ALBERT

Description: This decade marks the centenary of the discovery of X-ray diffraction. The development of mineralogy as a scientific discipline in which the properties of minerals are understood in terms of their atomic-scale structures has paralleled the development of diffraction crystallography. As diffraction crystallography revealed more precise details of mineral structures, more subtle questions about mineral properties could be addressed and a deeper understanding of the relationship between the two could be attained. We review the developments in X-ray single-crystal diffraction crystallography over the last century and show how its power to provide fundamental information about the structures of minerals has evolved with the improvements in data quality and the increased technological capacity to handle the data. We show that modern laboratory X-ray diffraction data are of the quality such that mineralogical results are no longer limited by the data quality, but by the physical validity of the refinement models used to interpret the data.

Description: A synthetic single crystal of pure orthoenstatite (MgSiO 3 , space group Pbca ) has been investigated at high pressure for structural determinations by in situ single-crystal X-ray diffraction using a diamond-anvil cell. Ten complete intensity data collections were performed up to 9.36 GPa. This study significantly improved the accuracy of structural parameters in comparison to a previous high-pressure structural study, allowing a more detailed examination of structural behavior of orthoenstatite at high pressures and a comparison to other more recent structural studies performed on orthopyroxenes with different compositions. The structural evolution determined in this work confirms the high-pressure evolution found previously for other orthopyroxenes and removes some ambiguities originating from the less accurate published data on the MgSiO 3 structure at high pressure. The structural compression is mostly governed by significant volume decrease of the Mg1 and Mg2 octahedra, affecting in turn the kink of the tetrahedral chains, especially the TB chain of larger SiO 4 tetrahedra. The Mg2 polyhedron undergoes the largest volume variation, 8.7%, due especially to the strong contraction of the longest bond distance (Mg2-O3B), whereas Mg1 polyhedral volume decreases by about 7.4%. The compressional behavior of the tetrahedral sites is quite different from previously published data. The TA and TB tetrahedral volumes decrease by about 2.8 and 1.8%, respectively, and no discontinuities can be observed in the pressure range investigated. Using the data on the pure orthoenstatite as reference, we can confirm the basic influences of element substitutions on the evolution of the crystal structure with pressure.

Description: 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.

Description: This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo. Two C 2/ c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively. The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/(Mg + Fe 2+ ) atomic ratio]. The analysis of the thermal behavior gives similar thermal expansion volume coefficients, α V , for the Mg-rich and Fe-rich samples, with α V = 2.72(8) and 2.72(7) x 10 –5 K –1 , respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples. Our data suggest that MERTIS spectra will be able to provide indications of C 2/ c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Description: Minerals trapped as inclusions within other host minerals can develop residual stresses on exhumation as a result of the differences between the thermo-elastic properties of the host and inclusion phases. The determination of possible entrapment pressures and temperatures from this residual stress requires the mutual elastic relaxation of the host and inclusion to be determined. Previous estimates of this relaxation have relied on the assumption of linear elasticity theory. We present a new formulation of the problem that avoids this assumption. We show that for soft inclusions such as quartz in relatively stiff host materials such as garnet, the previous analysis yields entrapment pressures in error by the order of 0.1 GPa. The error is larger for hosts that have smaller shear moduli than garnet.

Description: The crystal chemistry and crystal structure of hambergite from the Anjanabonoina mine, Madagascar [Be 2 BO 3 (OH) 0.96 F 0.04 , Z = 8, a = 9.762(2), b = 12.201(2), c = 4.430(1) Å, V = 527.6(2) Å 3 , space group Pbca ], were reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, secondary-ion mass spectrometry, single-crystal X-ray and neutron Laue diffraction. Chemical analyses show only a small amount of F (0.7–0.8 wt%, approximately 0.04 atoms per formula unit) substituting OH and no other substituent at a significant level. An anisotropic neutron structural refinement has been performed with final agreement index R 1 = 0.0504 for 76 refined parameters and 1430 unique reflections with F o 〉 4( F o ). The geometry of the hydroxyl group and hydrogen bonding in hambergite is now well defined: (1) only one independent H site was located and the O4-H distance, corrected for "riding motion," is ~0.9929 Å; (2) only one hydrogen bond appears to be energetically favorable, with a symmetry-related O4 as acceptor and with O4···O4 = 2.904(1) Å, H···O4 = 1.983(1) Å, and O4-H···O4 = 157.5(1)°. In other words, O4 sites act both as donor and as acceptor of the hydrogen bond, with a zigzag chain of H-bonds along [001]. The hydrogen-bonding scheme in hambergite found in this study is consistent with the pleochroic scheme of the infrared spectra previously reported, with two intensive modes ascribable to stretching vibrations of the hydroxyl group, at 3415 and 3520 cm –1 , respectively. The two modes suggest at least two distinct hydrogen-bonding environments, ascribable to the presence of oxygen and fluorine at the acceptor site.

Description: In this work we report the first finding of CaCl 2 β·4H 2 O, long known as a synthetic phase. The mineral, called ghiaraite, was discovered in 2011 in a sample belonging to the Real Museo Mineralogico di Napoli (Italy), that had been collected in 1872 at Vesuvius volcano and stored in a glass sealed vial. It is associated with chlorocalcite (KCaCl 3 ), hematite, sylvite, and halite. The mineral was found inside an ejecta of 5 m in size transported by a lava flow to the locality of Massa di Somma. Here with the ejecta still hot the sample was collected and rapidly stored in a sealed glass vial to preserve it from the atmospheric conditions. Ghiaraite is triclinic, space group P , with unit-cell parameters: a = 6.3660(5), b = 6.5914(5), c = 8.5568(6) Å, α = 93.504(6)°, β = 97.778(7)°, = 110.557(6)°, V = 330.802(9) Å 3 , Z = 2. The calculated density is 1.838 g/cm 3 using the ideal formula and the powder X-ray diffraction data. It occurs as euhedral isometric grains up to 5–6 μm long intimately intermixed with chlorocalcite. The eight strongest reflections in the X-ray powder diffraction pattern [listed as d (Å)( I )( hkl )] are: 2.628(100)(02); 2.717(88)(10); 4.600(88)(1 ); 2.939(77)(200); 2.204(75)(121), 5.874(73)(100), 6.124(47)(010); 3.569(46)(11). Ghiaraite was approved by the Commission on New Minerals, Nomenclature and Classification with IMA number 2012-072. The mineral was named in honor of Maria Rosaria Ghiara (b. 1948), Head of Real Museo Mineralogico of Napoli and Centro Musei delle Scienze Naturali e Fisiche dell’Università degli Studi di Napoli Federico II for her important work in promoting the scientific research focused on the mineralogy of Vesuvius volcano.

Description: The pressure-volume equation of state for the two spinel end-member compositions chromite FeCr 2 O 4 and magnesiochromite MgCr 2 O 4 was determined for flux-grown synthetic single crystals at room temperature up to 8.2 and 9.2 GPa, respectively, by single-crystal X-ray diffraction using a diamond-anvil cell. The pressure-volume data show that the linear volume compressibility (here used only for purpose of comparison), calculated as β V = |[( V / V 0 )/ P ]|, is 0.00468 and 0.00470 GPa –1 , for chromite and magnesiochromite, respectively, with a negligible difference below 0.5%. The experimental data were fitted to a third-order Birch-Murnaghan equation of state (BM3) allowing a simultaneous refining of the following coefficients: V 0 = 588.47(4) Å 3 , K T0 = 184.8(1.7) GPa, and K' = 6.1(5) for chromite and V 0 = 579.30(4) Å 3 , K T0 = 182.5(1.4) GPa, and K' = 5.8(4) for magnesiochromite. The difference in K T0 is reduced to 〈1.5% going from Fe to Mg end-member composition, whereas the first pressure derivative seems not to be affected by the chemical variability. The limited difference in the equation of state coefficients recorded for FeCr 2 O 4 and MgCr 2 O 4 allowed us to fit the pressure-volume data of both to a single BM3 equation, resulting in a K T0 = 184.4(2.2) GPa and K' = 5.7(6).

Description: The new mineral species ichnusaite, Th(MoO 4 ) 2 ·3H 2 O, has been discovered in the Mo-Bi mineralization of Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as colorless thin {100} tabular crystals, up to 200 μm in length, associated with muscovite, xenotime-(Y), and nuragheite, Th(MoO 4 ) 2 ·H 2 O. Luster is pearly adamantine. Ichnusaite is brittle, with a perfect {100} cleavage. Owing to the very small quantity of available material and its intimate association with nuragheite, density and optical properties could not be measured. Electron microprobe analysis gave (mean of 4 spot analyses in wt%): MoO 3 47.86(1.43), ThO 2 43.40(79), total 91.26(87). On the basis of 8 O atoms per formula unit and assuming 3 H 2 O groups, in agreement with the crystal structure data, the chemical formula of ichnusaite is Th 0.99 Mo 2.01 O 8 ·3H 2 O. Main diffraction lines, corresponding to multiple hkl indices, are [ d (Å), relative visual intensity]: 5.66 (m), 3.930 (m), 3.479 (s), 3.257 (s), 3.074 (m). Ichnusaite is monoclinic, space group P 2 1 /c , with a = 9.6797(12), b = 10.3771(13), c = 9.3782(12) Å, β = 90.00(1)°, V = 942.0(2) Å 3 , Z = 4. The crystal structure has been solved and refined to a final R 1 = 0.051 on the basis of 2008 observed reflections [with F o 〉 4( F o )]. It consists of electroneutral [Th(MoO 4 ) 2 (H 2 O) 2 ] 0 (100) sheets of polymerized ThO 7 (H 2 O) 2 and MoO 4 polyhedra; successive sheets, stacked along [100], are connected through hydrogen bonds. Ichnusaite brings new understanding about the crystal chemistry of actinide molybdates, that may form during the alteration of spent nuclear fuel and influence the release of radionuclides under repository conditions.