properties, with little asymmetry, modest excess entropies, and excess enthalpies approximately proportional to the difference in the molar volumes of the end-members. Received: 11 February 1998 / Accepted: 29 June 1998     

Crystallographic studies of some olivine-related (Ni,Mg)3(PO4)2 solid solutions

Olivine-related (Ni, Mg)₃(PO₄)₂ solid solutions were prepared and equilibrated at 1070 K. Accurate monoclinic unit cell dimensions were determined from Guinier-Hägg photographic data. Structural refinements based on the X-ray profile-fitting technique after Rietveld were carried out for pure nickel (II) orthophosphate and for three Ni/Mg solid solutions. (Ni_1-x Mg _x )₃(PO₄)₂ phases with 0.40≦x≦0.60 are probably isostructural with Ni₃(PO₄)₂ (P2₁/a) while phases with low magnesium contents (<27 atom % Mg) deviate structurally from Ni₃(PO₄)₂. The results also show that Ni²⁺ is partially ordered at the octahedralM(1) sites, withK _D (Ni, Mg)=4.0±0.2     

The kinetics of nucleation of solid solutions from aqueous solutions: a new model for calculating non-equilibrium distribution coefficients

The nucleation kinetics of binary solid solutions, with general formula B_xC_1−xA, crystallising from aqueous solution can be described using a generalised expression for the nucleation rate: the function, J(x), in which supersaturation, interfacial free energy and other parameters of the classical nucleation rate equation are considered as functions of the solid composition. As an example, we studied the behaviour of such J(x) functions for the case of the (Ba,Sr)SO₄ and (Ba,Sr)CO₃ solid solutions. J(x) functions are very sensitive to slight changes in the composition of the aqueous solution, which result in strong modifications of the nucleation kinetics. The implications of the relationship between supersaturation and nucleation rate functions for the general nucleation behaviour in solid solution-aqueous solution (SS-AS) systems are discussed. Finally, we present a method for constructing non-equilibrium Roozeboom diagrams based on the nucleation kinetics in SS-AS systems. Our Roozeboom diagrams calculated for different departures from equilibrium conditions are consistent with previous experimental work and they can be used to predict actual distribution coefficients.     

Dissolution of quartz into dilute alkaline solutions at 90°C: A kinetic study

The rate of dissolution of Fontainebleau sand (pure quartz) into sodium hydroxide solutions (from 0.001 M to 0.5 M) has been determined at 90°C in well-stirred vessels. Dissolution leads to an equilibrium state, controlled by the solubility of quartz in pure water as undissociated silicic acid H₄SiO₄. As long as the initial molality of sodium hydroxide does not exceed 0.02 mol · kg⁻¹, the dissolution leads only to the formation of the three monomeric species H₄SiO₄, H₃SiO₄⁻ and H₂SiO₄²⁻, while polymers appear in the silica-rich solutions obtained in more alkaline media. The rate of dissolution can be represented by an adaptation of Stöber's model to alkaline solutions; the basic assumption is that the quartz surface is partially covered by a layer of adsorbed silicate ions, which represent an intermediate species between solid and dissolved silica.     

Coprecipitation in the barite isostructural family: 2. Numerical simulations of reactions and mass transport

Coprecipitation of barite with trace constituents was simulated with consideration of aqueous speciation and complexation, mixing properties for the binary solid solutions (Zhu, this issue), precipitation and dissolution kinetics, and advective-dispersive transport. Speciation-solubility modeling was used to reproduce BaSO₄-RaSO₄ coprecipitation experimental results, and to calculate CrO₄²⁻ aqueous concentrations in equilibrium with a Ba(SO₄,CrO₄) solid solution. Kinetic reaction path modeling was used to simulate the coprecipitation of barite with RaSO₄ to form an onion-like chemically zoned solid upon the cooling of oil field brine.A one-dimensional coupled reactive mass transport model shows a strikingly different transport pattern for the tracer Ra²⁺, when the dominant attenuation reaction is with solid solution (Ba, Ra) SO₄ as compared to the case when it is controlled by pure RaSO₄ and barite solids under local equilibrium conditions. A self-enrichment of Ra²⁺ in the groundwater and aquifer solid matrix—higher concentrations of Ra²⁺ downstream from the reaction front—results from the coprecipitation reaction and advective-dispersive transport. This self-enrichment process generates a secondary tracer source, which has tracer concentrations higher than that of the original source. On the other hand, coprecipitation reactions can reduce Ra²⁺ concentrations in groundwater to a much lower level (below ppb) than that of pure RaSO₄(c) solubility (near ppm), which has been used to establish the Ra²⁺ concentration limits in groundwater, soil, and nuclear waste repositories.     

The composition and evolution of primordial solutions on Mars, with application to other planetary bodies

We examine a model for Mars involving bulk primordial solutions (oceans and lakes) that were relatively Mg-rich and SO₄/(SO₄ + Cl)-rich. Such solutions could be produced when (1) volatiles leached a planet (or portions of a planet) with an ultramafic-mafic composition in a process called “planetary leaching”; and/or by (2) “impactor leaching” where meteoritic and/or cometary impactor fragments were leached. When Mg-SO₄/(SO₄ + Cl)-rich solutions are concentrated, we predict that the following sequence of salts precipitates: phosphates; carbonates; gypsum; epsomite; bloedite; halite; hexahydrite; and, finally bischofite. This sequence is modified slightly if appreciable Fe-, Mg- or Na-carbonates, Fe-sulfates, Mg-phosphate, or other halide salts crystallized before the Mg-Na-sulfate salts, or if HCO₃+CO₃ concentrations vary due to other effects (e.g., atmosphere CO₂ levels change).On Mars, the initial primordial solutions would have been relatively salt-rich and water-poor; therefore, the surface solutions formed Mg-Na-SO₄-Cl salts (cements, veneers, and dust) and subsurface solutions or ice (solid H₂O). This model is supported by the compositions of cements in the regolith on Mars (high Mg, Na, S, and Cl) and geochemical and petrographic evidence that the salts precipitated in the predicted sequence. We suggest that the partial pressure of oxygen was above the hematite-magnetite buffer where Fe³⁺-(hydrous)-oxides are stable and SO₄^2- or HSO₄^- are solutes in any solution. Such a partial pressure of oxygen may have been attained via H₂-loss.In contrast, on the Galilean satellites (Europa, Ganymede, and Callisto) surface solutions were relatively water-rich and formed ice, Mg-SO₄-rich salts, and solutions, thereby producing surface features dependent on the initial water content and the crystallization path. Unlike the Na-Cl-rich oceans on Earth, the solutions of these planetary bodies likely did not change greatly from their bulk primordial Mg-rich, SO₄/(SO₄ + Cl)-rich compositions; hence they did not attain compositions similar to modern seawater.     

High pressure phase equilibria in the system CaTiO3-CaSiO3: stability of perovskite solid solutions

Phase relations in the system CaTiO₃-CaSiO₃ were experimentally examined at 5.3–14.7 GPa and 1200–1600 °C with a 6–8 type multianvil apparatus. As pressure increases, stability field of perovskite solid solution extends from CaTiO₃ to CaSiO₃, and the perovskite becomes stable for the entire composition range above about 12.3 GPa. The stability field of Ca(Ti_1?X, Si_X)₂O₅ (0.78<x≦1) titanite solid solution +Ca₂SiO₄ larnite exists in the CaSiO₃-rich composition range at 9.3–12.3 GPa and 1200 °C. Perovskite solid solutions containing CaSiO₃ component of 0 to 66 mol% could be quenched to 1 atm. The composition-molar volume relationship of perovskite solid solution showed that molar volume of perovskite solid solution linearly reduces from the value of CaTiO₃ to that of CaSiO₃.     

Henry’s and non-Henry’s law behavior of Br in simple marine systems

Experimental studies for the partitioning of Br as a trace element between aqueous and solid solutions were carried out in simple marine systems. The evaporation experiments were performed at 25°C and 1 atm in the systems of halite (NaCl), sylvite (KCl), kainite (KMgClSO₄ · 2.75H₂O), carnallite (KMgCl₃ · 6H₂O), and bischofite (MgCl₂ · 6H₂O). The partition coefficients for the systems investigated are constant only at a restricted concentration range. For concentrations lower than 100 to 300 μg Br/g aqueous solutions, D_Br increases with decreasing concentrations. Various evaporation experiments indicate that this observation is not due to kinetic effects (evaporation rates). To find a link between the partition coefficient and the Henry’s law behavior, the activity coefficients of the trace components in the solid solutions were recalculated from the experimentally derived data. It can be shown from these calculations that constant activity coefficients or Henry’s law behavior is reached for higher mole fractions of the trace component in the solid solution in halite and sylvite and thus correspond to constant partition coefficients. For bischofite and carnallite, Henry’s law behavior is restricted to the lower mole fractions, where D_Br is not constant. This behavior is caused by the activity of the trace component in the aqueous solution, which is powered by the stoichiometric factor of this component in the Br-end-member solid solution. For halite, sylvite, and kainite, this factor equals 1 and is 2 for bischofite and 3 for carnallite. However, it is thus impossible to correlate Henry’s law behavior with constant partition coefficients for solid solution systems where the stoichiometric factor of the trace component is greater than 1.     

Thermochemistry of phases in the system MgGa2O4-Mg2GeO4

The enthalpies of solution in molten 2PbO · B₂O₃ of phases synthesized at one atmosphere in the system MgGa₂O₄-Mg₂GeO₄ have been measured. A spinel solid solution, which is stable at 1400 °C from the MgGa₂O₄ end-member to 27 mole percent Mg₂GeO₄, shows endothermic heats of mixing of up to 10 kJ/mole at the solubility limit. The spinelloid phase, Mg₃Ga₂GeO₈, is energetically less stable than a mixture of terminal spinel solid solutions (0.73 MgGa₂O₄·0.27 Mg₂GeO₄(sp)+Mg₂GeO₄(sp)), by 3.63±3.64 kJ/mole. This indicates that the spinelloid is a high-entropy phase.The volume of the spinel solid solution, MgGa₂O₄-Mg₂GeO₄, shows a positive deviation from Vegard's Law. Modeling of the cation distribution in the solid solution indicates that this V is due to a change in the spinel type from inverse towards normal as the Mg₂GeO₄ content increases.     

Preiswerkite and Na-(Mg,Fe)-margarite in eclogites

Preiswerkite and Na-(Mg,Fe)-margarite are two unusual micas very rare in Nature. They have been observed together in two eclogite occurrences (La Compointrie, France; Liset, Norway) as retrogression products in coronae or symplectites around kyanite. The chemical compositions and some physical properties of these micas are presented. The possible solid solutions and the conditions of stability are discussed. The preiswerkites display slight solid solution towards phengitic muscovite and Na-phlogopite. On the other hand, there is negligible solid solution towards more aluminous compositions; Al^IV ≤ 4 appears to be a composition limit for natural (K,Na)-micas. The margarites have an unusual Na-(Mg,Fe)-rich composition. They can be considered as a solid solution of about 2/3 mol% of margarite and 1/3 mol% of the theoretical end-member Na₂(Mg,Fe)₁Al^VI ₄[Si₄Al^IV ₄]O₂₀(OH)₄ (“Mica L”), with a possible substitution towards paragonite. The Marg_2/3 Mica L_1/3 composition (i.e. NaCa₂(Mg,Fe)_0.5 Al^VI ₆ [Si₆Al^IV ₆]O₃₀(OH)₆) might represent a particularly stable crystallographic configuration and could be considered as a true end-member. Many “sodian” margarites described in the literature are, in fact, complex solid solutions between margarite, paragonite and Marg_2/3 Mica L_1/3. The rarity of these micas is not related to extreme or unusual P-T conditions. They seem to be related to unusual chemical compositions, appearing in H₂O-saturated Na-Al-rich Si-poor systems, principally, if not only, at greenschist- or amphibolite-facies P-T conditions. Moreover, they are subject to crystallographic constraints whereby the high proportion of Al-tetrahedra create considerable distortion which prevents the entry of K into the interlayer site, thus necessitating Na (preiswerkite or ephesite) or Ca (margarite or clintonite) instead. Received: 21 April 1998 / Accepted: 25 January 1999     

Thermodynamic modeling of REE behavior in oxidized hydrothermal fluids of high sulfate sulfur concentrations

Thermodynamic calculations using the HCh software were made for mineral equilibriums including REEs in the fluoride–sulfide–chloride–carbonate–sulfate–system in the presence of Na, Ca, and P with fluids of various acidities–alkalinities [11]. The obtained thermodynamic characteristics of thenardite allowed us to carry out the calculations for this phase under complicated hydrothermal conditions simulating the presence of oxidized fluids at 500–100°C and 2000–125 bar. Among other solid phases, REEs–fluorite, monazite, and REE–F–apatite were formed as CaF₂–(Ln,Y)F₃, LnPO₄, and Ca₅(PO₄)₃F–(Ln,Y)₃(PO₄)₃ ideal solid solutions, respectively, where Ln is La, Ce, Pr, Nd, Sm, Eu, and Gd. Xenotime, anhydrite, elemental sulfur, and calcite were found as well.     

A contribution to the crystal chemistry of the voltaite group: solid solutions, Mössbauer and infrared spectra, and anomalous anisotropy

Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K₂Fe₅ ²⁺Fe₃ ³⁺Al(SO₄)₁₂·18H₂O and the nominal symmetry is cubic, $Fd\overline{3}c$ . The tetragonal (I4₁/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe²⁺ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH₄ ⁺ instead of K⁺. The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M³⁺ and M²⁺ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M²⁺. In the Fe²⁺-Mg-Zn solid solutions, the Fe²⁺-Mg and Fe²⁺-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M²⁺-O bonds within the chains. In the Mg-Zn solid solution, some of the M²⁺-O bonds expand while none contract. In the other solid solutions, expansion of some M²⁺-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data.     

Electrical conductivity of alkali feldspar solid solutions at high temperatures and high pressures

The electrical conductivities of alkali feldspar solid solutions ranging in chemical composition from albite (NaAlSi₃O₈) to K-feldspar (KAlSi₃O₈) were measured at 1.0 GPa and temperatures of 873–1,173 K in a multi-anvil apparatus. The complex impedance was determined by the AC impedance spectroscopy technique in the frequency range of 0.1–10⁶ Hz. Our experimental results revealed that the electrical conductivities of alkali feldspar solid solutions increase with increasing temperature, and the linear relationship between electrical conductivity and temperature fits the Arrhenius formula. The electrical conductivities of solid solutions increase with the increasing Na content at constant temperature. At 1.0 GPa, the activation enthalpy of solid solution series shows strong dependency on the composition, and there is an abrupt increase from the composition of Or₄₀Ab₆₀ to Or₆₀Ab₄₀, where it reaches a value of 0.96 eV. According to these results in this study, it is proposed that the dominant conduction mechanism in alkali feldspar solid solutions under high temperature and high pressure is ionic conduction. Furthermore, since the activation enthalpy is less than 1.0 eV for the alkali feldspar solid solutions, it is suggested to be a model where Na⁺ and K⁺ transport involves an interstitial mechanism for electrical conduction. The change of main charge carriers can be responsible for the abrupt increase in the activation energy for Or₆₀Ab₄₀. All electrical conductivity data were fitted by a general formula in order to show the dependence of activation enthalpy and pre-exponential factor on chemical composition. Combining our experimental results with the effective medium theory, we theoretically calculated the electrical conductivity of alkali feldspar granite, alkali feldspar quartz syenite, and alkali feldspar syenite with different mineral content and variable chemical composition of alkali feldspar at high temperatures at 1.0 GPa, and the calculated results are almost in agreement with previous experimental studies on silicate rocks.     

Hydrotalcite-like solid solutions with variable SO 4 2− and CO 3 2− contents at 50° C

Hydrotalcite-like solid solutions have been synthesized by coprecipitation in basic solutions with variable SO ₄ ^2? /CO ₃ ^2? ratios. Chemical determination of CO ₃ ^2? in the interlayer was impossible because of the presence of minor hydromagnesite. SO ₄ ^2? was determined both by chemical analysis and X-ray photoelectron spectroscopy (XPS), the two methods giving similar results. A Raman spectrometry gave additional data on the SO ₄ ^2? /CO ₃ ^2? ratio. Then, the stoichiometry of the anionic interlayers, X _s , X _c , and X _OH were determined, and the influence of X _s on the c′ parameter (increasing from c′=7.97 Å to c′=8.63 Å between X _s =0 and X _s =1) was characterized. In addition, a partitioning curve of SO ₄ ^2? and CO ₃ ^2? between aqueous solutions and hydrotalcite-like compounds was established. Its general shape strongly suggests a miscibility gap between a sulfate-rich end and a carbonate-rich solid solution (maximum SO ₄ ^2? /CO ₃ ^2? about 0.2). This result explains why most of the hydrotalcites synthesized during experimental alteration of basaltic glasses by sea-water (a sulfate-rich solution) are CO ₃ ^2? -rich solid solutions.     

Metastable phenomena on calcite {101̄4} surfaces growing from Sr2+–Ca2+–CO32− aqueous solutions

In situ atomic force microscopy (AFM) experiments, scanning electron microscopy (SEM) imaging and composition analysis, and X-ray diffraction have provided information about the growth, dissolution and transformation processes promoted by Sr²⁺–Ca²⁺–CO₃²⁻ aqueous solutions in contact with calcite {101̄4} surfaces. Experiments have shown a wide variety of surface phenomena, such as the influence of the Sr-bearing newly-formed surface on the subsequent growth (template effect), the growth and subsequent dissolution of surfaces and the nucleation of secondary three-dimensional nuclei on calcite surfaces. These phenomena reveal the metastability of the crystallisation system and are a consequence of the interplay between thermodynamics (the relative stability of the two calcite and aragonite structure solid solutions that can be formed), supersaturation of the aqueous solution with respect to the two possible solid solutions, and the crystallographic control of the surfaces on cation incorporation.