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A single crystal of natrojarosite, NaFe3(SO4)2(OH)6, was investigated by single-crystal X-ray diffraction at high-pressure conditions (up to 8.8 GPa) using a diamond-anvil cell. The unit-cell parameters were determined at 11 different pressures and no indications of a phase transition were found up to the maximum pressure reached. The volume and axial moduli were fitted to a third-order Birch-Murnaghan equation-of-state which gave the following values: V0 = 769.6(2) Å3, KT0 = 50.6(9) GPa, K' = 9.9(4); a = 7.3172(6) Å, KT' = 104(2), K' = 7.6(9); c = 16.5965(20) Å, KT0 = 24.6(4) and K' = 7.1(2). The crystal structure of natrojarosite was refined at seven different pressures up to 8.779(11) GPa [a = 7.3170(4), c = 16.5955(5) Å and V = 769.46(9) Å 3 in R3m at 0.00010(1) GPa and a = 7.1594(8), c = 15.6003(17) Å and V = 692.49(8) Å 3 at 8.779(11) GPa]. The structural analysis shows that the 12-fold Na polyhedron accommodates most of the deformation by a large volume decrease (14%) and strong polyhedral distortion (63%). Our results indicate that natrojarosite has the most compressible structure of the supergroup studied sofar, and has a very strong axial anisotropy. [ABSTRACT FROM AUTHOR]

This paper describes single-crystal measurements on a crystal plate cut from a naturally-occurring almandine-rich single crystal (AlmPyrSpeGro) from Wrangell Alaska. The objective was to measure the mean-squared-displacement (msd) tensor precisely using Mössbauer spectroscopy. Parallel quantum-mechanical calculations based on X-ray determined atomic displacement parameters and Mössbauer parameters from polycrystalline measurements indicated that the msd tensor should display significant anisotropy, easily measurable within the precision of the Mössbauer experiment. For each single-crystal orientation the observed Mössbauer spectrum represents a macroscopic quantity that is the average over six symmetry-related (local) dodecahedral sites in which the high-spin Fe ions reside. The anisotropy in the measured msd tensor is, nevertheless, unequivocal. Furthermore, the magnitudes of the Mössbauer-determined msd principal values exceed those of the corresponding X-ray-determined quantities by a factor 3.7. The equivalent recoilless fractions are also anisotropic and consequently one observes the Gol'danskii-Karyagin Effect (GKE), as manifested by an asymmetric quadrupole doublet in polycrystalline absorbers. Moreover, the line widths of the two quadrupole lines are markedly different but angle invariant. This is interpreted as implying that, in addition to anisotropy in the msd tensor, differential spin-spin relaxation is present in the $$ m = \pm 3/2 \leftrightarrow \pm 1/2 $$ and $$ m = \pm 1/2 \leftrightarrow \pm 1/2,\, \mp 1/2 $$ nuclear transitions. While both effects contribute to the quadrupole asymmetries observed in Mössbauer spectra of polycrystalline almandine, the GKE is apparently predominant. [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]

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]

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]