ALBERT

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: The crystal structure of Fe2SiO4 spinel at room temperature was investigated at seven different pressures by X-ray diffraction, using a diamond anvil cell to examine the influence of Fe substitution on ringwoodite behaviour at high pressure. The results compared with those of a pure Mg endmember show that the substitution of Fe into the spinel structure causes only small changes in the compression rate of coordination polyhedra and the distortion of the octahedron. The data show that the compression rate for the octahedron and tetrahedron in (Mg,Fe)2SiO4 can be considered statistically equal for FeO6 and MgO6, as well as for SiO4 in both the endmembers. This shows why almost identical bulk moduli are reported along the solid solution in recent literature.

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: Raberite, ideally Tl5Ag4As6SbS15, is a new mineral from Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs very rarely as euhedral crystals up to 150 μm across associated with yellow needle-like smithite, realgar, hatchite and probable trechmannite, edenharterite, jentschite and two unidentified sulfosalts. Raberite is opaque with a metallic lustre and has a dark brown–red streak. It is brittle with a Vickers hardness (VHN10) of 52 kg mm−2 (range 50–55) corresponding to a Mohs hardness of 2½–3. In reflected light raberite is moderately bireflectant and very weakly pleochroic from light grey to a slightly greenish grey. It is very weakly anisotropic with greyish to light blue rotation tints between crossed polars. Internal reflections are absent. Reflectance percentages for the four COM wavelengths [listed as Rmin, Rmax, (λ)] are 30.6, 31.8 (471.1 nm), 28.1, 29.3 (548.3 nm), 27.1, 28.0 (586.6 nm), and 25.8, 26.9 (652.3 nm).Raberite is triclinic, space group P1İ, with a = 8.920(1), b = 9.429(1), c = 20.062(3) Å, α = 79.66(1), β = 88.84(1), γ = 62.72(1)°, V = 1471.6(4) Å3 and Z = 2. The crystal structure [R1 = 0.0827 for 2110 reflections with I 〉 2σ(I)] consists of columns of nine-coordinate Tl atoms forming irregular polyhedra extending along [001] and forming sheets parallel to (010). The columns are decorated by corner-sharing MS3 pyramids (M = As, Sb) and linked by AgS3 triangles. Of the seven M positions, one is dominated by Sb and the others by As; the mean M–S bond distances reflect As↔Sb substitution at these sites.The eight strongest lines in the powder diffraction pattern [dcalc in Å (I) (hkl)] are: 3.580 (100) (11İ3); 3.506 (58) (1İ2İ3); 3.281 (73) (006); 3.017 (54) (1İ23); 3.001 (98) (133); 2.657 (51) (226); 2.636 (46) (300); 2.591 (57) (330). A mean of 9 electron microprobe analyses gave Tl 39.55(13), Ag 18.42(8), Cu 0.06(2), As 17.08(7), Sb 5.61(6), S 19.15(11); total 99.87 wt.%, which corresponds to Tl4.85Ag4.28Cu0.02As5.72Sb1.16S14.97 with 31 atoms per formula unit. The new mineral has been approved by the IMA-CNMNC Commission (IMA 2012-017) and is named for Thomas Raber, an expert on Lengenbach minerals.

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: The volumes of a disordered An20 (Qod = 0.15), a disordered An78 (Qod = 0.55) and an ordered An78 (Qod = 0.81) were determined up to 9.569(10) GPa, 8.693(5) GPa and 9.765(10) GPa, respectively, using single-crystal X-ray diffraction. The volume variations with pressure for these samples are described with 4th-order Birch Murnaghan equations of state with V0 = 669.88(7) A3, K0 = 59.7(7) GPa, K' = 5.7(5), K'' = -0.8(2) GPa-1 for disordered An20, V0 = 1340.48(10) A3, K0 = 77.6(5) GPa, K0' = 4.0(3), K'' = -0.59(9) GPa-1 for disordered An78 and V0 = 1339.62(6) A3, K0 = 77.4(6) GPa, K' = 4.2(4), and K'' = -0.7(1) GPa-1 for ordered An78. Along with data from previous studies (An0 ordered, An0 disordered and An20 ordered), the volumes for the disordered samples were found to be up to [~]0.3% larger than the ordered samples of the same composition. The disordered samples are softer than the ordered samples of the same composition by 4(1)% for An0, 2.5(9)% for An20 and essentially zero for An78. The relationship between volume increase, density decrease, and decreasing bulk modulus with increasing disorder is in accordance with Birch's Law.

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.