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Three natural olivines with Fo80Fa20, Fo71Fa29, and Fo62Fa38 compositions were investigated in situ at high pressure by single-crystal X-ray diffraction using a diamond-anvil cell up to ~8 GPa at room temperature. The bulk modulus, KT0, and its first pressure derivative, K', do not show any significant variation among the compositions investigated and, using the data on a further sample with Fo92Fa8 composition recently investigated in the same laboratory and using the same experimental technique, we obtain, for the first time, a single equation of state for the entire Fo92Fa8-Fo62Fa38 compositional range. The equation has the following coefficients: KT0 = 124.7(9) GPa and K' = 5.3(3) and can be used for thermodynamic calculations involving the most common mantle olivine compositions. [ABSTRACT FROM AUTHOR]

Four synthetic crystals belonging to the jadeite (Jd, NaAlSi2O6)-hedenbergite (Hd, CaFeSi2O6) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to Pmax = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. V0, KT0, and K′ were simultaneously refined by fitting a third-order Birch-Murnaghan equation of state to pressure-volume data for all samples. The following relationship between bulk modulus and molar fraction of jadeite is observed: KT0 = 108.7(2) (GPa) + 0.191(9) × [% molar Jd] + 0.0006(1) × [% molar Jd]2 The bulk modulus of hedenbergite is 19% lower than jadeite with a strong axial anisotropy that increases with increasing the Hd content. In particular, the compressibility along the b axis (the most compressible in pyroxenes) increases by about 35% going from Jd to Hd while along the c axis the increase in compressibility is about 24%. The a axis does not show any variation in the deformation rate along the join. The analysis of the crystal structure behavior with pressure for all samples clearly indicates that the main cause of the strong anisotropy on the b-c plane is related to the narrowing of the M1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd. [ABSTRACT FROM AUTHOR]

The crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite [a = 12.7334(4), b = 13.6298(5), c = 7.3467(3) Å, V = 1275.04 Å3, space group Pnma)] and eudidymite [a = 12.6188(10), b = 7.3781(5), c = 13.9940(9) Å, β = 103.762(5)°, V = 1265.47 Å3, space group C2/c] from Malosa, Malawi, has been reinvestigated by means of energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, inductively coupled plasma-optical emission spectroscopy and single-crystal neutron diffraction. Two anisotropic structure refinements have been performed with final agreement index R1 = 0.0317 for 137 refined parameters and 2261 unique reflections with Fo > 4σ(Fo) for epididymite, and R1 = 0.0478 for 136 refined parameters and 1732 unique reflections with Fo > 4σ(Fo) for eudidymite. The analysis of the difference-Fourier maps of the nuclear density of the two dimorphs confirms the presence of extra-framework water molecules in both, and not hydroxyl groups as wrongly reported in previous studies and in several crystal-structure databases. The correct chemical formula of edipidymite and eudidymite is Na2Be2Si6O15·H2O (Z = 4). The configuration of the water molecules and the hydrogen bonds are fully described for both the dimorphs. The chemical analyses show that a small, but significant, amount of Al and Fe (most likely substituting for Si in the tetrahedral sites) and K (substituting for Na as an extra-framework cation) occurs in both dimorphs. [ABSTRACT FROM AUTHOR]

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) ų, KT0 = 193(1) GPa, K' = 5.6(3) for the ordered sample and V0 = 528.39(2) ų, KT0 = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the MgAl2O4 spinel. [ABSTRACT FROM AUTHOR]

Synthetic NaGaSi2O6-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θD, and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi2O6 at about 200 K did not occur for NaGaSi2O6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t2g orbitals, which do not exist in NaGaSi2O6. [ABSTRACT FROM AUTHOR]

Synthetic NaGaSi2O6-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θD, and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi2O6 at about 200 K did not occur for NaGaSi2O6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t2g orbitals, which do not exist in NaGaSi2O6. [ABSTRACT FROM AUTHOR]

The high-pressure behavior of protoamphibole (space group Pnmn) was studied by in situ single-crystal X-ray diffraction on a sample of protomangano-ferro-anthophyllite with formula (Mn1.39Fe0.59)(Fe3.98Mg1.02)Si8O22(OH)2, from Yokone-Yama, Awano Town, Tochigi Prefecture, Japan. Unit-cell parameters were collected at various pressures up to 9 GPa, and structural refinements were obtained from data collected at several pressures up to 7 GPa. Fitting the P-V data to a third-order Birch-Murnaghan equation of state (EoS) gives the following parameters: KT0 = 64(1) GPa, K' = 7.0(4), and V0 = 926.4(4) ų. Axial moduli are: K0a = 30.7(8) GPa, K'a = 10.8(5), and a0 = 9.430(2) Å; K0b = 109(4) GPa, K'b = 2.7(8), and b0 = 18.364(4) Å; K0c = 94(5), GPa, K'c = 4(1), and c0 = 5.354(2) Å. The corresponding axial compressibilities (10-3 GPa-1) are βa = 10.9(3), βb = 3.1(1), and βc = 3.5(2), and indicate that the HP behavior of protomangano-ferro-anthophyllite is highly anisotropic, the highest compressibility being along [100]. No discontinuous behavior or polymorphic transitions were observed in the pressure range studied. Structural refinements show that M1, M2, and M3 polyhedra have similar compressibilities, owing to their similar composition. M4 (72% Mn, 28% Fe) is a highly distorted site and is slightly softer than the other octahedra. The major movements in the tetrahedral ribbon concern kinking of the double chain, bending along the [100] direction through the empty A site, and tetrahedral rotation, necessary to maintain coherence with the octahedral layer. The kinking angle of O5-O6-O5, which in air is 179.1(2)°, decreases to 174(2)° at 6.9 GPa. The T1-O7-T1 angle changes from 143.8(3)° to 134(5)° at 6.9 GPa, and the tetrahedral rotation α increases from 0.2(2)° to 4(2)°. [ABSTRACT FROM AUTHOR]

We present the results of a single-crystal X-ray diffraction structural study on pyroxmangite in a diamond-anvil cell up to 5.6 GPa. The sample comes from Yokone-Yama, Awano Town, Tochigi Prefecture, Japan. Crystals are triclinic, centrosymmetric, with composition [Mn0.576(2)Fe0.284(5) Ca0.044(3)Mg0.089(2)]Si1.003(4)O3. Structure refinements were performed with intensity data collected at 1.24 and 3.57 GPa on a CCD-equipped diffractometer. Lattice parameters were accurately measured with the point-detector mounted on the same instrument. The bulk modulus of pyroxmangite fitting data to a second-order Birch-Murnaghan equation of state is K0 = 109.6(7) GPa. Axial compressibility values were βa = 2.2(1), βb = 3.3(1), and βc = 2.6(1) 10-3 GPa-1 showing slightly anisotropic behavior, with the most compressible direction along the b axis, as commonly found in the related family of pyroxene. Silicon tetrahedra are almost incompressible in the pressure range investigated. M polyhedra are more compressible: the volume change is smaller in the more regular octahedra M1-M4 (-3.3%) and greater in the more irregular polyhedra M5-M7 (-5.2%). Owing to the different contraction of Si tetrahedra and cation polyhedra, the sevenfold tetrahedral chains in pyroxmangite must kink to avoid misfit between chains and octahedral bands. This results in shortening of 1.2% of the c axis and a decrease in both Obr -Obr-Obr and Si-Obr-Si angles. The behavior of pyroxmangite at high P is approximately inverse to that observed at high T. Compressibility data may be combined with those on thermal expansion to formulate the approximate equation of state: V = V0 (1 - 9.12 × 10-3 ΔP + 3.26 × 10-5 ΔT), where P is in GPa and T in degrees Celsius. [ABSTRACT FROM AUTHOR]

A single crystal of a natural orthopyroxene with composition M2[Fe0.8182+Mg0.156Ca0.010Mn0.016]M1[Fe0.0812+ Mg0.767Al0.084Fe0.0683+]TA[Si]TB[Si0.848Al0.152]O6 and space group Pbca (sample S95) was investigated at high pressure by X-ray diffraction using a diamond-anvil cell up to 9.56 GPa. No phase transitions were detected in the pressure range investigated. The unit-cell parameters, a, b, and c, decrease non-linearly with pressure and show an axial compression anisotropy with a ratio βa:βb:βc = 1.00:1.64:1.16. The unit-cell volume decreases non-linearly as well and with a negative variation, by about 6.3% up to 9.56 GPa. The equation of state calculated using high-accuracy volume-pressure data up to 5.5 GPa gave the following coefficients: V0 = 846.02(4) ų, KT0 = 115.4(6) GPa, K' = 7.7(3). Among the Mg-orthopyroxenes investigated at high pressure so far S95 shows the highest bulk modulus. Six complete intensity data were collected at 0, 0.16, 1.72, 3.95, 8.03, and 9.56 GPa. The results confirm previous conclusions regarding the compressional mechanism in orthopyroxenes. At lower pressures, compression is mostly connected to a decrease in volume of the two M-coordination octahedra, accompanied by an increased kink in the B-tetrahedral chain. At higher pressures, compression of the M sites decreases, the kink of the tetrahedral chains stops to change, and reduction in unit-cell volume is accompanied mainly by compression of tetrahedra. This change in compressional trends results in a relatively large K' parameter and a pronounced stiffening of the structure with pressure. The presence of Al in the TB tetrahedral site influences the kink of the B tetrahedral chain, which depends on the ratio of sizes of the M2 and TB coordination polyhedra. The increased stiffness of the M polyhedra, caused by the presence of Fe, is the main reason for the high bulk modulus of S95 and its resistance to shortening of the c axis. This explains the limited shortening of the c axis in S95 and the different compressional axial anisotropy with respect to other orthopyroxenes investigated under high pressure. [ABSTRACT FROM AUTHOR]

The crystal-structures of seven synthetic pyroxenes along the jadeite--hedenbergite (Jd57Hd43, Jd26Hd74, Jd0Hd100) and jadeite--aegirine (Jd100Ae0, Jd76Ae24, Jd35Ae65, Jd0Ae100) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, R4σ between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and hedenbergite content, moreover the Ca for Na substitution along the jadeite.hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend UeqM2 > UeqO2 > UeqO3 > UeqO1 > UeqM1 ≈ UeqT for all samples belonging to the jadeite.aegirine join and for pure hedenbergite; in contrast,, for pyroxenes with intermediate compositions between hedenbergite and jadeite the trend is UeqO1 > UeqO2 > UeqM2 > UeqO3 > UeqM1 ≈ UeqT, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural configuration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deformation with respect of the double substitution at both the M1 and M2 sites in (Na,Ca)(M3+,M2+)Si2O6 pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site. [ABSTRACT FROM AUTHOR]

The crystal chemistry of a natural topaz [with OH/(OH + F) < 0.5] was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, single-crystal X-ray diffraction (at 298 K) and neutron diffraction (at 298 and 10 K), and polarized infrared spectroscopy to define unambiguously the real symmetry of topaz, the location of the proton and its thermal displacement parameters at room and low temperatures, the hydrogen-bonding and the vibration modes (stretching and bending) of the OH dipole. X-ray and neutron structural refinements allow us to infer that the crystal structure of natural topaz with OH/(OH + F) < 0.5 can be described with the Pbnm space group. Violating reflections, found in the previous investigations and in this study, are likely due to Renninger effect (double diffraction phenomenon). The nuclear density Fourier map shows that the proton is located at Wyckoff 8d position and the refined coordinates are: x = 0.495(2), y = 0.252(1), z = 0.1629(7). The O-H bond lies on the (010)-plane and forms an angle of about 28.9° with the c-axis. Neutron structural refinements at 298 and 10 K show that the displacement ellipsoid of the proton is highly anisotropic. The H-bonding arrangement appears to be complex, with at least four potential H···O/F interactions (distances < 2.38 Å). The topological configuration of the O-H group described by the neutron structural refinements is confirmed by the infrared investigation: the OH stretching mode (at 3640 cm-1) has no component of vibration parallel to the b axis (i.e., the O-H direction is perpendicular to [010]). The OH bending mode (at 1161 cm-1) shows components along the three crystallographic axes, which appear to be more prominent along the a and b-axes. The possible distribution into the crystal structure of topaz of the minor/trace elements found (Na, Ca, Fe Cr, V, Ti, B), and the implied topological effects, is discussed. [ABSTRACT FROM AUTHOR]

The crystal structure of alkali/water-poor beryl (H2O + Na2O + Cs2O < 1.2 wt%) was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction, and polarized infrared spectroscopy to determine the real topological configuration of the extra-framework content in the six-membered ring channels. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the sixfold axis at the 2a site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogen atoms are distributed in 6 x 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) Å and the H-O-H bond angle is 106.9(2.2)º. The H⋯H vector is oriented at ∼4° from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H⋯H vector is perpendicular to [001]. Na is probably located, with the H2O oxygen, at the 2a site. According to the chemical analysis, which shows that the amounts of other alkali and earth-alkali cations are negligible (Rb, K, Mg, Mn ≤ 110 ppm, Ca ≤ 225 ppm, Cs ≤ 430 ppm), no effect of other cations on the extra-framework population was observed in the structural refinement. The final agreement index (R1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with Fo 4σ(Fo). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H2O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H⋯H vector parallel (or quasi-parallel) to [001]. [ABSTRACT FROM AUTHOR]