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Author(s): F. Nestola (corresponding author) [1]; N. Korolev [2, 3]; M. Kopylova [2]; N. Rotiroti [4]; D. G. Pearson [5]; M. G. Pamato [6]; M. Alvaro [7]; L. Peruzzo [8]; [...]

The ultimate origin of water in the Earth's hydrosphere is in the deep Earth--the mantle. Theory (1) and experiments (2-4) have shown that although the water storage capacity of olivine-dominated [...]

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 ([Mn.sub.1.39][Fe.sub.0.59]) ([Fe.sub.3.98][Mg.sub.102])[Si.sub.8][O.sub.22][(OH).sub.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: [K.sub.T0] 64(1) GPa, K' = 7.0(4), and [V.sub.0] = 926.4(4) [[Angstrom].sup.3]. Axial moduli are: [K.sub.0a] = 30.7(8) GPa, [K'.sub.a] = 10.8(5), and [a.sub.0] 9.430(2) [Angstrom]; [K.sub.0b] = 109(4) GPa, [K'.sub.b] = 2.7(8), and [b.sub.0] = 18.364(4) [Angstrom]; [K.sub.0c], = 94(5), GPa, [K'.sub.c] = 4(1), and [C.sub.0] = 5.354(2) [Angstrom]. The corresponding axial compressibilities ([10.sup.-3] [GPa.sup.-1])are [[beta].sub.a] = 10.9(3), [[beta].sub.b] = 3.1(1), and [[beta].sub.c] = 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 05-06-05, which in air is I79.1(2)[degrees], decreases to 174(2)[degrees] at 6.9 GPa. The T1-O7-T1 angle changes from 143.8(3)[degrees] to 134(5)[degrees] at 6.9 GPa, and the tetrahedral rotation [alpha] increases from 0.2(2)[degrees] to 4(2)[degrees]. Keywords: Protoamphibole, protomangano-ferro-anthophyllite, compressibility, equation of state, high-pressure structure DOI: 10.2138/am.2010.3565

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 [[Mn.sub.0.576(2)][Fe.sub.0.284(5)][Ca.sub.0.044(3)] [Mg.sub.0.089(2)]][Si.sub.1.003(4)][O.sub.3]. 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 [K.sub.0]= 109.6(7) GPa. Axial compressibility values were [[beta].sub.a]= 2.2(1), [[beta].sub.b]= 3.3(1), and [[beta].sub.c] 2.6(1) [10.sup.-3] G[Pa.sup.-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 [O.sub.br] -[O.sub.br]-[O.sub.br] and Si-[O.sub.br]-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 = [V.sub.0] (1 - 9.12 x [10.sup.-3] [DELTA]P + 3.26 x [10.sup.-5] [DELTA]T), where P is in GPa and T in degrees Celsius. Keywords: Pyroxmangite, pyroxenoid, equation of state, compressibility

The crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite [a = 12.7334(4), b = 13.6298(5), c = 7.3467(3) [Angstrom], V = 1275.04 [[Angstrom].sup.3], space group Pnma)] and eudidymite [a = 12.6188(10), b 7.3781(5), c = 13.9940(9) [Angstrom], [beta] = 103.762(5)[degrees], V = 1265.47 [[Angstrom].sup.3,], 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 [R.sub.1] = 0.0317 for 137 refined parameters and 2261 unique reflections with [F.sub.o] > 4[sigma]([F.sub.o]) for epididymite, and [R.sub.1] = 0.0478 for 136 refined parameters and 1732 unique reflections with [F.sub.o] > 4[sigma]([F.sub.o]) 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 [Na.sub.2][Be.sub.2][Si.sub.6][O.sub.15] x [H.sub.2]O (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. Keywords: Epididymite, eudidymite, crystal chemistry, single-crystal neutron diffraction, hydrogen bonding

Four synthetic crystals belonging to the jadeite (Jd, NaAl[Si.sub.2][O.sub.6])-hedenbergite (Hd, CaFe[Si.sub.2][O.sub.6]) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to [P.sub.max] = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. [V.sub.0], [K.sub.T0], 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: [K.sub.T0] = 108.7(2) (GPa) + 0.191(9) x [% molar Jd] + 0.0006(1) x [[% molar Jd].sup.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 M 1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd. Keywords: High pressure, solid solution, crystal structure, X-ray

A single crystal of a natural orthopyroxene with composition [sup.M2][[[Fe.sup.2+.sub.818][Mg.sub.0.156][Ca.sub.0.010] [Mn.sub.0.016]].sup.Ml][[Fe.sup.2+.sub.0.081][Mg.sub.0.0767] [Al.sub.0.84][Fe.sup.3+.sub.0.068]sup.TA][Si][sup.TA] [Si.sub.0.848][Al.sub.0.152][O.sub.6] and space group Pbca(sampleS95) was investigated a thigh 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 [[beta].sub.a]:[[beta].sub.b]: [[beta].sub.c] = 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: [V.sub.o] = 846.02(4) [[angstrom].sup.3], [K.sub.T0] = 115.4(6) GPa, K' = 7.7(3). Among the Mgorthopyroxenes 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.[O.sub.3], 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 oral 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 e 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. Keywords: Orthopyroxene, high pressure, crystal structure, X-ray diffraction

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural Mg[Al.sub.2][O.sub.4] 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, [V.sub.0], the bulk modulus, [K.sub.T0], and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: [V.sub.0] = 529.32(2) [[Angstrom].sup.3], [K.sub.T0] = 193(1) GPa, K' = 5.6(3) for the ordered sample and [V.sub.0] = 528.39(2) [[Angstrom].sup.3], [K.sub.T0] = 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 Mg[Al.sub.2][O.sub.4] spinel. Keywords: X-ray single-crystal diffraction, spinel, cation ordering, high pressure

The crystal-structures of seven synthetic pyroxenes along the jadeite hedenbergite ([Jd.sub.57][Hd.sub.43], [Jd.sub.26][Hd.sub.74], [Jd.sub.0][Hd.sub.100]) and jadeite-aegirine ([Jd.sub.100][Ae.sub.0][Jd.sub.76][Ae.sub.24], [Jd.sub.35][Ae.sub.65], [Jd.sub.0][Ae.sub.100]) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, [R.sub.4[sigma]] 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 [U.sub.eq]M2 > [U.sub.eq]O2 > [U.sub.eq]O3 > [U.sub.eq]O1 > [U.sub.eq]M1 [approximately equal to] [U.sub.eq]T for all samples belonging to the jadeite-aegirine join and for pure hedenbergite; in contrast, for pyroxenes with intermediate compositions between hedenbergite andjadeite the trend is [U.sub.eq]O1 > [U.sub.eq]O2 > [U.sub.eq]M2 > [U.sub.eq]O3 > [U.sub.eq]M1 [approximately equal to] [U.sub.eq]T, 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)([M.sup.3+],[M.sup.2+])[Si.sub.2][O.sub.6] 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. Keywords: Single crystal, X-ray diffraction, crystal-structure, clinopyroxenes

Synthetic NaGa[Si.sub.2][O.sub.6]-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, [[theta].sub.D], and the Gruneisen parameter, [gamma], 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-P1 phase transition observed in a previous work for NaTi[Si.sub.2][O.sub.6] at about 200 K did not occur for NaGa[Si.sub.2][O.sub.6] 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 [t.sub.2g] orbitals, which do not exist in NaGa[Si.sub.2][O.sub.6]. Keywords: Clinopyroxene, crystal structure, X-ray diffraction, low temperature