Thermal expansion and high-temperature P21/c–C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6

A synthetic clinopyroxene with composition LiFe³⁺Ge₂O₆, monoclinic s.g. P2₁/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) Å, β = 108.844(6)°, V = 442.75(16) ų, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P2₁/c to C2/c at a transition temperature T _tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T _tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T _tr the thermal volume expansion is due to dilatation of the structure in the $(\bar{1}\,0\,1) A synthetic clinopyroxene with composition LiFe³⁺Ge₂O₆, monoclinic s.g. P2₁/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) ?, β = 108.844(6)°, V = 442.75(16) ?³, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P2₁/c to C2/c at a transition temperature T _tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T _tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T _tr the thermal volume expansion is due to dilatation of the structure in the ([`1] 0 1)(\bar{1}\,0\,1) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P2₁/c expands more than the C2/c phase. The evolution of Q ² (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q ₀² = 0.538(2) at T _tr, with T _c of 481(7) K, b/a = −2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe³⁺Si₂O₆) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P2₁/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > T _tr (<1‰), nor a change of tilting of the basal plane. No change in coordination is observed at T > T _tr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at T _tr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T < T _tr [206.8(5)° at 703 K] and extends by 12° at the transition.     

High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

 An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca_0.8Mg_1.2Si₂O₆ clinopyroxene (Di₈₀En₂₀) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic, with β_b ⋙ β_c > β_a > β_asinβ for any pressure range and for both diopside and Di₈₀En₂₀. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di₈₀En₂₀ than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di₈₀En₂₀. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di₈₀En₂₀ the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3) for Di₈₀En₂₀; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich orthopyroxenes. Received: 3 January 2000 / Accepted: 21 May 2000     

The molar volume of FeO–MgO–Fe2O3–Cr2O3–Al2O3–TiO2 spinels

We define and calibrate a new model of molar volume as a function of pressure, temperature, ordering state, and composition for spinels in the supersystem (Mg, Fe²⁺)(Al, Cr, Fe³⁺)₂O₄ ? (Mg, Fe²⁺)₂TiO₄. We use 832 X-ray and neutron diffraction measurements performed on spinels at ambient and in situ high-P, T conditions to calibrate end-member equations of state and an excess volume model for this system. The effect on molar volume of cation ordering over the octahedral and tetrahedral sites is captured with linear dependence on Mg²⁺, Al³⁺, and Fe³⁺ site occupancy terms. We allow standard-state volumes and coefficients of thermal expansion of the end members to vary within their uncertainties during extraction of the mixing properties, in order to achieve the best fit. Published equations of state of the various spinel end members are analyzed to obtain optimal values of the bulk modulus and its pressure derivative, for each explicit end member. For any spinel composition in the supersystem, the model molar volume is obtained by adding excess volume and cation order-dependent terms to a linear combination of the five end-member volumes, estimated at pressure and temperature using the high-T Vinet equation of state. The preferred model has a total of 9 excess volume and order-dependent parameters and fits nearly all experiments to within 0.02 J/bar/mol, or better than 0.5 % in volume. The model is compared to the current MELTS spinel model with a demonstration of the impact of the model difference on the estimated spinel-garnet lherzolite transition pressure.     

Shear modulus, heat capacity, viscosity and structural relaxation time of Na2O–Al2O3–SiO2 and Na2O–Fe2O3–Al2O3–SiO2 melts

The configurational heat capacity, shear modulus and shear viscosity of a series of Na₂O–Fe₂O₃–Al₂O₃–SiO₂ melts have been determined as a function of composition. A change in composition dependence of each of the physical properties is observed as Na₂O/(Na₂O + Al₂O₃) is decreased, and the peralkaline melts become peraluminous and a new charge-balanced Al-structure appears in the melts. Of special interest are the frequency dependent (1 mHz–1 Hz) measurements of the shear modulus. These forced oscillation measurements determine the lifetimes of Si–O bonds and Na–O bonds in the melt. The lifetime of the Al–O bonds could not, however, be resolved from the mechanical spectrum. Therefore, it appears that the lifetime of Al–O bonds in these melts is similar to that of Si–O bonds with the Al–O relaxation peak being subsumed by the Si–O relaxation peak. The appearance of a new Al-structure in the peraluminous melts also cannot be resolved from the mechanical spectra, although a change in elastic shear modulus is determined as a function of composition. The structural shear-relaxation time of some of these melts is not that which is predicted by the Maxwell equation, but up to 1.5 orders of magnitude faster. Although the configurational heat capacity, density and shear modulus of the melts show a change in trend as a function of composition at the boundary between peralkaline and peraluminous, the deviation in relaxation time from the Maxwell equation occurs in the peralkaline regime. The measured relaxation times for both the very peralkaline melts and the peraluminous melts are identical with the calculated Maxwell relaxation time. As the Maxwell equation was created to describe the timescale of flow of a mono-structure material, a deviation from the prediction would indicate that the structure of the melt is too complex to be described by this simple flow equation. One possibility is that Al-rich channels form and then disappear with decreasing Si/Al, and that the flow is dominated by the lifetime of Si–O bonds in the Al-poor peralkaline melts, and by the lifetime of Al–O bonds in the relatively Si-poor peralkaline and peraluminous melts with a complex flow mechanism occurring in the mid-compositions. This anomalous deviation from the calculated relaxation time appears to be independent of the change in structure expected to occur at the peralkaline/peraluminous boundary due to the lack of charge-balancing cations for the Al-tetrahedra.     

High-pressure behaviour along the jadeite NaAlSi2O6–aegirine NaFeSi2O6 solid solution up to 10 GPa

The volume variation as a function of pressure along the jadeite–aegirine solid solution was determined at room temperature up to pressures between 6.5 and 9.7 GPa by single-crystal X-ray diffraction. The unit-cell volumes collected at room pressure for the different compositions indicate a slight deviation from linearity along the join. The pressure–volume data have been fitted using a third-order Birch-Murnaghan equation of state (BM3-EoS). The bulk modulus, K _T0, varies from 134.0(7) GPa for pure jadeite to 116.1(5) GPa for pure aegirine. Its evolution with composition along the join is not linear and can be described by the following second order polynomial:

Phase Equilibria in the Ternary Systems KBr–K2B4O7–H2O and KCl–K2B4O7–H2O at 373 K

According to the compositions of the underground gasfield brines in the west of Sichuan Basin,the phase equilibria in the ternary systems KBr-K2B4O7-H2O and KCl-K2B4O7-H2O at 373 K were studied using the isothermal dissolution equilibrium method.The solubilities of salts and the densities of saturated solutions in these ternary systems were determined.Using the experimental data,phase diagrams and density-composition diagrams were constructed.The two phase diagrams were simple co-saturation type,each having an invariant point,two univariant curves and two crystallization regions.The equilibrium solid phases in the ternary system KBr-K2B4O7-H2O are potassium bromide （KBr） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）,and those in the ternary system KCl-K2B4O7-H2O are potassium chloride （KCl） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）.Comparisons of the phase diagrams of the two systems at different temperatures show that there is no change in the crystallization phases,but there are changes in the size of the crystallization regions.As temperature increases,the solubility of K2B4O7·4H2O increases rapidly,so the crystallization field of K2B4O7·4H2O becomes smaller.     

The thermal stability of sideronatrite and its decomposition products in the system Na2O–Fe2O3–SO2–H2O

The thermal stability of sideronatrite, ideally Na₂Fe³⁺(SO₄)₂(OH)·3(H₂O), and its decomposition products were investigated by combining thermogravimetric and differential thermal analysis, in situ high-temperature X-ray powder diffraction (HT-XRPD) and Fourier transform infrared spectroscopy (HT-FTIR). The data show that for increasing temperature there are four main dehydration/transformation steps in sideronatrite: (a) between 30 and 40 °C sideronatrite transforms into metasideronatrite after the loss of two water molecules; both XRD and FTIR suggest that this transformation occurs via minor adjustments in the building block. (b) between 120 and 300 °C metasideronatrite transforms into metasideronatrite II, a still poorly characterized phase with possible orthorhombic symmetry, consequently to the loss of an additional water molecule; X-ray diffraction data suggest that metasideronatrite disappears from the assemblage above 175 °C. (c) between 315 and 415 °C metasideronatrite II transforms into the anhydrous Na₃Fe(SO₄)₃ compound. This step occurs via the loss of hydroxyl groups that involves the breakdown of the [Fe³⁺(SO₄)₂(OH)] _∞ ^2? chains and the formation of an intermediate transient amorphous phase precursor of Na₃Fe(SO₄)₃. (d) for T > 500 °C, the Na₃Fe(SO₄)₃ compound is replaced by the Na-sulfate thenardite, Na₂SO₄, plus Fe-oxides, according to the Na₃Fe³⁺(SO₄)₃ → 3/2 Na₂(SO₄) + 1/2 Fe₂O₃ + SO_x reaction products. The Na–Fe sulfate disappears around 540 °C. For higher temperatures, the Na-sulfates decomposes and only hematite survives in the final product. The understanding of the thermal behavior of minerals such as sideronatrite and related sulfates is important both from an environmental point of view, due to the presence of these phases in evaporitic deposits, soils and sediments including extraterrestrial occurrences, and from the technological point of view, due to the use of these materials in many industrial applications.     

Thermal expansion along the NaAlSi2O6–NaFe3+Si2O6 and NaAlSi2O6–CaFe2+Si2O6 solid solutions

The high temperature volume and axial parameters for six C2/c clinopyroxenes along the NaAlSi₂O₆–NaFe³⁺Si₂O₆ and NaAlSi₂O₆–CaFe²⁺Si₂O₆ joins were determined from room T up to 800°C, using integrated diffraction profiles from in situ high temperature single crystal data collections. The thermal expansion coefficient was determined by fitting the experimental data according to the relation: ln(V/V ₀) = α(T − T ₀). The thermal expansion coefficient increases by about 15% along the jadeite–hedenbergite join, whereas it is almost constant between jadeite and aegirine. The increase is related to the Ca for Na substitution into the M2 site; the same behaviour was observed along the jadeite–diopside solid solution, which presents the same substitution at the M2 site. Strain tensor analysis shows that the major deformation with temperature occurs in all samples along the b axis; on the (010) plane the higher deformation occurs in jadeite and aegirine at a direction almost normal to the tetrahedral–octahedral planes, and in hedenbergite along the projection of the longer M2–O bonds. The orientation of the strain ellipsoid with temperature in hedenbergite is close to that observed with pressure in pyroxenes. Along the jadeite–aegirine join instead the high-temperature and high-pressure strain are differently oriented.     

Hydrogen incorporation into forsterite in Mg2SiO4–K2Mg(CO3)2–H2O and Mg2SiO4–H2O–C at 7.5–14.0 GPa

Experiments on water solubility in forsterite in the systems Mg₂SiO₄–K₂Mg(CO₃)₂–H₂O and Mg₂SiO₄–H₂O–C were conducted at 7.5–14.0 GPa and 1200–1600 °C. The resulting crystals contain 448 to 1480 ppm water, which is 40–70% less than in the forsterite–water system under the same conditions. This can be attributed to lower water activity in the carbonate-bearing melt. The water content of forsterite was found to vary systematically with temperature and pressure. For instance, at 14 GPa in the system forsterite–carbonate–H₂O the H₂O content of forsterite drops from 1140 ppm at 1200 °C to 450 ppm at 1600 °C, and at 8 GPa it remains constant or increases from 550 to 870 ppm at 1300–1600 °C. Preliminary data for D-H-bearing forsterite are reported. Considerable differences were found between IR spectra of D-H- and H-bearing forsterite. The results suggest that CO₂ can significantly affect the width of the olivine-wadsleyite transition, i.e., the 410-km seismic discontinuity, which is a function of the water content of olivine and wadsleyite.     

Hydrogen incorporation in enstatite in the system MgO–SiO2–H2O–NaCl

The incorporation of hydrogen in enstatite in a hydrous system containing various amounts of NaCl was investigated at 25 kbar. The hydrogen content in enstatite shows a clear negative correlation to the NaCl-concentration in the system. The most favourable explanation is the reduction of water fugacity due to dilution. Other reasons for the limited hydrogen incorporation at high NaCl levels, such as a significant influence of Na⁺ on the defect chemistry or an exchange between OH^- and Cl⁻in enstatite, appear much less important. A partition coefficient D _Na ^En/Fluid = 0.0013 could be determined, demonstrating that Na is less incompatible in enstatite than H. The new results support the idea that dissolved components have to be considered when the total hydrogen storage capacity in nominally anhydrous minerals is estimated, especially in geological settings with high levels of halogens, such as subduction zones.     

Solid-liquid Equilibria for the Quaternary NaCl–Na2SO4–Na2B4O7–H2O System at 348 K

正1 Introduction China has very abundant liquid mineral resources.Especially,the brine resources in the west of Sichuan Basin are pushed into the first place in China,whose K and B contents are unusually high.These rare liquid mineral resources have very good exploitation prospect(Lin,2001;2006).Generally speaking,phase equilibrium     

The high-temperature P2/c<Subscript>1</Subscript>–C2/c phase transition in Fe-free Ca-rich P2<Subscript>1</Subscript>/c clinopyroxenes

An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca_0.40Mg_1.60Si₂O₆, Ca_0.52Mg_1.46Al_0.05Si_1.98O₆, Ca_0.59Mg_1.41Si₂O₆ and Ca_0.70Mg_1.30Si₂O₆, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ and Ca_0.59Mg_1.41Si₂O₆, evidence of for the P2₁/c-C2/c displacive phase transition was seen in changes in lattice parameters at T 550 and 300 °C respectively. Landau modelling of the phase transition behaviour for the sample with composition Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ shows a tricritical behaviour [T _c =547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside–enstatite indicates that a decrease in T _{c ,} and a switch from first-order to tricritical behavior occurs with increasing Ca content. The change in the transition behaviour was related to an interaction with the antiphase domains at the nanoscale.     

The MgCr2O4–MgFe2O4 solid solution series: effects of octahedrally coordinated Fe3+ on T–O bond lengths

The influence on the spinel structure of Fe³⁺ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr₂O₄–MgFe₂O₄) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe₂O₄), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm^?1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe³⁺ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a ₀ and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002) influence of Fe³⁺ at the M site on T–O bond lengths in the spinel structure.     

Raman study of MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 synthetic series: the effects of Fe2+ and Fe3+ on Raman shifts

Two synthetic series of spinels, MgCr₂O₄–Fe²⁺Cr₂O₄ and MgCr₂O₄–MgFe₂ ³⁺O₄ have been studied by Raman spectroscopy to investigate the effects of Fe²⁺ and Fe³⁺ on their structure. In the first case, where Fe²⁺ substitutes Mg within the tetrahedral site, there is a continuous and monotonic shift of the Raman modes A_1g and E_g toward lower wavenumbers with the increase of the chromite component into the spinel, while the F_2g modes remain nearly in the same position. In the second series, for low Mg-ferrite content, Fe³⁺ substitutes for Cr in the octahedral site; when the Mg-ferrite content nears 40 %, a drastic change in the Raman spectra occurs as Fe³⁺ starts entering the tetrahedral site as well, consequently pushing Mg to occupy the octahedral one. The Raman spectral region between 620 and 700 cm^?1 is associated to the octahedral site, where three peaks are present and it is possible to observe the Cr–Fe³⁺ substitution and the effects of order–disorder in the tetrahedral site. The spectral range at 500–620 cm^?1 region shows that there is a shift of modes toward lower values with the increase of the Mg-ferrite content. The peaks in the region at 200–500 cm^?1, when observed, show little or negligible Raman shift.     

Phase relations in the MgO–P2O5–H2O system and the stability of phosphoellenbergerite: petrological implications

The polymorphic relations for Mg₃(PO₄)₂ and Mg₂PO₄OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg₃(PO₄)₂-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg₃(PO₄)₂-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg₂PO₄OH polymorphs only althausite, holtedahlite, β-Mg₂PO₄OH (the hydroxyl analogue of wagnerite) and ɛ-Mg₂PO₄OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (<600 °C), ɛ-Mg₂PO₄OH the high-temperature, low-pressure phase and β-Mg₂PO₄OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (K_D ^F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO₂. Synthetic phosphoellenbergerite has a composition close to (Mg_0.9□_0.1)₂Mg₁₂P₈O₃₈H_8.4. It is a high-pressure phase, which breaks down to Mg₂PO₄OH + Mg₃(PO₄)₂ + H₂O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H₂O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react. Received: 7 April 1995 / Accepted: 12 November 1997     

Solubility and partitioning of water in synthetic forsterite and enstatite in the system MgO–SiO2–H2O±Al2O3

The solubility of water in coexisting enstatite and forsterite was investigated by simultaneously synthesizing the two phases in a series of high pressure and temperature piston cylinder experiments. Experiments were performed at 1.0 and 2.0 GPa at temperatures between 1,100 and 1,420°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on orientated single crystals of each phase. Phase water contents were estimated using the calibration of Libowitzky and Rossman (Am Mineral 82:1111–1115, 1997). Enstatite crystals, synthesized in equilibrium with forsterite and an aqueous phase at 1,350°C and 2.0 GPa, contain 114 ppm H₂O. This is reduced to 59 ppm at 1,100°C, under otherwise identical conditions, suggesting a strong temperature dependence. At 1,350°C and 1.0 GPa water solubility in enstatite is 89 ppm, significantly lower than that at 2.0 GPa. In contrast water solubility in forsterite is essentially constant, being in the range 36–41 ppm for all conditions studied. These data give partition coefficients in the range 2.28–3.31 for all experiments at 1,350°C and 1.34 for one experiment at 1,100°C. The incorporation of Al₂O₃ in enstatite modifies the OH stretching spectrum in a systematic way, and slightly increases the water solubility.     

Experimental study of the K2ZrSi3O9 (wadeite)–K2TiSi3O9 and K2(Zr,Ti)Si3O9–phlogopite systems at 2–3 GPa

Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K₂ZrSi₃O₉ (wadeite) and K₂TiSi₃O₉ cyclosilicates under upper mantle conditions. The K₂ZrSi₃O₉–K₂TiSi₃O₉ join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K₂ZrSi₃O₉ is the more refractory end member. At 2 GPa, in the more complex K₂ZrSi₃O₉–K₂TiSi₃O₉–K₂Mg₆Al₂Si₆O₂₀(OH)₄ system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO₂ partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K₂(Zr,Ti)Si₃O₉ phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution ^VITi+^VI□=2^VIMg. Received: 26 January 1999 / Accepted: 10 January 2000     

Vibrational analysis of dioptase Cu6Si6O18 · 6(H2O) and its puckered six-membered ring

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R or C _3i ² ) consists of puckered six-membered silicate rings (Si₆O₁₈) linked by Cu²⁺ ions and H₂O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si⁴⁺ and Cu²⁺ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H₂O motions with those of the ring is found for the Raman-active modes below 430 cm^-1. The normal modes and frequencies of the hypothetical isolated ring with C _3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H₂O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.