The dynamics of H2O in minerals

The application of infrared, proton nuclear magnetic resonance, and dielectric spectroscopy and incoherent neutron scattering for the elucidation of the dynamics of H₂O incorporated into minerals is reviewed. The examples given include beryl, cordierite, gypsum, bassanite, layer silicates and zeolites. It is demonstrated that for such structures static models may be inappropriate, and dynamic models have to be used to describe the role and behavior of the H₂O molecules.     

Gypsum: a review of its role in the deterioration of building materials

The deterioration of buildings and monuments by gypsum is the result of crystallization cycles of this salt. Although gypsum can dehydrate to a hemihydrate, the mineral bassanite, and to an anhydrate, the mineral anhydrite, this reaction occurs in nature on a geological time scale and therefore it is unlikely to occur when gypsum is found on and in building materials. The CaSO₄–H₂O system appears deceptively simple, however there are still discrepancies between the experimental and thermodynamically calculated data. The reason for the latter can be attributed to the slow crystallization kinetics of anhydrite. Apart from this, the large numbers of studies carried out on this system have focused on industrially important metastable phases, such as the hemihydrate and soluble anhydrite. The paper presents a review of the studies dealing with the phase equilibria of the CaSO₄–H₂O system as well as the influence of other salts on the solubility of gypsum. It tries to glean out the relevant information that will serve to explain the deterioration observed on building materials by the crystallization of gypsum and thus allows developing improved conservation methods.     

The heat capacity of hydrous cordierite above 295 K

The heat capacity of synthetic hydrous cordierite (Mg₂Al₄Si₅O₁₈·nH₂O) has been determined by differential scanning calorimetry (DSC) from 295 to 425 K as a function of H₂O content. Six samples with H₂O contents ranging from 0 to 0.82 per formula unit were examined. The partial molar heat capacity of H₂O in cordierite over the measured temperature interval is independent of composition and temperature within experimental uncertainty and is equal to 43.3 ±0.8 J/mol/ K. This value exceeds the molar heat capacity of gaseous H₂O by 9.7 J/mol/K, but is significantly smaller than the heat capacity of H₂O in several zeolites and liquid H₂O. A statistical-mechanical model of the heat capacity of adsorbed gas species (Barrer 1978) is used to extrapolate the heat capacity of hydrous cordierite to temperatures greater than 425 K. In this model, the heat capacity of hydrous cordierite (Crd·nH₂O) is represented as follows: Cp(Crd · nH₂O) = Cp(Crd)+ n{Cp(H₂O, gas)+ R(gas constant)} (1) An examination of calorimetric data for hydrous beryl, analcime, mordenite, and clinoptilolite (Hemingway et al. 1986; Johnson et al. 1982, 1991, 1992) demonstrates the general applicability of the statistical-mechanical model for the extrapolation of heat capacity data of zeolitic minerals. The heat capacity data for cordierite are combined with the data of Carey and Navrotsky (1992) to obtain the molar enthalpy of formation and enthalpy of hydration of hydrous cordierite as a function of temperature.     

An assessment of selenate co-precipitation with gypsum

In this study, we assessed the co-precipitation of selenate (SeO₄²⁻) with gypsum (CaSO₄·2H₂O) in controlled laboratory experiments. Batch testing was used to quantify the ability of CaSO₄·2H₂O to co-precipitate dissolved SeO₄²⁻ over a range of dissolved SeO₄²⁻-Se concentrations (0–50 mg/L) and under slightly acidic (pH ∼5.5–6.1) and oxic (Eh ∼416−501 mV) conditions. Aqueous samples were analyzed using inductively coupled plasma optical emission spectrometry, solid samples using X-ray diffraction and Raman spectroscopy, and digests of selected CaSO₄·2H₂O precipitates using inductively coupled plasma-mass spectrometry. The concentration of Se co-precipitated in CaSO₄·2H₂O increased linearly with dissolved SeO₄²⁻-Se concentration. The aqueous analyses and calculations based on the CaSO₄·2H₂O digest data show between 14–19 % of the dissolved Se was removed during the co-precipitation experiments. The strong linear relationship between SeO₄²⁻-Se added to the test solutions and Se co-precipitated in CaSO₄·2H₂O can be used to estimate the concentration of co-precipitated SeO₄^2- if the concentration of SeO₄^2- in the associated porewater is known, and vice versa. Results indicate that <1% of SeO₄²-Se was removed from the test solutions during co-precipitation and the mass of Se in CaSO₄·2H₂O solids was low, ranging between 0−120 μg/g. These results were used in conjunction with field- and model-derived data to show co-precipitation of SeO₄^2- with CaSO₄·2H₂O should be a minor SeO₄^2- sequestration mechanism. The findings of this study should be applicable to mined rock dumps in North America and elsewhere.     

The thermal behaviour of γ-CaSO<Subscript>4</Subscript>

Thermal behaviour of γ-anhydrite (γ-CaSO₄, soluble anhydrite) has been investigated in situ real-time using laboratory parallel-beam X-ray powder diffraction data. Thermal expansion has been analysed from 303 to 569 K with temperature steps of 4 K. Lattice parameters and volume were fitted with a second-order polynomial to calculate thermal expansion coefficients. Thermal expansion of γ-anhydrite is anisotropic being larger along the c axis. Within the 343–383 K thermal range, γ-anhydrite has been found to partially re-hydrate to bassanite CaSO₄·0.5H₂O. At 455 K the transformation γ-CaSO₄ → β-CaSO₄, insoluble anhydrite, starts reaching completion at 653 K.     

Geochemical studies of northern taiga (gypsum) karst ecosystems and their high vulnerability to natural and anthropogenic hazards

In the taiga gypsum karst ecosystems, gypsum soils formed on the hard gypsum substrates predominate in the soil cover. In these soils, the mineral horizons consist of 95–99% gypsum (CaSO₄·2H₂O) and the litter is the main horizon for nutrient accumulation. For this reason, the ecosystems are vulnerable to fire and erosion by walkers, from which they only recover slowly. Gypsum mining for industrial uses is also leading to the destruction of this unique ecosystem.     

Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction

Thermal behaviour and kinetics of dehydration of gypsum in air have been investigated using in situ real-time laboratory parallel-beam X-ray powder diffraction data evaluated by the Rietveld method. Thermal expansion has been analysed from 298 to 373 K. The high-temperature limits for the cell edges and for the cell volume, calculated using the Einstein equation, are 4.29 × 10⁻⁶, 4.94 × 10⁻⁵, 2.97 × 10⁻⁵, and 8.21 × 10⁻⁵. Thermal expansion of gypsum is strongly anisotropic being larger along the b axis mainly due to the weakening of hydrogen bond. Dehydration of gypsum has been investigated in isothermal conditions within the 348–403 K range with a temperature increase of 5 K. Dehydration proceeds through the CaSO₄·2H₂O → CaSO₄·0.5H₂O → γ-CaSO₄ steps. Experimental data have been fitted with the Avrami equation to calculate the empirical activation energy of the process. No change in transformation mechanism has been observed within the analysed temperature range and the corresponding E _a is 109(12) kJ/mol.     

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.     

Spatial relationships of salt distribution and related physical changes of underlying rocks on naturally weathered sandstone exposures (Bohemian Switzerland National Park,Czech Republic)

Efflorescence, case hardening, and granular disintegration represent common weathering features of Upper Cretaceous quartz sandstones exposed in the Bohemian Switzerland National Park (NW Bohemia, Czech Republic). Salt species (sulphates: gypsum (CaSO₄·2H₂O), potassium alum (KAl(SO₄)₂·12H₂O), tschermigite (NH₄Al(SO₄)₂·12H₂O), alunite (K(Al₃(SO₄)₂(OH)₆), and alunogen (Al₂(SO₄)₃·17H₂O), minor nitrates: nitrammite (NH₄NO₃)) determined by X-ray diffraction exhibit vertical and geographic zoning. More soluble salts (chlorides, nitrates, tschermigite) crystallize preferentially on the cliffs exposed to the south, whereas the north face is characterized by the presence of less soluble phases: gypsum and K(Al₃(SO₄)₂(OH)₆. Vertical zoning of salt distribution on natural outcrops differs from the salt distribution in masonry. Salt distribution near the base of the cliff (profile to about 2–2.5 m above the ground) is affected by capillary rise from the ground level (first maximum of water-soluble salts at the level of 1–1.5 m above the ground) and by percolation of precipitation through the overhanging rock sequence (second maximum of 2–2.5 m above the ground). Percolation of salt solution from higher parts is affected by the asperity of the rock surface. The concentration of salts (determined by ion exchange chromatography) correlates to the changes of physical properties: bulk porosity, microporosity and water absorption. The porosity, microporosity, moisture content and absorption generally increase with the increasing volume of sulphates and nitrates.     

The dehydration process of gypsum under high pressure

The effects of pressure on the dehydration of gypsum materials were investigated up to 633 K and 25 GPa by using Raman spectroscopy and synchrotron X-ray diffraction with an externally heated diamond anvil cell. At 2.5 GPa, gypsum starts to dehydrate around 428 K, by forming bassanite, CaSO₄ hemihydrate, which completely dehydrates to γ-anhydrite at 488 K. All the sulphate modes decrease linearly between 293 and 427 K with temperature coefficients ranging from −0.119 to −0.021 cm⁻¹ K⁻¹, where an abrupt change in the ν₃ mode and in the OH-stretching region indicates the beginning of dehydration. Increasing the temperature to 488 K, the OH-stretching modes completely disappear, marking the complete dehydration and formation of γ-anhydrite. Moreover, the sample changes from transparent to opaque to transparent again during the dehydration sequence gypsum-bassanite-γ-anhydrite, which irreversibly transforms to β-anhydrite form at 593 K. These data compared with the dehydration temperature at room pressure indicate that the dehydration temperature increases with pressure with a ΔP/ΔT slope equal to 230 bar/K. Synchrotron X-ray diffraction experiments show similar values of temperature and pressure for the first appearance of bassanite. Evidence of phase transition from β-anhydrite structure to the monazite type was observed at about 2 GPa under cold compression. On the other hand at the same pressure (2 GPa and 633 K), β-anhydrite was found, indicating a positive Clausis-Clayperon slope of the transition. This transformation is completely reversible as showed by the Raman spectra on the sample recovered after phase transition.     

Weathering of the black limestone of historical monuments (Ljubljana,Slovenia): Oxygen and sulfur isotope composition of sulfate salts

The black limestone widely used in Slovenian monuments, particularly in the baroque architecture, is deteriorating extensively due to salt crystallization. Samples of soluble salts from two important historical monuments (in Ljubljana, Slovenia) were investigated in terms of their mineral and isotopic (S and O) compositions. Results revealed the presence of gypsum and soluble salts of the MgSO₄·nH₂O series, such as starkeyite (MgSO₄·4H₂O), pentahydrite (MgSO₄·5H₂O) and hexahydrite (MgSO₄·6H₂O). Whereas black crusts and subflorescences consisted of gypsum, efflorescences appeared to be an assemblage of gypsum and MgSO₄ hydrates. Sample δ¹⁸O_sulfate values varied from ?1.9‰ to +5.5‰ vs. V-SMOW and δ³⁴S_sulfate values from ?19.8‰ to +3.2‰ vs. V-CDT. The respective isotopic composition of analysed outdoor and indoor monument samples indicated different sources of contamination.     

Geochemical models of the impact of acidic groundwater and evaporative sulfate salts on Boulder Creek at Iron Mountain,California

During dry season baseflow conditions approximately 20% of the flow in Boulder Creek is comprised of acidic metals-bearing groundwater. Significant amounts of efflorescent salts accumulate around intermittent seeps and surface streams as a result of evaporation of acid rock drainage. Those salts include the Fe-sulfates — rhomboclase ((H₃O)Fe³⁺(SO₄)₂·3H₂O), ferricopiapite (Fe³⁺₅(SO₄)₆O(OH)·20H₂O), and bilinite (Fe²⁺Fe₂³⁺(SO₄)₄·22H₂O); Al-sulfates — alunogen (Al₂(SO₄)₃·17H₂O) and kalinite (KAl(SO₄)₂·11H₂O); and Ca- and Mg-sulfates — gypsum (CaSO₄·2H₂O), and hexahydrite (MgSO₄·6H₂O). The dissolution of evaporative sulfate salt accumulations during the first major storm of the wet season at Iron Mountain produces a characteristic hydrogeochemical response (so-called “rinse-out”) in surface waters that is subdued in later storms. Geochemical modeling shows that the solutes from relatively minor amounts of dissolved sulfate salts will maintain the pH of surface streams near 3.0 during a rainstorm. On a weight basis, Fe-sulfate salts are capable of producing more acidity than Al- or Mg-sulfate salts. The primary mechanism for the production of acidity from salts involves the hydrolysis of the dissolved dissolved metals, especially Fe³⁺. In addition to the lowering of pH values and providing dissolved Fe and Al to surface streams, the soluble salts appear to be a significant source of dissolved Cu, Zn, and other metals during the first significant storm of the season.     

The high-pressure phase diagram of synthetic epsomite (MgSO4·7H2O and MgSO4·7D2O) from ultrasonic and neutron powder diffraction measurements

We present an ultrasonic and neutron powder diffraction study of crystalline MgSO₄·7H₂O (synthetic epsomite) and MgSO₄·7D₂O under pressure up to ~3 GPa near room temperature and up to ~2 GPa at lower temperatures. Both methods provide complementary data on the phase transitions and elasticity of magnesium sulphate heptahydrate, where protonated and deuterated counterparts exhibit very similar behaviour and properties. Under compression in the declared pressure intervals, we observed three different sequences of phase transitions: between 280 and 295 K, phase transitions occurred at approximately 1.4, 1.6, and 2.5 GPa; between 240 and 280 K, only a single phase transition occurred; below 240 K, there were no phase transformations. Overall, we have identified four new phase fields at high pressure, in addition to that of the room-pressure orthorhombic structure. Of these, we present neutron powder diffraction data obtained in situ in the three phase fields observed near room temperature. We present evidence that these high-pressure phase fields correspond to regions where MgSO₄·7H₂O decomposes to a lower hydrate by exsolving water. Upon cooling to liquid nitrogen temperatures, the ratio of shear modulus G to bulk modulus B increases and we observe elastic softening of both moduli with pressure, which may be a precursor to pressure-induced amorphization. These observations may have important consequences for modelling the interiors of icy planetary bodies in which hydrated sulphates are important rock-forming minerals, such as the large icy moons of Jupiter, influencing their internal structure, dynamics, and potential for supporting life.     

Hydrogen bonding in coquimbite, nominally Fe2(SO4)3·9H2O, and the relationship between coquimbite and paracoquimbite

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite ( $ P {\bar 3}1c $ , a?=?10.924(2)/10.882(2) Å, c?=?17.086(3) / 17.154(3) Å, V?=?1765.8(3)/1759.2(5) ų, at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O–H distances in coquimbite at 50 K vary between 0.98 and 1.01 Å. In addition to H₂O molecules coordinated to the Al³⁺ and Fe³⁺ ions, there are rings of six “free” H₂O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 Å. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe₂(SO₄)₃·9H₂O. Coquimbite, on the other hand, has a composition approximating Fe_1.5Al_0.5(SO₄)₃·9H₂O. Hence, even a “simple” sulfate Fe_2-x Al _x (SO₄)₃·9H₂O may be structurally rather complex.     

Thermoluminescence,fluorescence and electron paramagnetic resonance properties of synthetic hydrothermal scheelites

Hydrothermal scheelite was synthesized using Na₂WO₄ · 2 H₂O mixed with CaCl₂ · H₂O, CaSO₄ · 2 H₂O or CaF₂ at different temperatures (270–720° C) and 10⁸ Pa. The morphology of the crystals depends on the starting products. The observed faces include the {112}, {114}, {011}, and {013} forms. Pure or REE doped scheelites were studied by thermoluminescence (TL), fluorescence and electron paramagnetic resonance (EPR). The main TL peaks are located near 88, 149, 216, 277, and 315 K. Results obtained with EPR or optical fluorescence have been correlated with TL measurements and show that the trivalent lanthanide elements substitute for calcium ions without site distortion. The differences in TL observed between Eu and the other doping elements are related to the greater stability of Eu²⁺ caused by X-irradiation.     

室温条件下青海湖水中镁离子浓度对沉淀碳酸钙同质多像类型的调控

青海湖是我国唯一报道过的现代湖底沉积物中白云石、方解石和文石等多种碳酸盐矿物共存的高原内陆咸水湖泊.以青海湖水和除菌青海湖水作为载体,以CaCl2和MgCl2·6 H2O作为反应原料,在实验室常温条件下采取控制变量法制备出不同浓度Mg2+参与下的钙质沉淀物,探讨Mg2+浓度对沉淀物类型的影响.仅添加CaCl2时,青海湖...     

Cordierite as a sensor of fluid conditions in high-grade metamorphism and crustal anatexis

Cordierite H₂O and CO₂ volatile saturation surfaces derived from recent experimental studies are presented for P–T conditions relevant to high‐grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation. The volatile saturation surfaces and saturation isopleths for both H₂O and CO₂ in cordierite are strongly pressure dependent. In contrast, the uptake of H₂O by cordierite in equilibrium with melts formed through biotite dehydration melting, controlled by the distribution of H₂O between granitic melt and cordierite, D_w[D_w = wt% H₂O (melt)/wt% H₂O(Crd)], is mainly temperature dependent. D_w = 2.5–6.0 for the H₂O contents (0.4–1.6 wt percentage) typical of cordierite formed through biotite dehydration melting at 3–7 kbar and 725–900 °C. This range in D_w causes a 15–30% relative decrease in the total wt% of melt produced from pelites. Cordierite in S‐type granites are H₂O‐rich (1.3–1.9 wt%) and close to or saturated in total volatiles, signifying equilibration with crystallizing melts that achieved saturation in H₂O. In contrast, the lower H₂O contents (0.6–1.2 wt percentage) preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid‐absent conditions during anatexis. In several cases, including the Cooma migmatites and Broken Hill granulites, the cordierite volatile compositions yield a_H2O values (0.15–0.4) and melt H₂O contents (2.2–4.4 wt%) compatible with model dehydration melting reactions. In contrast, H₂O leakage is indicated for cordierite from Prydz Bay, Antarctica that preserve H₂O contents (0.5–0.3 wt%) which are significantly less than those required (1.0–0.8 wt%) for equilibrium with melt at conditions of 6 kbar and 860 °C. The CO₂ contents of cordierite in migmatites range from negligible (< 0.1 wt%) to high (0.5–1.0 wt%), and bear no simple relationship to preserved cordierite H₂O contents and a_H2O. In most cases the cordierite volatile contents yield total calculated fluid activities (a_H2O + a_CO2) that are significantly less than those required for fluid saturation at the P–T conditions of their formation. Whether this reflects fluid absence, dilution of H₂O and CO₂ by other components, or leakage of H₂O from cordierite is an issue that must be evaluated on a case‐by‐case basis.     

Cordierite as an indicator of thermodynamic conditions of petrogenesis

A refined thermodynamic model of H₂O and CO₂ bearing cordierite based on recent data on volatile incorporation into cordierite (Thompson et al. in Contrib Mineral Petrol 142:107–118, 2001; Harley and Carrington in J Petrol 42:1595–1620, 2001) reflects non-ideality of channel H₂O and CO₂ mixing. The dependence of cordierite H₂O and CO₂ contents on P, T and equilibrium fluid composition has been calculated for the range 600–800°C and 200–800 MPa. It has been used for establishing thermodynamic conditions of cordierite formation and the following retrograde P–T paths of cordierite rocks from many localities. Estimates of the H₂O and CO₂ activities have shown that cordierites in granites, pegmatites and high-pressure granulites were formed in fluid-saturated conditions and wide range of H₂O/CO₂ relations. Very low cordierite H₂O contents in many migmatites may be caused not only by fluid-undersaturated conditions at rock formation and H₂O leakage on retrograde P–T paths but also by the presence of additional volatile components like CH₄ and N₂. The pressure dependence of cordierite-bearing mineral equilibria on fluid H₂O/CO₂ relations has been evaluated.     

Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

The H₂O and CO₂ content of cordierite was analysed in 34 samples from successive contact metamorphic zones of the Etive thermal aureole, Scotland, using Fourier‐transform infrared spectroscopy (FTIR). The measured volatile contents were used to calculate peak metamorphic H₂O and CO₂ activities. Total volatile contents are compared with recently modelled cordierite volatile saturation surfaces in order to assess the extent of fluid‐present v. fluid‐absent conditions across the thermal aureole. In the middle aureole, prior to the onset of partial melting, calculated aH₂O values are high, close to unity, and measured volatile contents intersect modelled H₂O–CO₂ saturation curves at the temperature of interest, suggesting that fluid‐present conditions prevailed. Total volatile contents and aH₂O steadily decrease beyond the onset of partial melting, consistent with the notion of aH₂O being buffered to lower values as melting progresses once free hydrous fluid is exhausted. All sillimanite zone samples record total volatile contents that are significantly lower than modelled H₂O–CO₂ saturation surfaces, implying that fluid‐absent conditions prevailed. The lowest recorded aH₂O values lie entirely within part of the section where fluid‐absent melting reactions are thought to have dominated. Samples within 30 m of the igneous contact appear to be re‐saturated, possibly via a magmatically derived fluid. In fluid‐absent parts of the aureole, cordierite H₂O contents yield melt–H₂O contents that are compatible with independently determined melt–H₂O contents. The internally consistent cordierite volatile data and melt–H₂O data support the conclusion that the independent P–T estimates applied to the Etive rocks were valid and that measured cordierite volatile contents are representative of peak metamorphic values. The Etive thermal aureole provides the most compelling evidence, suggesting that the cordierite fluid monitor can be used to accurately assess the fluid conditions during metamorphism and partial melting in a thermal aureole.     

The thermodynamic properties of H2O in magnesium and iron cordierite

 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H₂O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)₂Al₄Si₅O₁₈+H₂O ⇄ (Fe,Mg)₂Al₄Si₅O₁₈.H₂O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ^° _r =–37141±3520 J and ΔS ^° _r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n _H2O=1/[1+1/(K ⋅ f _H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H₂O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996