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The high-pressure behavior of dravite tourmaline [Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)] has been studied using luminescence spectroscopy and synchrotron-based single-crystal diffraction up to ∼65 and ∼24 GPa, respectively. Two emission bands associated with Cr3+/V2+ substitution are constant in energy up to ∼9.0 GPa, and they shift to longer wavelength at higher pressures, suggesting that a change in compressional mechanism could occur at this pressure. Single-crystal diffraction data show subtle changes in ring ditrigonality occur near 9.0 GPa, which could cause the observed change in luminescence. Near 15 GPa, a splitting of one of the emission bands is observed, suggesting that a phase transition occurs at this pressure and that two unique octahedral sites are present in the high-pressure phase. Hysteresis is not observed on decompression, which indicates that this is a second-order transition, and the high-pressure structure appears to be metastable up to ∼65 GPa. Single-crystal diffraction measurements show that a phase transition from rhombohedral R3m to rhombohedral R3 occurs at pressures near 15.4 GPa. The high-pressure phase is characterized by a distorted Si6O18 ring (e.g., the Si-Si-Si angles deviate from 120°), and the Si, Al, O6, O7, and O8 sites of the low-pressure phase split, implying that the high-pressure phase of tourmaline is a higher entropy phase. The large X-site exerts the primary control on compressibility, and the substitution of larger cations into this site will likely lower the pressure at which this transition occurs. Dravite tourmaline shows anisotropic compression with the c-axis being more compressible than the a-axis. The pressure and volume data up to ∼15.4 GPa were fit with second- and third-order Birch-Murnaghan equations of state. We obtain a bulk modulus, K0 = 109.6(3.2) GPa, and a pressure derivative, K0' = 4.6(8) GPa, and with the pressure derivative set to 4, a bulk modulus of 112.0(1.0) GPa is derived. Moreover, our high-pressure results show that massive overbonding of the X and Y sites can be accommodated by the tourmaline structure. This unexpected result may explain the extraordinary structural tolerance with respect to chemical substitution on the X, Y, and Z sites.

High-pressure single-crystal X-ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg3Al6Si6O18(BO3)3(OH)3OH, K-dravite KMg3Al6Si6O18(BO3)3(OH)3OH, magnesio-foitite ∎(Mg2Al)Al6Si6O18(BO3)3(OH)3OH, oxy-uvite CaMg3Al6Si6O18(BO3)3(OH)3O, and olenite NaAl3Al6Si6O18(BO3)3O3OH, where ∎ represents an X-site vacancy] were collected to 60 GPa at 300 K using a diamond-anvil cell and synchrotron radiation. No phase transitions were observed for any of the investigated compositions. The refined unit-cell parameters were used to constrain third-order Birch-Murnaghan pressure-volume equation of states with the following isothermal bulk moduli (K0 in GPa) and corresponding pressure derivatives (K'0 = δK0/δP)T: dravite K0 = 97(6), K'0 = 5.0(5); K-dravite K0 = 109(4), K'0 = 4.3(2); oxy-uvite K0 = 110(2), K'0 = 4.1(1); magnesio-foitite K0 = 116(2), K'0 = 3.5(1); olenite K0 = 116(6), K'0 = 4.7(4). Each tour-maline exhibits highly anisotropic behavior under compression, with the c axis 2.8-3.6 times more compressible than the a axis at ambient conditions. This anisotropy decreases strongly with increasing pressure and the c axis is only 14% more compressible than the a axis near 60 GPa. The octahedral Y- and Z-sites' composition exerts a primary control on tourmaline's compressibility, whereby Al content is correlated with a decrease in the c-axis compressibility and a corresponding increase in K0 and K'0. Contrary to expectations, the identity of the X-site-occupying ion (Na, K, or Ca) does not have a demonstrable effect on tourmaline's compression curve. The presence of a fully vacant X site in magnesio-foitite results in a decrease of K'0 relative to the alkali and Ca tourmalines. The decrease in K'0 for magnesio-foitite is accounted for by an increase in compressibility along the a axis at high pressure, reflecting increased compression of tourmaline's ring structure in the presence of a vacant X site. This study highlights the utility of synthetic crystals in untangling the effect of composition on tourmaline's compression behavior.

We report the crystal structure of dolomite-IV, a high-pressure polymorph of Fe-dolomite stabilized at 115 GPa and 2500 K. It is orthorhombic, space group Pnma, a = 10.091(3), b = 8.090(7), c = 4.533(3) A, V = 370.1(4) A3 at 115.2 GPa and ambient temperature. The structure is based on the presence of threefold C3O9 carbonate rings, with carbon in tetrahedral coordination. The starting Fe-dolomite single crystal during compression up to 115 GPa transforms into dolomite-II (at 17 GPa) and dolomite-IIIb (at 36 GPa). The dolomite-IIIb, observed in this study, is rhombohedral, space group R3, a = 11.956(3), c = 13.626(5) A, V = 1686.9(5) A3 at 39.4 GPa. It is different from a previously determined dolomite-III structure, but topologically similar. The density increase from dolomite-IIIb to dolomite IV is ca. 3%. The structure of dolomite-IV has not been predicted, but it presents similarities with the structural models proposed for the high-pressure polymorphs of magnesite, MgCO3 A ring-carbonate structure match with spectroscopic analysis of high-pressure forms of magnesite-siderite reported in the literature, and, therefore, is a likely candidate structure for a carbonate at the bottom of the Earth's mantle, at least for magnesitic and dolomitic compositions.

The structure and isothermal equation of state of aragonite were determined to 40 GPa using synchrotron single-crystal X-ray techniques. In addition, powder diffraction techniques were used to determine thermal expansion between 298-673 K. At room temperature, aragonite has orthorhombic Pnma structure to 40 GPa, with an isothermal bulk modulus of 66.5(7) GPa and K' = 5.0(1). Between 25-30 GPa the aragonite unit cell begins to distort due to a stiffening of the c-axis compressibility, which is controlled by the orientation and distortion of the carbonate groups. The ambient pressure thermal expansion measurements yielded thermal expansion coefficients a0 = 4.9(2) × 10-5 and a1 = 3.7(5) × 10-8. The combined results allow the derivation of a thermal equation of state. The new data provide constraints on the behavior of carbonates and carbon cycling in the Earth's crust and mantle.

The single-crystal elastic moduli of minerals and related materials with cubic symmetry have been collected and evaluated. The compiled data set covers measurements made over an approximately 70 year period and consists of 206 compositions. More than 80% of the database is comprised of silicates, oxides, and halides, and approximately 90% of the entries correspond to one of six crystal structures (garnet, rocksalt, spinel, perovskite, sphalerite, and fluorite). Primary data recorded are the composition of each material, its crystal structure, density, and the three independent nonzero adiabatic elastic moduli (C11, C12, and C44). From these, a variety of additional elastic and acoustic properties are calculated and compiled, including polycrystalline aggregate elastic properties, sound velocities, and anisotropy factors. The database is used to evaluate trends in cubic mineral elasticity through consideration of normalized elastic moduli (Blackman diagrams) and the Cauchy pressure. The elastic anisotropy and auxetic behavior of these materials are also examined. Compilations of single-crystal elastic moduli provide a useful tool for investigation structure-property relationships of minerals.

Allanite-(Y), ideally CaY(Al2Fe2+)(Si2O7)(SiO4)O(OH), is a valid species with the type locality in the Åskagen pegmatite, Värmland, Sweden. The mineral occurs as an accessory phase in the blocky zone of the NYF granitic pegmatite near Åskagen, Värmland, Sweden. It forms rims together with iimoriite-(Y), gadolinite-(Y) and allanite-(Nd) around altered crystals of thalénite-(Y). Allanite-(Y) replaced primary thalénite-(Y) during an episode of early post-magmatic hydrothermal activity. Allanite-(Y) forms euhedral crystals with size up to 1 mm, black with a vitreous lustre, conchoidal fracture and greyish brown streak. It has a Mohs hardness of ca. 6, the calculated density of 3.945 g.cm-3 and is biaxial (-) with α = 1.760(3), β = 1.799(2) and γ = 1.784(3) in 589 nm light; pleochroism is weak pale yellowish brown in all directions. Allanite-(Y) has monoclinic symmetry, with the space group P21/m, a = 8.8520(8) Å, b = 5.6959(5) Å, c = 10.0543(9) Å, β = 115.510(2)°, V = 457.52(7) Å3 and Z = 2. Crystal-chemical analysis resulted in the empirical formula: A1(Ca0.900Mn0.090Na0.010)Σ1.000A2(Y0.323Ca0.260Nd0.118Sm0.0 87Gd0.098Dy0.044Ce0.034Pr0.014Tb0.012Er0.005La0.003Ho0.002Yb0.00 1)Σ1.001M1(Al0.921Fe2+0.070Ti0.003)Σ0.994M2(Al1.000)M3(Fe2 +0.638Fe3+0.262Al0.072Mg0.028)Σ1.000T1(Si1.000)T2(Si1.000)T3( Si1.003)O12.000(OH)1.000. Allanite-(Y) belongs to the allanite group of the epidote supergroup. The closest end-member compositions of valid allanite group species are allanite-(Ce), allanite-(La) and allanite-(Nd) related via the simple exchange mechanism Y ⇌ Ln. The allanite-(Y) origin during metasomatic replacement of the thalénite-(Y) was mainly affected by local system composition and structural constraints rather than Ln+Y fluoride complexation in hydrothermal solution.

The high-temperature behaviour of a feldspar-group mineral, filatovite (with the simplified formula: K(Al,Zn)2(As,Si)2O8), in which the Al:As:Si ratio is close to 2:1:1), was studied by in situ high-temperature single-crystal X-ray diffraction and in situ high-temperature (hot stage) Raman spectroscopy up to 600°C. In the temperature range studied (25-600°C) filatovite does not undergo any phase transition, whereas at 800°C it decomposes to X-ray amorphous phase(s). The evolution of 12 main Raman bands was traced during heating, which indicates a gradual change in the crystal structure. The thermal expansion coefficients of filatovite demonstrate a sharply anisotropic character of thermal expansion: the maximal expansion is close to the a axis (α11 = 17.7(1) × 10-6 °C-1), whereas along the b and c axes the thermal expansion coefficients are close to zero. Such behaviour is typical for minerals with a similar crystal structure topology; it indicates the dominant role of structure geometry in the thermal behaviour of the mineral.

The high-pressure behaviour of inderborite [ideally CaMg[B3O3(OH)5]2(H2O)4·2H2O, space group C2/c with a ≈ 12.14, b ≈ 7.43, c ≈ 19.23 Å and β ≈ 90.3° at room conditions] has been studied by two in situ single-crystal synchrotron X-ray diffraction experiments up to ∼10 GPa, using He as pressure-transmitting fluid. Between 8.11(5) and 8.80(5) GPa, inderborite undergoes a first-order phase transition to its high-pressure polymorph, inderborite-II (with a ≈ 11.37, b ≈ 6.96, c ≈ 17.67 Å, β ≈ 96.8° and ΔV ≈ 7.0%, space group unknown). The isothermal bulk modulus (KV0 = β-1P0,T0, where βP0,T0 is the volume compressibility coefficient) of inderborite was found to be KV0 = 41(1) GPa. The destructive nature of the phase transition prevented any structure resolution of inderborite-II or even the continuation of the experiments at pressures higher than 10.10(5) GPa. In the pressure range 0-8.11(5) GPa, the compressional anisotropy of inderborite, indicated by the ratio between the principal components of the Eulerian finite unit-strain ellipsoid, is ε1:ε2:ε3 = 1.4:1.05:1. The deformation mechanisms at the atomic scale in inderborite are here described. Our findings support the hypothesis of a quasi-linear correlation between the total H2O content and P-stability range in hydrated borates, as the pressure at which inderborite undergoes the phase transition falls in line with most of the hydrate borates studied at high-pressure so far.

The thermal behaviour of fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)·H2O, from the charoitites of the Severny district at the Malyy Murun massif, Murun complex, NW Aldan Shield, Siberia, Russia, has been investigated in order to understand the temperature-induced changes in the crystal structure of this rare silicate. The study has been carried out combining in situ high-temperature single-crystal X-ray diffraction (T range 25-550°C), ex situ high-temperature Fourier-transform infrared spectroscopy (25-700°C) and ab initio calculations. An increasing trend of lattice parameters and cell volume was observed in the 150-550°C temperature range, when the mineral underwent a progressive dehydration process. At 550°C ∼40% water loss was detected. If compared with the fluorcarletonite structure at room temperature, the partially dehydrated fluorcarletonite shows: the same space group (P4/mbm); increased distances between the oxygens of the H2O molecules (O11w and O12w) and their Na-centred octahedral cations (Na1 and Na2, respectively); distortion of the four- and six-member tetrahedral rings of the double silicate layer. The dehydration process mainly involves the oxygen at the O11w site which has a different local environment with respect to the oxygen at the O12w site. At T > 600°C, the complete dehydration is accompanied by deprotonation of the OH groups substituting for the F atoms and by the collapse of the structure when the CO2 is released. The adopted approach allowed definition of the temperature thresholds at which modifications occur in the fluorcarletonite crystal structure when subjected to controlled heating conditions. Our findings contribute to assessment of stability, reactivity and, more generally, the thermal behaviour of sheet silicates with fluorcarletonite-like topology.

The elasticity of single-crystal hydrous pyrope with ∼900 ppmw H2O has been derived from sound velocity and density measurements using in situ Brillouin light spectroscopy (BLS) and synchrotron X-ray diffraction (XRD) in the diamond-anvil cell (DAC) up to 18.6 GPa at room temperature and up to 700 K at ambient pressure. These experimental results are used to evaluate the effect of hydration on the single-crystal elasticity of pyrope at high pressure and high temperature (P-T) conditions to better understand its velocity profiles and anisotropies in the upper mantle. Analysis of the results shows that all of the elastic moduli increase almost linearly with increasing pressure at room temperature, and decrease linearly with increasing temperature at ambient pressure. At ambient conditions, the aggregate adiabatic bulk and shear moduli (KS0, G0) are 168.6(4) and 92.0(3) GPa, respectively. Compared to anhydrous pyrope, the presence of ∼900 ppmw H2O in pyrope does not significantly affect its KS0 and G0 within their uncertainties. Using the third-order Eulerian finite-strain equation to model the elasticity data, the pressure derivatives of the bulk [(δKS/δP)T] and shear moduli [(δG/δP)T] at 300 K are derived as 4.6(1) and 1.3(1), respectively. Compared to previous BLS results of anhydrous pyrope, an addition of ∼900 ppmw H2O in pyrope slightly increases the (δKS/δP)T, but has a negligible effect on the (δG/δP)T within their uncertainties. The temperature derivatives of the bulk and shear moduli at ambient pressure are (δKS/δT)P = -0.015(1) GPa/K and (δG/δT)P = -0.008(1) GPa/K, which are similar to those of anhydrous pyrope in previous BLS studies within their uncertainties. Meanwhile, our results also indicate that hydrous pyrope remains almost elastically isotropic at relevant high P-T conditions, and may have no significant contribution to seismic anisotropy in the upper mantle. In addition, we evaluated the seismic velocities (δP and δS) and the δP/δS ratio of hydrous pyrope along the upper mantle geotherm and a cold subducted slabs geotherm. It displays that hydrogen also has no significant effect on the seismic velocities and the δP/δS ratio of pyrope at the upper mantle conditions.