Crystal structures of Ca-rich clinopyroxenes on the CaMgSi2O6-Mg2Si2O6 join

Summary The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2 position which is solely occupied by Mg. The maximum conversion of M2 to M2 found in this study is 7%. The closest parallel to the M2 geometry is found in the ZnSiO₃ pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2 atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO₃ pyroxene (C2/c).

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Kristallstrukturen Ca-reicher Klinopyroxene der CaMgSi₂O₆-Mg₂Si₂O₆-Reihe

Zusammenfassung Es wurden die strukturellen Änderungen von Klinopyroxenen der Zusammensetzungen Di100, Di90En10 und Di80En20, die durch den Mg-Ersatz für Ca verursacht werden, untersucht. Es zeigt sich, daß mit steigendem Mg-Gehalt ein kleiner Teil der Atome der M2-Position zu einer neuen M2-Position wechselt; diese wird ausschließlich durch Mg besetzt. Der größte in dieser Arbeit gefundene Übergang von M2 nach M2 beträgt ca. 7%. Die stärksten Parallelen zur Geometrie um M2 werden im Pyroxen ZnSiO₃ (C2/c) gefunden. Die Besetzung dieser neuen Position verursacht bedeutende Änderungen im Tetraederverband, da die M2-Atome nicht an O3 gebunden sind. Die Pyroxenstrukturen mit den intermediären Zusammensetzungen Di90En10 und Di80En20 können als Überlagerung zweier Modelle betrachtet werden: Diopsid und ZnSiO₃-Pyroxen (C2/c).

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Abbreviations En Enstatite - Di Diopside - Hd Hedenbergite - Fs Ferrosilite - ClEn Clinoenstatite - Di100 pure diopside - Di90 Di90En10 (mol.-%) - Di80 Di80En20 - brg bridging With 6 Figures     

Ca−Sr substitution in clinopyroxenes along the join CaMgSi2O6−SrMgSi2O6

Summary In order to define the limits of expansion of the M2 polyhedron in theC2/c clinopyroxenes of formulaX ^M2Mg^M1 [Si₂O₆] as the mean ionic radius in the M2 site increases, the join CaMgSi₂O₆–SrMgSi₂O₆ (Di–SrPx) has been investigated atP=1 atm and between 1090°C and 1350°C. The extent of the clinopyroxene solid solutions is limited to the compositional range Di₁₀₀–Di₇₀SrPx₃₀. Within this range the unit-cell parameters of the clinopyroxenes show a linear variation with the increase of Sr content. The comparison of the variations caused in the unit-cell dimensions by the increase of the mean ionic radius in the M2 site (Di–SrPx series) with those caused by the decrease of the mean ionic radius in M2 (Di–En series) displays a different trend ofb in the two series. This different trend ofb suggests a different mechanism of the structure deformation in the two solid solution series. The narrow extent of the Di–SrPx solid solutions atT=1200°C shows that the increase of the mean ionic radius in the M2 site is restricted to the range 1.12–1.16 Å.

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La substitution Ca–Sr dans les clinopyroxènes le long du joint CaMgSi₂O₆–SrMgSi₂O₆

Résumé Le joint CaMgSi₂O₆–SrMgSi₂O₆ (Di–SrPx) a été étudié entre 1090°C et 1350°C à 1 atm dans le but d'établir quelles sont les limites de l'expansibilité du polyhèdre M2 dans les clinopyroxènesX ^M2Mg^M1 [Si₂O₆] (group spatialC2/c) avec l'augmentation du rayon jonique moyen dans le site M2. La solution solide est limitée à l'intervalle de composition Di₁₀₀–Di₇₀ SrPx₃₀. Dans ce domaine les paramètres de la maille varient d'une façon linéaire avec la teneur croissante de Sr. Si on compare les variations de la maille, déterminées par le rayon jonique moyen croissant dans le site M2 (série Di–SrPx), avec celles causées par la diminution du rayon jonique moyen dans le site M2 (série Di–En), on observe une tendance différente du paramètreb dans les deux séries. Ceci indique un mécanisme différent de la déformation structuralle dans les deux séries de solutions solides. Puisque àT=1200°C le domaine des solutions solides Di–SrPx est étroit, l'augmentation du rayon ionique moyen dans le site M2 est bornée à l'intervalle 1.12–1.16 Å.

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With 5 Figures     

Clinopyroxene on the join CaMgSi2O6-CaFe3+AlSiO6-CaTiAl2O6 at low oxygen fugacity

Subsolidus phase relations on the join CaMgSi₂O₆-CaFe³⁺ AlSiO₆-CaTiAl₂O₆ were studied by the ordinary quenching method at \(f_{O_2 } = 10^{ - 11} \) atm and 1,100°C. Crystalline phases encountered are clinopyroxene_ss (ss:solid solution) (Cpx_ss), melilite (Mel), perovskite (Pv), spinel_ss (Sp_ss), magnetite_ss (Mt_ss) and anorthite (An). There is no Cpx_ss single phase field, and the following assemblages were found; Cpx_ss+Mel, Cpx_ss+Mel+Sp_ss, Cpx_ss+Mel+Pv, Cpx_ss+Mel+Sp_ss+Pv, Cpx_ss+Pv+Sp_ss+An, Sp_ss+Pv+Mel+An+Cpx_ss, Mel+Mt_ss+An+Sp_ss+Cpx_ss+liquid and Mel+Mt_ss+An+Sp_ss+Cpx_ss+Pv. Mössbauer spectral study revealed that Cpx_ss contains both Fe²⁺ and Fe³⁺ in the octahedral site, and it was confirmed that the CaFe³⁺ AlSiO₆ content in the Cpx_ss at low \(f_{O_2 } \) is considerably less than that in the Cpx_ss crystallized in air, whereas the CaFe²⁺Si₂O₆ component increases. The maximum solubility of CaTlAl₂O₆ component in the Cpx_ss at low \(f_{O_2 } \) is higher than that in air. The decrease of CaFe³⁺ AlSiO₆ in the Cpx_ss at low \(f_{O_2 } \) may cause increase of CaTial₂O₆ in the Cpx_ss.     

Thermochemical study of glasses in the system CaMgSi2O6-CaAl2SiO6

High temperature solution calorimetry of glasses in the system CaMgSi₂O₆ (Di)-CaAl₂SiO₆ (CaTs) show them to have negative enthalpies of mixing with a regular enthalpy parameter, W_H, of -11.4 ± 0.7 kcal. Negative heats of mixing between alumina-rich and alumina-poor glasses seem to be a general phenomenon in aluminosilicates and are not confined only to glassy systems containing anorthite as a component. The thermodynamic behavior of glasses in the system SiO₂-Ca_0.5;AlO₂-CaMgO₂ appears to vary in a smooth fashion, with small positive heats of mixing near SiO₂ and substantial negative heats of mixing for other compositions. The exothermic behavior with increasing $\text{A1}\text{(Al + Si)}$ may be related to local charge balance of M²⁺ and Al³⁺. The negative heats of mixing in MgCaSi₂O₆-CaAl₂SiO₆, MgCaSi₂O₆-CaAl₂Si₂O₈ and NaAlSi₃O₈-CaAl₂Si₂O₈ glasses are in contrast to the positive heats of mixing found in MgCaSi₂O₆-CaAl₂SiO₆ (pyroxene) and NaAlSi₃O₈-CaAl₂Si₂O₈ (high plagioclase) crystalline solid solutions.     

Viscosity of silicate melts in CaMgSi2O6–NaAlSi2O6 system at high pressure

In situ X-ray viscometry of the silicate melts was carried out at high pressure and at high temperature. The viscosity of the silicate melts in the diopside(Di)–jadeite(Jd) system was determined in the pressure range from 1.88 GPa to 7.9 GPa and in the temperature range from 2,003 K to 2,173 K. The viscosity of the Di 25%–Jd 75% melt decreases continuously to 5.0 GPa, whereas the viscosity of the Di 50%–Jd 50% melt increases over 3.5 GPa. The viscosity of the Di50%–Jd 50% melt reaches a minimum around 3.5 GPa. Since the amounts of silicon in the two melts are the same, the difference in the pressure dependence of the viscosity may be controlled by another network-forming element, i.e., aluminum. The difference in the pressure dependence of the viscosities in the melts with two intermediate compositions in the Di–Jd system is estimated to be due to the difference in the melt structures at high pressures and high temperatures.     

Synthesis of kosmochlor and phase equilibria in the join CaMgSi2O6-NaCrSi2O6

Kosmochlor (NaCrSi₂O₆) was synthesized by the flux method from melts along the join Na₂O·2 SiO₂-Na₂O·Cr₂O₃·4 SiO₂ at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi₂O₆-NaCrSi₂O₆ was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr₂O₃) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.     

Surface tension of melts in the system CaMgSi2O6-CaAl2Si2O8 and its structural significance

The surface tension between silicate melts and air has been measured for melt compositions lying on the diopside-anorthite (Di-An) join from 1300° C to 1580° C. It ranges from 300 dyne/cm to 400 dyne/cm, and decreases with increasing temperature, except for a pure diopside composition. At relatively high temperatures, the surface tension decreases as the anorthite content increases, whereas at lower temperature it is almost constant. These results suggest that diopside melt has a more discrete structure at higher temperatures, whereas, anorthite-bearing melts do not dissociate in the temperature range studied. They also suggest that the structure of both the surface and interior parts of the melt are almost identical at lower temperatures, but at higher temperatures, the surface part has a more polymerized structure with Al₂O₃ enrichment. The surface energy, obtained from the relationship between surface tension and temperature, increases from 294 erg/cm² (Di composition) to 1013 erg/cm² (Di40An60) with increasing anorthite content.     

Experimental study of the CaMgSi2O6–CO2 system at 3–8 GPa

The melting relationships in the system CaMgSi₂O₆ (Di)–CO₂ have been studied in the 3–8 GPa pressure range to determine if there is an abrupt decrease in the temperature of the solidus accompanying the stabilization of carbonate as a subsolidus phase. Such a decrease has been observed previously in peridotitic and some eclogitic systems. In contrast, the solidus in the Di–CO₂ system was found to decrease in a gradual fashion from 3 to 8 GPa. This decrease accompanies an evolution in the composition of the melt at the solidus from silicate-rich with minor CO₂ at 3 GPa to carbonatitic at 5.5 GPa, where the carbonation reaction Diopside + CO₂ = Dolomite (Dol) + Coesite (Cst) intersects the solidus. The near-solidus melt remains carbonatitic at higher pressure, consistent with carbonate being the dominant contributor to the melt. Based on previous studies in both eclogitic and peridotitic systems, this conclusion can be extended to more complicated systems: once carbonate is a stable subsolidus phase, it plays a major role in controlling both the temperature of melting and the composition of the melt produced.     

Densities of melts in the system CaMgSi2O6-CaAl2Si2O8 at low and high pressures,and their structural significance

Density measurements have been carried out on the melt system diopside-anorthite from room temperature to 1600° C at 1 atm, and from 1400° C to 1800° C at pressures up to 20 Kb. The densities were determined based on the dilatometric curve and density at 22° C for lowtemperatures, the double-bob Archimedean method for high-temperatures at 1 atm, and on the sinking and floating spheres method for high-pressure conditions.The results at 1 atm indicate that the thermal expansion coefficient of the glassy state is almost constant, while that of the liquid state decreases with increasing temperature. Density decreased with increasing anorthite content for both glassy and liquid states. Melts in the liquid-state mix ideally with respect to volume, while the glassy state exhibits a maximum excess volume at Di30An70. Density-pressure relations clearly show a density reversion between diopsiderich and anorthite-rich melts; the anorthite-rich melt becomes denser than diopside-rich melt at pressures above 8 kb.The free volumes of both the liquid and glassy states decreased with increasing anorthite content.Isothermal compressibilities and the hard-sphere diameter have been calculated based on the hard-sphere liquid model using thermal expansion coefficients and surface tension data. Calculated compressibilities for diopside-rich melt (Di:>Di60) agreed well with the experimental data, while calculated and observed compressibilities for anorthite-rich melt did not. This evidence indicates that diopside melt may be regarded as a discrete-melt composed of small constituent units (about 10 Å in average diameter) and much interstitial space, while anorthite melt is a three-dimensional network melt with little interstitial space. The critical composition Di60An40 is similar to that of the eutectic and corresponds to breaks between composition and other physical properties. It is proposed that the composition may reflect a kind of critical state in the substitution of the continuous structure of anorthite melt for the discrete structure of diopside melt. The critical state may be interpreted based on the site-percolation theory.     

Crystal-field spectra for blue and green diopsides synthesized in the join CaMgSi2O6-CaCrAlSiO6

Synthetic diopsides in the join CaMgSi₂O₆ CaCrAlSiO₆ have been studied by means of crystal-field theory. These diopsides are either blue or pale green in colour. The former forms at lower temperatures and the latter at higher temperatures. The optical spectra and colours can be unequivocally explained based on the assumption that Cr³⁺ions occupy both tetrahedral and octahedral sites in the diopsides. In the blue diopsides Cr³⁺ions are present in two kinds of spin state, i.e., tetrahedrally coordinated low spin and octahedrally coordinated high spin. On heating the blue diopsides, tetrahedral occupancy of chromium decreases sharply due to spin transformation from tetrahedral low spin to octahedral high spin. Above 1,160° C nearly all chromium ions are present in octahedral sites with high spin state and the diopsides become pale green in colour. Some petrogenetic applications of the resultes are discussed.     

Raman spectroscopic study of clinopyroxenes in the system CaScAlSiO6 - CaAl2SiO6

Seven clinopyroxenes in the system CaScAlSiO₆- CaAl₂SiO₆ synthesized at 1 atm and under high pressure have been studied by Raman spectroscopy. The T-O-T stretching band of CaScAlSiO₆ pyroxene can be deconvoluted into three bands corresponding to Al-O-Al, Al-O-Si, and Si-O-Si stretching vibrations, although that of CaAl₂SiO₆ can be deconvoluted into the two bands (Al-O-Al+Al-O-Si) and Si-O-Si. The Al-O-Si Raman shifts of CaScAlSiO₆ and CaAl₂SiO₆ pyroxenes are found to fall on the linear plot of the relationship between T-T distance and Raman shifts in ABSi₂O₆-type pyroxenes, suggesting that the Al-O-Si chains are relatively long. Variation of areal fractions of the Raman bands demonstrates that the partial disordering of Al/Si depends on the ionic radius and electronegativity of the octahedral ion.     

Thermodynamic parameters of CaMgSi2O6-Mg2Si2O6 pyroxenes based on regular solution and cooperative disordering models

Thermodynamic parameters for the reaction: $$\begin{gathered} Mg_2 Si_2 O_6 = Mg_2 Si_2 O_6 \hfill \\ enstatite clinopyroxene \hfill \\ \end{gathered} $$ in the system CaO-MgO-SiO₂ have been deduced from phase equilibrium and enthalpy of solution data. From the regular solution theory, the seventeen currently available reversed experimental compositions of coexisting enstatite and clinopyroxene, presumed to be ordered diopside, lead, by a statistical regression, to the following best fit parameters: ΔH ^o=6.80 kJ ΔS ^o=2.75 J/K W _H ^Cpx =24.47 kJ (regular solution enthalpy parameter) W _V ^Cpx =0.105 J/bar (regular solution volume parameter). The derived parameters are not significantly affected by the (necessary) choice of W ^Opx in the range 20–50 kJ. The above values are in very good agreement with deductions from the solution calorimetry on synthetic CaMgSi₂O₆-Mg₂Si₂O₆ clinopyroxenes of Newton et al. (1979), which also places bounds on possible departures from the optimal values of these parameters. The calorimetric data may also be interpreted in terms of a Bragg-Williams cooperative-disordering model (Navrotsky and Loucks, 1977), in which diopside-structure clinopyroxene and a ‘relaxed’ low-Ca clinopyroxene (‘Fe-free pigeonite’) approach each other in composition, structural state, and stability with increasing temperature. The ΔH ^o parameter deduced from the regular solution theory is reinterpreted as the enthalpy change of enstatite to Mg₂Si₂O₆ pigeonite; the ΔH ^o of the transformation of enstatite to the diopside structure would, in this case, be considerably larger than 6.8 kJ. The curvature of the enthalpy of solution data, explained by the regular solution theory in terms of M2-site energetics (involving W _H ^cpx ), is reinterpreted as due to disordering and ‘relaxation’ in the Navrotsky-Loucks model. Although the regular solution theory with the best-fit parameters accounts for all of the reversed enstatite and diopside compositions to within 18 ° C, and is a convenient representation of the phase equilibria for purposes of geothermometry, the disordering model is, at the present level of knowledge, equally valid and allows for a region of stability of two coexisting clinopyroxenes.     

X-ray determination and equilibrium composition of clinopyroxenes in the system CaO-MgO-Al2O3-SiO2

Ternary clinopyroxenes have been synthesized in the plane Di-CaTs-En. The variation of their crystallographic parameters has allowed the development of three determinative grids, which utilize b-, 2 _22¯1–2 ₃₁₀ and 2 ₃₃₀ –2 ₂₀₂ respectively. These grids show significant differences in comparison with the previously proposed ones. Present results have been used to review some data on clinopyroxenes equilibria in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) (Biggar 1969; Bruno and Facchinelli 1978; Herzberg 1978; O'Hara and Schairer 1963; Boyd 1969). In particular the petrogenic grid correlating P, T and CaTs content of clinopyroxenes in spinel-lherzolite assemblage (Herzberg 1978) has been revised, and consequently equilibrium temperatures rise by one hundred degrees.     

Experimental Investigation and Optimization of Thermodynamic Properties and Phase Diagrams in the Systems CaO-SiO2, MgO-SiO2, CaMgSi2O6-SiO2 and CaMgSi2O6-Mg2SiO4 to 1{middle dot}0 GPa

Using experimental results at 1·0 GPa for the systemsCaO–SiO₂, MgO–SiO₂, CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–Mg₂SiO₄,and all the currently available phase equilibria and thermodynamicdata at 1 bar, we have optimized the thermodynamic propertiesof the liquid phase at 1·0 GPa. The new optimized thermodynamicparameters indicate that pressure has little effect on the topologyof the CaO–SiO₂, CaMgSi₂O₆–SiO₂, and CaMgSi₂O₆–Mg₂SiO₄systems but a pronounced one on the MgO–SiO₂ binary. Themost striking change concerns passage of the MgSiO₃ phase fromperitectic melting at 1 bar to eutectic melting at 1·0GPa. This transition is estimated to occur at 0·41 GPa.For the CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–Mg₂SiO₄ pseudo-binaries,the size of the field clinopyroxene + liquid increases withincreasing pressure. This change is related to the shift ofthe piercing points clinopyroxene + silica + liquid (from 0·375mol fraction SiO₂ at 1 bar to 0·414 at 1·0 GPa)and clinopyroxene + olivine + liquid (from 0·191 molfraction SiO₂ at 1 bar to 0·331 at 1·0 GPa) thatbound the clinopyroxene + liquid field in the CaMgSi₂O₆·SiO₂and CaMgSi₂O₆·Mg₂SiO₄ pseudo-binaries, respectively. KEY WORDS: CaO–SiO₂; CaMgSi₂O₆–Mg₂SiO₄; CaMgSi₂O₆–SiO₂; experiments; MgO–SiO₂     

Experimental study on the phase equilibria in the join CaMgSi2O6-CaCrCrSiO6 with special reference to the blue diopside

The behaviour of tetrahedrally coordinated and octahedrally coordinated Cr³⁺ ions in diopside is discussed from the study on the join CaMg-Si₂O₆-CaCrCrSiO₆. The molecule CaCrCrSiO₆ decomposes into uvarovite+eskolaite and its maximum solubility in diopside is 6.7 wt percent at 940 ° C. Crystalline phases are diopside ss (ss is abbreviation of solid solution), uvarovite ss, wollastonite ss, spinel and eskolaite. The diopside ss is blue in colour. Its optical spectra were measured in the wavelenght range of 325–2600 nm, and assigned after tetrahedral configuration Td and octahedral configuration Oh. It is estimated that octahedral Cr³⁺ ions are in high spin state, while tetrahedral Cr³⁺ ions may be probably in low spin state. The _t and B are 10,300–10,370 cm^–1 and 429–432 cm^–1. The CFSE for tetrahedral low spin Cr³⁺ ions is nearly the same as that for octahedral high spin Cr³⁺ ions. The ionic radii of tetrahedral low spin Cr³⁺ ions calculated are 0.47–0.53 Å, shrinked from the radius of octahedral high spin Cr³⁺ ion (0.615 Å) as much as 14–24 percent. Petrologic implications of the result are also discussed.The first half of the D. Sc. dissertation of K. Ikeda presented to Hokkaido University in June, 1976     

Electron microprobe measurement of pyroxenes coexisting with H2O-undersaturated liquid in the join CaMgSi2O6-Mg2Si2O6-H2O at 30 kilobars,with applications to geothermometry

Mixtures of synthetic crystalline enstatite and diopside were reacted with small water contents in sealed capsules in piston-cylinder apparatus at 30 kb between 1000° C and 1700° C. The compositions of coexisting enstatite and diopside solid solutions were measured with an ARL-EMX electron microprobe between 1000° C and 1500° C. Between 1100° C and 1500° C the pyroxenes coexisted with H₂O-undersaturated liquid which quenched to inhomogeneous pyroxene crystals. The presence of liquid facilitated growth of pyroxene crystals suitable for microprobe determinations. The solvus of Davis and Boyd (1966) is generally used in geothermometry; our enstatite solvus limb is a few mol-% richer in Mg₂Si₂O₆ in the temperature range 1000–1400° C; our diopside solvus limb is a few mol-% richer in Mg₂Si₂O₆ below 1100°C, in close agreement between 1100° C and 1200° C, but richer in CaMgSi₂O₆ between 1200° C and 1500° C. Estimated equilibration temperatures for a diopside with composition 78.7% Di is 1300° C according to our results compared with 1210° C for the Davis and Boyd solvus.     

57Fe mössbauer study of clinopyroxenes in the join CaFe3+ AlSiO6 - CaTiAl2O6

Synthetic clinopyroxenes of compositions between CaFe³⁺AlSiO₆ and CaFe _0.85 ³⁺ Ti_0.15Al_1.15Si_0.85O₆ have been studied by ⁵⁷Fe Mössbauer spectroscopy. The spectra consist of two doublets assigned to Fe³⁺ in M1 and T sites. From the area ratios of the doublets the site occupancies of Fe³⁺ and Al were determined. Si decreases from 1.00 to 0.85 and Al+Fe³⁺ increases from 1.00 to 1.15 per formula unit with increasing CaTiAl₂O₆ component of the clinopyroxene. The atomic ratio of Fe³⁺(T)/Fe³⁺(total) is 0.11–0.16; 4.5–7.5 percent of the T sites are occupied by Fe³⁺. Thus the presence of Si-O-Fe³⁺, Al-O-Fe³⁺, and Fe³⁺-O-Fe³⁺ bonds is expected in addition to Si-O-Si, Si-O-Al and Al-O-Al bonds. However, the possibility of the former bonds being present would be small, because the amount of Fe³⁺(T) is far less than that of Si and Al. The isomer shift of Fe³⁺(T) is one of the largest in the values found previously for Fe³⁺(T) in silicates. It increases with increasing CaTiAl₂O₆ component and seems to be correlated to the ionic character of the cation — anion bonds calculated from electronegativity. The quadrupole splittings of Fe³⁺(M1) and Fe³⁺(T) decrease with the substitution of Fe³⁺?Ti⁴⁺ in the M1 and of Si?Al in the T sites.     

Displacive phase transitions in C-centred clinopyroxenes: spodumene, LiScSi2O6 and ZnSiO3

The clinopyroxenes spodumene (LiAlSi₂O₆), LiScSi₂O₆ and ZnSiO₃, all with space group C2/c at ambient conditions, were studied under high pressures by single-crystal X-ray diffraction in a diamond-anvil cell. Changes in the evolution of the unit-cell parameters, optical properties and the appearance of h + k odd reflections characteristic of a primitive lattice, indicate that all three pyroxenes undergo phase transitions. The transitions are mostly displacive in character, and are non-quenchable. Transition pressures are 3.19 GPa in spodumene, ∼0.6 GPa in LiScSi₂O₆ and 1.92 GPa in ZnSiO₃. The space group of all three high-pressure phases was determined to be P2₁/c by structure refinement to single-crystal X-ray intensity data collected in the DAC. In the ZnSiO₃ clinopyroxene the intermediate P2₁/c phase further transforms to a second C2/c phase (HP-C2/c) at 4.9 GPa (confirmed by structure refinement). The volume change at this transition is about 2.6%, three times larger than in the first phase transition, and typical of the P2₁/c→ HP-C2/c phase transitions found previously in MgSiO₃, FeSiO₃, etc. These results therefore provide the first direct evidence that the HP-C2/c and the HT-C2/c structures of pyroxenes are distinct polymorphs with the same space group. The phase transition from C2/c to P2₁/c symmetry in spodumene and LiScSi₂O₆ therefore occurs because the polymorphs stable at ambient conditions are isotypic to the high-temperature C2/c phases of clinopyroxenes such as pigeonite and clinoenstatite. Received: 22 December 1999 / Accepted: 7 June 2000     

Compressional behaviour of CaNiSi2O6 clinopyroxene: bulk modulus systematic and cation type in clinopyroxenes

A single-crystal of composition CaNiSi₂O₆ (space group C2/c) was investigated at high pressure up to about 7.8 GPa by X-ray diffraction. The unit-cell parameters were measured at 18 different pressures. The P-V data were fitted by a third-order Birch-Murnaghan equation of state V₀=435.21(1) ų, K ₀=117.6(3) GPa and K^=6.4(1). The linear axial compressibilities _a, _b, _c and a sin are 2.14(1), 3.00(1), 2.43(1) and 1.63(1) × 10–3 GPa^–1. Comparing the compressibility data with other CaM1Si₂O₆ pyroxenes we suggest that the empirical K × V = constant relationships are followed in C2/c pyroxenes only if the same valence electron character is shared.     

Stability relations in the system CaSiO3-CaMnSi2O6-CaFeSi2O6

In the system CaSiO₃-CaMnSi₂O₆-CaFeSi₂O₆ extensive miscibility gaps between pyroxenoids and clinopyroxenes are observed. The miscibility gap between Mn-bustamite and Mn-wollastonite has been determined experimentally by a hydrothermal technique between 400° and 1200° C at P _f= 2 kbar. Further experiments have been performed at P _f=9 kbar, which revealed a shifting of the miscibility gap towards more Ca-rich compositions. The bustamite phase is stabilized by high pressures and the wollastonite structure is the stable phase at high temperatures.Similar phase relations as along the join CaSiO₃-CaMnSi₂O₆ exist along the join CaSiO₃-CaFeSi₂O₆ but with a more extensive two-phase field of bustamite-clinopyroxene.Possible phase relations along the joins CaSiO₃-CaMnSi₂O₆, CaSiO₃-CaFeSi₂O₆ and CaFeSi₂O₆-CaMnSi₂O₆ are given in temperature-composition diagrams for low pressures, based on natural and experimental data.