High-T phase transition of synthetic ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study

The synthetic amphibole Na_0.95(Li_0.95Mg_1.05)Mg₅Si₈O₂₂(OH)₂ was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P2₁ /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm⁻¹, respectively, and two minor bands at 3,667 and 3,687 cm⁻¹. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (^A□_0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T _c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P2₁ /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ^ANa_0.8^B(Na_0.8Mg_1.2)^CMg₅Si₈O₂₂(OH)₂ indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T _c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.     

Synthetic ANaB(Na x Li1 − x Mg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study

The high-pressure behavior of three synthetic amphiboles crystallized with space group P2₁/m at room conditions in the system Li₂O–Na₂O–MgO–SiO₂–H₂O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions ^ANa ^B(Na _x Li_1 − x Mg₁) ^CMg₅ Si₈ O₂₂(OH)₂ with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O–H dipoles in the P2₁/m structure, bonded to the same local environment ^M1M3Mg₃–OH–^ANa, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2₁/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole ^B(Na_0.2Li_0.8Mg₁) in the frequency range 50 to 1,400 cm⁻¹ also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479–482, 2006a) regarding a new high-pressure amphibole polymorph.     

High-<Emphasis Type="Italic">T</Emphasis> behaviour of gedrite: thermoelasticity,cation ordering and dehydrogenation

The thermoelastic behaviour of a natural gedrite having the crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.05 Fe_0.86²⁺ Mn_0.02 Ca_0.04) ^C(Mg_3.44 Fe_0.36²⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69)O₂₂ ^W(OH)₂ has been studied by single-crystal X-ray diffraction to 973 K (Stage 1). After data collection at 973 K, the crystal was heated to 1,173 K to induce dehydrogenation, which was registered by significant changes in unit-cell parameters, M1–O3 and M3–O3 bond lengths and refined site-scattering values of M1 and M4 sites. These changes and the crystal-chemical formula calculated from structure refinement show that all Fe²⁺ originally at M4 migrates into the ribbon of octahedrally coordinated sites, where most of it oxidises to Fe³⁺, and there is a corresponding exchange of Mg from the ribbon into M4. The resulting composition is that of an oxo-gedrite with an inferred crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.93 Ca_0.04) ^C(Mg_2.56 Mn_0.02²⁺ Fe_0.10²⁺ Fe_1.22³⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69) O₂₂ ^W[O_1.12²⁻ (OH)_0.88]. This marked redistribution of Mg and Fe is interpreted as being driven by rapid dehydrogenation at the H3A and H3B sites, such that all available Fe in the structure orders at M1 and M3 sites and is oxidised to Fe³⁺. Thermoelastic data are reported for gedrite and oxo-gedrite; the latter was measured during cooling from 1,173 to 298 K (Stage 2) and checked after further heating to 1,273 K (Stage 3). The thermoelastic properties of gedrite and oxo-gedrite are compared with each other and those of anthophyllite.     

EPR study of chromium-doped forsterite crystals: Cr<Superscript>3+</Superscript>(<Emphasis Type="Italic">M</Emphasis>1) with and without associated nearest-neighbor Mg<Superscript>2+</Superscript>(<Emphasis Type="Italic">M</Emphasis>2) vacancy

Electron paramagnetic resonance (EPR) study of single crystals of chromium-doped forsterite grown by the Czochralski method in two different research laboratories has revealed, apart from the known paramagnetic centers Cr³⁺(M1), Cr³⁺(M2) and Cr⁴⁺, a new center \textCr ³⁺ (M 1)-V_{\textMg ²⁺} (M 2) {\text{Cr}}^{ 3+ } (M 1){-}V_{{{\text{Mg}}^{ 2+ } }} (M 2) formed by a Cr³⁺ ion substituting for Mg²⁺ at the M1 structural position with a nearest-neighbor Mg²⁺ vacancy at the M2 position. For this center, the conventional zero-field splitting parameters D and E and the principal g values and A values of the ⁵³Cr hyperfine splitting have been determined as follows: D = 33.95(3) GHz, E = 8.64(1) GHz, g = [1.9811(2), 1.9787(2), 1.9742(2)], A = [51(3), 52(2), 44(3)] MHz. The center has been identified by comparing EPR spectra with those of the charge-uncompensated ion Cr³⁺(M1) and the ion pair Cr³⁺(M1)–Li⁺(M2) observed in forsterite crystals codoped with chromium and lithium. It has been found that the concentration of the new center decreases to zero, whereas that of the Cr³⁺(M1) and Cr³⁺(M1)–Li⁺(M2) centers increases with an increase of the Li content from 0 up to ~0.03 wt% (at the same Cr content ~0.07 wt%) in the melt. The known low-temperature luminescence data pertinent to the centers under consideration are also discussed.     

Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130°C and 2.2 GPa

The following article presents constraints of the stability of Mg-rich (Mg/(Mg + Fe²⁺) > 0.5) calcic amphibole in both calc-alkaline and alkaline magmas, testing of previous thermobarometers, and formulation of new empirical equations that take into consideration a large amount of literature data (e.g. more than one thousand amphibole compositions among experimental and natural crystals). Particular care has been taken in choosing a large number of natural amphiboles and selecting quality experimental data from literature. The final database of experimental data, composed of 61 amphiboles synthesized in the ranges of 800–1,130°C and 130–2,200 MPa, indicates that amphibole crystallization occurs in a horn-like P–T stability field limited by two increasing curves (i.e. the thermal stability and an upper limit), which should start to bend back to higher pressures. Among calcic amphiboles, magnesiohornblendes and tschermakitic pargasites are only found in equilibrium with calc-alkaline melts and crystallize at relatively shallow conditions (P up to ~1 GPa). Kaersutite and pargasite are species almost exclusively found in alkaline igneous products, while magnesiohastingsite is equally distributed in calc-alkaline and alkaline rocks. The reliability of previous amphibole applications was checked using the selected experimental database. The results of this testing indicate that none of the previous thermobarometers can be successfully used to estimate the P, T and fO₂ in a wide range of amphibole crystallization conditions. Multivariate least-square analyses of experimental amphibole compositions and physico-chemical parameters allowed us to achieve a new thermobarometric model that gives reasonably low uncertainties (T ± 23.5°C, P ± 11.5%, H₂O_melt ± 0.78wt%) for calc-alkaline and alkaline magmas in a wide range of P–T conditions (up to 1,130°C and 2,200 MPa) and ∆NNO values (±0.37 log units) up to 500 MPa. The ^AK-^[4]Al relation in amphibole can be readily used to distinguish crystals of calc-alkaline liquids from those of alkaline magmas. In addition, several chemometric equations allowing to estimate the anhydrous composition of the melts in equilibrium with amphiboles of calc-alkaline magmas were derived.     

Time-resolved luminescence of Fe3+ and Mn2+ ions in hydrous volcanic glasses

 Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different Fe, Mn, and H₂O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most probably due to the ⁴T₂(⁴D) →⁶A₁(⁶S) and ⁴A₁(⁴G) →⁶A₁(⁶S) ligand field transitions of Fe³⁺. The green luminescence (500–560 nm) arises from the Mn²⁺ transition ⁴T₁(⁴G) →⁶A₁(⁶S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn²⁺ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied. The red luminescence (700–760 nm) arising from the ⁴T₁(⁴G) →⁶A₁(⁶S) transition of Fe³⁺ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated. Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions of the Fe³⁺ polyhedra. Received: 30 July 2001 / Accepted: 15 November 2001     

Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes

This work focuses on a rigorous analysis of the physical–chemical, compositional and textural relationships of amphibole stability and the development of new thermobarometric formulations for amphibole-bearing calc-alkaline products of subduction-related systems. Literature experimental results (550–1,120°C, <1,200 MPa, −1 ≤ ΔNNO ≤ +5), H₂O–CO₂ solubility models, a multitude of amphibole-bearing calc-alkaline products (whole-rocks and glasses, representing 38 volcanoes worldwide), crustal and high-P (1–3 GPa) mantle amphibole compositions have been used. Calcic amphiboles of basalt-rhyolite volcanic products display tschermakitic pargasite (37%), magnesiohastingsite (32%) and magnesiohornblende (31%) compositions with aluminium number (i.e. Al# = ^[6]Al/Al_T) ≤ 0.21. A few volcanic amphiboles (~1%) show high Al# (>0.21) and are inferred to represent xenocrysts of crustal or mantle materials. Most experimental results on calc-alkaline suites have been found to be unsuitable for using in thermobarometric calibrations due to the high Al# (>0.21) of amphiboles and high Al₂O₃/SiO₂ ratios of the coexisting melts. The pre-eruptive crystallization of consistent amphiboles is confined to relatively narrow physical–chemical ranges, next to their dehydration curves. The widespread occurrence of amphiboles with dehydration (breakdown) rims made of anhydrous phases and/or glass, related to sub-volcanic processes such as magma mixing and/or slow ascent during extrusion, confirms that crystal destabilization occurs with relatively low T–P shifts. At the stability curves, the variance of the system decreases so that amphibole composition and physical–chemical conditions are strictly linked to each other. This allowed us to retrieve some empirical thermobarometric formulations which work independently with different compositional components (i.e. Si^*, Al_T, Mg^*, ^[6]Al^*) of a single phase (amphibole), and are therefore easily applicable to all types of calc-alkaline volcanic products (including hybrid andesites). The Si^*-sensitive thermometer and the fO₂–Mg^* equation account for accuracies of ±22°C (σ_est) and 0.4 log units (maximum error), respectively. The uncertainties of the Al_T-sensitive barometer increase with pressure and decrease with temperature. Near the P–T stability curve, the error is <11% whereas for crystal-rich (porphyritic index i.e. PI > 35%) and lower-T magmas, the uncertainty increases up to 24%, consistent with depth uncertainties of 0.4 km, at 90 MPa (~3.4 km), and 7.9 km, at 800 MPa (~30 km), respectively. For magnesiohornblendes, the ^[6]Al^*-sensitive hygrometer has an accuracy of 0.4 wt% (σ_est) whereas for magnesiohastingsite and tschermakitic pargasite species, H₂O_melt uncertainties can be as high as 15% relative. The thermobarometric results obtained with the application of these equations to calc-alkaline amphibole-bearing products were finally, and successfully, crosschecked on several subduction-related volcanoes, through complementary methodologies such as pre-eruptive seismicity (volcano-tectonic earthquake locations and frequency), seismic tomography, Fe–Ti oxides, amphibole–plagioclase, plagioclase–liquid equilibria thermobarometry and melt inclusion studies. A user-friendly spreadsheet (i.e. AMP-TB.xls) to calculate the physical–chemical conditions of amphibole crystallization is also provided.     

Optical spectroscopic study of synthetic NaScSi<Subscript>2</Subscript>O<Subscript>6</Subscript>–CaNiSi<Subscript>2</Subscript>O<Subscript>6</Subscript> pyroxenes at normal and high pressures

Six synthetic NaScSi₂O₆–CaNiSi₂O₆ pyroxenes were studied by optical absorption spectroscopy. Five of them of intermediate (Na_1−x , Ca _x )(Sc_1−x , Ni _x )Si₂O₆ compositions show spectra typical of Ni²⁺ in octahedral coordination, more precise Ni²⁺ at the M1 site of the pyroxene structure. The common feature of all spectra is three broad absorption bands with maxima around 8,000, 13,000 and 24,000 cm⁻¹ assigned to ³ A _2g → ³ T _2g, ³ A _2g → ³ T _1g and →³ T _1g (³ P) electronic spin-allowed transitions of ^VINi²⁺. A weak narrow peak at ∼14,400 cm⁻¹ is assigned to the spin-forbidden ³ A _2g → ¹ T _2g (¹ D) transition of Ni²⁺. Under pressure the spin-allowed bands shift to higher energies and change in intensity. The octahedral compression modulus, calculated from the shift of the ³ A _2g → ³ T _2g band in the (Na_0.7Ca_0.3)(Sc_0.7Ni_0.3)Si₂O₆ pyroxene is evaluated as 85±20 GPa. The Racah parameter B of Ni²⁺(M1) is found gradually changing from ∼919 cm⁻¹ at ambient pressure to ∼890 cm⁻¹ at 6.18 GPa. The Ni end-member pyroxene [(Ca_0.93 Ni_0.07)NiSi₂O₆] has a spectrum different from all others. In addition to the above mentioned bands of Ni²⁺(M1) it displays several new relatively intense and broad extra bands, which were attributed to electronic transitions of Ni²⁺ at the M2 site. In difference to CaO₈ polyhedron geometry of an eightfold coordination, Ni²⁺(M2)O₈ polyhedra are assumed to be relatively large distorted octahedra. Due to different distortions and different compressibilities of the M1 and M2 sites the Ni²⁺(M1)- and Ni²⁺(M2)-bands display rather different pressure-induced behaviors, becoming more resolved in the high-pressure spectra than in that measured at atmospheric pressure. The octahedral compression modulus of Ni²⁺(M1) in this end-member pyroxene is evaluated as 150 ± 25 GPa, which is noticeably larger than in Ni_0.3 pyroxene. This is due to a smaller size and, thus, a stiffer character of Ni²⁺(M1)O₆ octahedron in the (Ca_0.93Ni_0.07)NiSi₂O₆ pyroxene compared to (Na_0.7Ca_0.3)(Sc_0.7Ni_0.3)Si₂O₆.

Thermal expansion and dehydroxylation of phengite micas

Phengite samples (2M ₁ and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M ₁ phengite (<α _V> ≈ 36.6 × 10⁻⁶ K⁻¹) was systematically greater than <α _V> of the 3T polytype (≈33.3 × 10⁻⁶ K⁻¹). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M ₁, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M ₁ is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H₂O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Experimentally derived thermochemical data for pargasite and reinvestigation of its stability with quartz in the system Na2O–CaO–>MgO–Al2O3–SiO2–H2O

 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na₂O– CaO–MgO–Al₂O₃–SiO₂–H₂O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An₅₀)+1.5 enstatite+H₂O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An₅₀)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An₅₀). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled Na^A-Al^T1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG ⁰ _f ,Pg=−12022.11±5.2 kJ mole^-1, and S ⁰ _Pg=591.7 ±7.9 JK^-1 mole^-1. Received: 31 July 1995/Accepted: 3 June 1996     

Thermoelasticity and high-<Emphasis Type="Italic">T</Emphasis> behaviour of anthophyllite

The thermoelastic behaviour of anthophyllite has been determined for a natural crystal with crystal-chemical formula ^ANa_0.01 ^B(Mg_1.30Mn_0.57Ca_0.09Na_0.04) ^C(Mg_4.95Fe_0.02Al_0.03) ^T(Si_8.00)O₂₂ ^W(OH)₂ using single-crystal X-ray diffraction to 973 K. The best model for fitting the thermal expansion data is that of Berman (J Petrol 29:445–522, 1988) in which the coefficient of volume thermal expansion varies linearly with T as α _V,T  = a ₁ + 2a ₂ (T − T ₀): α₂₉₈ = a ₁ = 3.40(6) × 10⁻⁵ K⁻¹, a ₂ = 5.1(1.0) × 10⁻⁹ K⁻². The corresponding axial thermal expansion coefficients for this linear model are: α _a ,₂₉₈ = 1.21(2) × 10⁻⁵ K⁻¹, a _2,a  = 5.2(4) × 10⁻⁹ K⁻²; α _b ,₂₉₈ = 9.2(1) × 10⁻⁶ K⁻¹, a _2,b  = 7(2) × 10⁻¹⁰ K⁻². α _c ,₂₉₈ = 1.26(3) × 10⁻⁵ K⁻¹, a _2,c  = 1.3(6) × 10⁻⁹ K⁻². The thermoelastic behaviour of anthophyllite differs from that of most monoclinic (C2/m) amphiboles: (a) the ε ₁ − ε ₂ plane of the unit-strain ellipsoid, which is normal to b in anthophyllite but usually at a high angle to c in monoclinic amphiboles; (b) the strain components are ε ₁ ≫ ε ₂ > ε ₃ in anthophyllite, but ε ₁ ~ ε ₂ ≫ ε ₃ in monoclinic amphiboles. The strain behaviour of anthophyllite is similar to that of synthetic C2/m ^ANa ^B(LiMg) ^CMg₅ ^TSi₈ O₂₂ ^W(OH)₂, suggesting that high contents of small cations at the B-site may be primarily responsible for the much higher thermal expansion ⊥(100). Refined values for site-scattering at M4 decrease from 31.64 epfu at 298 K to 30.81 epfu at 973 K, which couples with similar increases of those of M1 and M2 sites. These changes in site scattering are interpreted in terms of Mn ↔ Mg exchange involving M1,2 ↔ M4, which was first detected at 673 K.     

A low temperature X-ray single-crystal diffraction and polarised infra-red study of epidote

The effects of low-temperature on the crystal structure of a natural epidote [Ca_1.925Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6)? and β = 115.396(8)° at room conditions, Sp. Gr. P2₁ /m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe³⁺ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000–2,500 cm⁻¹ are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)–O(4). Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and Fe³⁺ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to protonation of O(10)–O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected for “rigid body motions”. A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra, especially when corrected for “non-correlated motion”. The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction of the magnitude of the thermal displacement ellipsoids, with a variation of U _eq (defined as one-third of the trace of the orthogonalised U _ij tensor) by ~40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U _eq of the octahedral and oxygen sites decrease similarly by ~35%, whereas those of the tetrahedral cations by ~22%.     

High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates: systematics in enthalpies of formation of A2+B4+O3 perovskites

Phase transitions in MgGeO₃ and ZnGeO₃ were examined up to 26 GPa and 2,073 K to determine ilmenite–perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite–perovskite boundaries have negative slopes and are expressed as P(GPa)=38.4–0.0082T(K) and P(GPa)=27.4−0.0032T(K), respectively, for MgGeO₃ and ZnGeO₃. Enthalpies of SrGeO₃ polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO₃ pseudowollasonite–walstromite and walstromite–perovskite transitions at 298 K were determined to be 6.0±8.6 and 48.9±5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO₃, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (ΔH _f°) of SrGeO₃ perovskite from the constituent oxides at 298 K was determined to be −73.6±5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite–perovskite transition boundaries in MgGeO₃ and ZnGeO₃ and the boundary of formation of SrSiO₃ perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The ΔH _f° of MgGeO₃, ZnGeO₃ and SrSiO₃ perovskites from constituent oxides were 10.2±4.5, 33.8±7.2 and −3.0±2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A²⁺B⁴⁺O₃ perovskites stable at both atmospheric and high pressures to explore the relationship between ΔH _f° and ionic radii of eightfold coordinated A²⁺ (R _A) and sixfold coordinated B⁴⁺ (R _B) cations. The results show that enthalpy of formation of A²⁺B⁴⁺O₃ perovskite increases with decreasing R _A and R _B. The relationship between the enthalpy of formation and tolerance factor ( R _o: O²⁻ radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A²⁺ and B⁴⁺ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A²⁺ and B⁴⁺ radii indicates a systematic change with equienthalpy curves. These relationships of ΔH _f° with R _A and R _B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.     

Single-crystal EPR and DFT study of a <Superscript>VI</Superscript>Al–O<Superscript>−</Superscript>–<Superscript>VI</Superscript>Al center in jeremejevite: electronic structure and <Superscript>27</Superscript>Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al₆B₅O₁₅(F, OH)₃, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an ²⁷Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g ₁ = 2.02899(1), g ₂ = 2.02011(2), g ₃ = 2.00595(1); A ₁/g _e β _e  = −0.881(1) mT, A ₂/g _e β _e  = −0.951(1) mT, and A ₃/g _e β _e  = −0.972(2) mT, with the orientations of the g ₃- and A ₃-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g ₁- and A ₁-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al³⁺ ions, after the removal of the proton (i.e., a ^VIAl–O⁻–^VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental ²⁷Al hyperfine coupling constants and directions, supporting the proposed structural model. The ^VIAl–O⁻–^VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the ^VIAl–O⁻–^VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001     

High-pressure and high-temperature in situ X-ray diffraction study of iron and corundum to 68 GPa using an internally heated diamond anvil cell

Using powder X-ray diffraction of heated solids to pressures reaching 68 GPa, the pressure-volume-temperature (PVT) data on corundum Al₂O₃ and ɛ-Fe were determined with the following results: *Corundum,*Iron, *Al₂O₃*ɛ-Fe Isothermal bulk*258 (2)*164 (3)  modulus K'_{300, 1} (GPa) Pressure derivative K_{300, 1}*4.88 (4)*5.36 (16) Temperature derivative*–0.020 (2)*–0.043 (3)  (∂K _T,1 /∂T) _P (GPa/K) Molar volume V_300,1*25.59 (2)*6.76 (2)  (cm³/mol) Isobaric thermal expansion at 1 atm (0.101 MPa) is given by (K^–1): α _T =2.6 (2) 10^–5+1.81 (9) 10^–9 T–0.67 (6)/T ² for corundum, and α _T =5.7 (4) 10^–5+4.2 (4) 10^–9 T–0.17 (7)/T ² for iron ɛ-Fe. Received: 1 March 1997 / Revised, accepted: 21 August 1997     

Behavior of epidote at high pressure and high temperature: a powder diffraction study up to 10 GPa and 1,200 K

The thermo-elastic behavior of a natural epidote [Ca_1.925 Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH)] has been investigated up to 1,200 K (at 0.0001 GPa) and 10 GPa (at 298 K) by means of in situ synchrotron powder diffraction. No phase transition has been observed within the temperature and pressure range investigated. P–V data fitted with a third-order Birch–Murnaghan equation of state (BM-EoS) give V ₀ = 458.8(1)ų, K _T0 = 111(3) GPa, and K′ = 7.6(7). The confidence ellipse from the variance–covariance matrix of K _T0 and K′ from the least-square procedure is strongly elongated with negative slope. The evolution of the “Eulerian finite strain” vs “normalized stress” yields Fe(0) = 114(1) GPa as intercept values, and the slope of the regression line gives K′ = 7.0(4). The evolution of the lattice parameters with pressure is slightly anisotropic. The elastic parameters calculated with a linearized BM-EoS are: a ₀ = 8.8877(7) Å, K _T0(a) = 117(2) GPa, and K′(a) = 3.7(4) for the a-axis; b ₀ = 5.6271(7) Å, K _T0(b) = 126(3) GPa, and K′(b) = 12(1) for the b-axis; and c ₀ = 10.1527(7) Å, K _T0(c) = 90(1) GPa, and K’(c) = 8.1(4) for the c-axis [K _T0(a):K _T0(b):K _T0(c) = 1.30:1.40:1]. The β angle decreases with pressure, β_P(°) = β_P0 −0.0286(9)P +0.00134(9)P ² (P in GPa). The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α₀ + α₁ T ^−1/2. The refined parameters for epidote are: α₀ = 5.1(2) × 10⁻⁵ K⁻¹ and α₁ = −5.1(6) × 10⁻⁴ K^1/2 for the unit-cell volume, α₀(a) = 1.21(7) × 10⁻⁵ K⁻¹ and α₁(a) = −1.2(2) × 10⁻⁴ K^1/2 for the a-axis, α₀(b) = 1.88(7) × 10⁻⁵ K⁻¹ and α₁(b) = −1.7(2) × 10⁻⁴ K^1/2 for the b-axis, and α₀(c) = 2.14(9) × 10⁻⁵ K⁻¹ and α₁(c) = −2.0(2) × 10⁻⁴ K^1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by α₀(a): α₀(b): α₀(c) = 1 : 1.55 : 1.77. The β angle increases continuously with T, with β_T(°) = β_T0 + 2.5(1) × 10⁻⁴ T + 1.3(7) × 10⁻⁸ T ². A comparison between the thermo-elastic parameters of epidote and clinozoisite is carried out.     

Excess heat capacity and entropy of mixing along the chlorapatite–fluorapatite binary join

The heat capacity at constant pressure, C _p, of chlorapatite [Ca₅(PO₄)₃Cl – ClAp], and fluorapatite [Ca₅(PO₄)₃F – FAp], as well as of 12 compositions along the chlorapatite–fluorapatite join have been measured using relaxation calorimetry [heat capacity option of the physical properties measurement system (PPMS)] and differential scanning calorimetry (DSC) in the temperature range 5–764 K. The chlor-fluorapatites were synthesized at 1,375–1,220°C from Ca₃(PO₄)₂ using the CaF₂–CaCl₂ flux method. Most of the chlor-fluorapatite compositions could be measured directly as single crystals using the PPMS such that they were attached to the sample platform of the calorimeter by a crystal face. However, the crystals were too small for the crystal face to be polished. In such cases, where the sample coupling was not optimal, an empirical procedure was developed to smoothly connect the PPMS to the DSC heat capacities around ambient T. The heat capacity of the end-members above 298 K can be represented by the polynomials: C _p^ClAp = 613.21 − 2,313.90T ^−0.5 − 1.87964 × 10⁷ T ⁻² + 2.79925 × 10⁹ T ⁻³ and C _p^FAp = 681.24 − 4,621.73 × T ^−0.5 − 6.38134 × 10⁶ T ⁻² + 7.38088 × 10⁸ T ⁻³ (units, J mol⁻¹ K⁻¹). Their standard third-law entropy, derived from the low-temperature heat capacity measurements, is S° = 400.6 ± 1.6 J mol⁻¹ K⁻¹ for chlorapatite and S° = 383.2 ± 1.5 J mol⁻¹ K⁻¹ for fluorapatite. Positive excess heat capacities of mixing, ΔC _p^ex, occur in the chlorapatite–fluorapatite solid solution around 80 K (and to a lesser degree at 200 K) and are asymmetrically distributed over the join reaching a maximum of 1.3 ± 0.3 J mol⁻¹ K⁻¹ for F-rich compositions. They are significant at these conditions exceeding the 2σ-uncertainty of the data. The excess entropy of mixing, ΔS ^ex, at 298 K reaches positive values of 3–4 J mol⁻¹ K⁻¹ in the F-rich portion of the binary, is, however, not significantly different from zero across the join within its 2σ-uncertainty.     

Electronic absorption and luminescence spectroscopic studies of kyanite single crystals: differentiation between excitation of FeTi charge transfer and Cr3+ dd transitions

A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R₁, R₂-sharp line luminescence and spectra of excitation of λ₃- and λ₄-components of R₁-line of Cr³⁺-emission had the following results: (1) The Fe²⁺–Ti⁴⁺ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm⁻¹ if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm⁻¹ in crystals with  ≥ . The energy difference is explained by an expansion of the O_f–O_k, and O_b–O_m edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr³⁺ substitutes for Al compared to the chromium-free case. (2) The Cr³⁺ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr^{3+ 4}A_2g→⁴T_2g(F)(I), →⁴T_1g(F)(II), and →⁴T_1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm⁻¹. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm⁻¹, Dq(b)=1600 cm⁻¹, B(a)=790 cm⁻¹, B(b)=620 cm⁻¹ (errors ca. ±10 cm⁻¹), again in agreement with values extracted from the λ³, λ⁴ excitation spectra. The CF-values of set a are close to those typical of Cr³⁺ substituting for Al in octahedra of other silicate minerals without constitutional OH⁻ as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr³⁺ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr³⁺ in the kyanite structural matrix. The CF-data of Cr³⁺ type b, expecially B, resemble those of Cr³⁺ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997