ALBERT

Description: We designed a plate impact shock recovery experiment to simulate the starting materials and shock conditions associated with the only known natural quasicrystals, in the Khatyrka meteorite. At the boundaries among CuAl5, (Mg0.75Fe2+0.25)2SiO4 olivine, and the stainless steel chamber walls, the recovered specimen contains numerous micron-scale grains of a quasicrystalline phase displaying face-centered icosahedral symmetry and low phason strain. The compositional range of the icosahedral phase is Al68–73Fe11–16Cu10–12Cr1–4Ni1–2 and extends toward higher Al/(Cu+Fe) and Fe/Cu ratios than those reported for natural icosahedrite or for any previously known synthetic quasicrystal in the Al-Cu-Fe system. The shock-induced synthesis demonstrated in this experiment reinforces the evidence that natural quasicrystals formed during a shock event but leaves open the question of whether this synthesis pathway is attributable to the expanded thermodynamic stability range of the quasicrystalline phase at high pressure, to a favorable kinetic pathway that exists under shock conditions, or to both thermodynamic and kinetic factors.

Description: The recent discovery in high-pressure experiments of compounds stable to 24–26 GPa with Fe4O5, Fe5O6, Fe7O9, and Fe9O11stoichiometry has raised questions about their existence within the Earth’s mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe0.71Ni0.24Cu0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth’s mantle. The microstructural relation of magnetite within a ferropericlase (Mg0.60Fe0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metalMoxide (M4O5) with a stoichiometry of (Fe3+2.15Fe2+1.59Ni2+0.17Cu+0.04)Σ=3.95O5. We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe4O5that subsequently decompose at shallowerP-Tconditions.

Description: Manganoquadratite, ideally AgMnAsS3, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as dark gray, anhedral to subhedral grains up 0.5 mm across, closely associated with alabandite, Mn-rich calcite, Mn-rich sphalerite, proustite, pyrite, pyrrhotite, tennantite, argentotennantite, stannite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S. Manganoquadratite is opaque with a metallic luster and possesses a reddish-brown streak. It is brittle, the Vickers microhardness (VHN10) is 81 kg/mm2 (range 75–96) (corresponding Mohs hardness of 2–2½). The calculated density is 4.680 g/cm3 (on the basis of the empirical formula). In plane-polarized reflected light, manganoquadratite is moderately bireflectant and very weakly pleochroic from dark gray to a blue gray. Internal reflections are absent. Between crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) for the four standard COM wavelengths are 29.5, 31.8 (471.1 nm), 28.1, 30.5 (548.3 nm), 27.3, 29.3 (586.6 nm), and 26.0, 28.2 (652.3 nm), respectively.Manganoquadratite is tetragonal, space group P4322, with unit-cell parameters: a = 5.4496(5), c = 32.949(1) Å, V = 978.5(1) Å3, c:a = 6.046, Z = 8. The structure, refined to R1 = 0.0863 for 907 reflections with Fo 〉 4σ(Fo), consists of a stacking along [001] of alabandite-like Mn2S2 layers connected to each to other by a couple of AgAsS2 sheets where As3+ forms typical AsS3 groups, whereas Ag+ cations are fivefold coordinated. The six strongest lines in the observed X-ray powder-diffraction pattern [d in Å (I/I0) (hkl)] are: 3.14 (60) (116), 2.739 (50) (0 0 12), 2.710 (100) (200), 1.927(70) (2 0 12 + 220), 1.645 (25) (3 0 16), and 1.573 (20) (22 12).Electron microprobe analyses gave the chemical formula (on the basis of six atoms) (Ag0.95Cu0.05)∑=1.00 (Mn0.96Pb0.04)∑=1.00(As0.87Sb0.14)∑=1.01S2.99, leading to the simplified formula AgMnAsS3.The name was chosen to indicate the close analogy of the formula and unit-cell dimensions with quadratite, Ag(Cd,Pb)(As,Sb)S3. The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2011-008.

Description: The crystal structure and chemical composition of a crystal of Na2MgSi5O12 garnet synthesized in the model system Mg3Al2Si3O12-Na2MgSi5O12 at 17.5 GPa and 1700 {degrees}C have been investigated. Quantitative analysis leads to the following formula: Na1.98Mg1.00Si5.01O12. Na2MgSi5O12 garnet was found to be tetragonal, space group I41/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) A, V = 1472.5(1) A3. The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 A for both the sites), whereas Si is ordered at both the Y (mean: 1.793 A) and Z sites (means: 1.630 and 1.624 A). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na2MgSi5O12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages.

Description: The mineral fettelite, [Ag6As2S7][Ag10HgAs2S8], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag6As2S7]2- and a second module layer (labeled B) with composition [Ag10HgAs2S8]2+. Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chanarcillo, Copiapo Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P[IMG]f1.gif" ALT="Formula" BORDER="0"〉m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.

Description: Icosahedrite, ideally Al63Cu24Fe13, is a new mineral from the Khatyrka River, southeastern Chukhotka, Russia. It occurs as dark gray-black anhedral to subhedral grains up to 100 {micro}m across, closely associated with spinel, diopside, forsterite, nepheline, sodalite, corundum, stishovite, khatyrkite, cupalite, and an unnamed phase of composition AlCuFe. Icosahedrite is opaque with a metallic luster, possesses a gray streak, and is brittle with an uneven fracture. The density could not be determined. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. In plane-polarized incident light, icosahedrite exhibits neither bireflectance nor pleochroism. Between crossed polars, it is isotropic. Reflectance percentages (Rmin = Rmax) for the four standard COM wavelengths are 62.3 (471.1 nm), 60.6 (548.3 nm), 58.1 (586.6 nm), and 56.0 (652.3 nm), respectively. The X-ray powder pattern was indexed on the basis of six integer indices, as conventionally used with quasicrystals, where the lattice parameter (in six-dimensional notation) is measured to be a6D = 12.64 A, with probable space group Fm[IMG]f1.gif" ALT="Formula" BORDER="0"〉 [IMG]f2.gif" ALT="Formula" BORDER="0"〉. The four strongest X-ray powder-diffraction lines [d in A (I/I0) (n1,n2,n3,n4,n5,n6)] are: 2.006 (100) (4[IMG]f3.gif" ALT="Formula" BORDER="0"〉0 042), 2.108 (90) (42[IMG]f3.gif" ALT="Formula" BORDER="0"〉 [IMG]f3.gif" ALT="Formula" BORDER="0"〉22), 1.238 (30) (60[IMG]f4.gif" ALT="Formula" BORDER="0"〉 064), and 3.41 (25) (31[IMG]f5.gif" ALT="Formula" BORDER="0"〉 [IMG]f5.gif" ALT="Formula" BORDER="0"〉11). Average results of 34 electron-microprobe analyses gave, on the basis of total atoms = 100, the formula Al63.11Cu24.02Fe12.78Si0.03Co0.01Ca0.01Zn0.01Cr0.02Cl0.01. The simplified formula is Al63Cu24Fe13, which requires the mass fractions Al 43.02, Cu 38.60, Fe 18.38, total 100.00 wt%. The new mineral is named for the icosahedral symmetry of its internal atomic structure, as observed in its diffraction pattern. Both the new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2010-042).

Description: Menchettiite, ideally AgPb2.40Mn1.60Sb3As2S12, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as black, anhedral to subhedral grains up to 200 µm across, closely associated with orpiment, tennantite/tetrahedrite, other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, and calcite. Menchettiite is opaque with a metallic luster and possesses a black streak. It is brittle, with uneven fracture; the Vickers microhardness (VHN100) is 128 kg/mm2 (range 119–136) (corresponding to a Mohs hardness of 2½–3). The calculated density is 5.146 g/cm3 (on the basis of the empirical formula). In plane-polarized incident light, menchettiite is weakly to moderately bireflectant and weakly pleochroic from dark gray to a dark green. Internal reflections are absent. Between crossed polarizers, the mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) for the four standard COM wavelengths are 33.1, 39.8 (471.1 nm), 31.8, 38.0 (548.3 nm), 30.9, 37.3 (586.6 nm), and 29.0, 35.8 (652.3 nm), respectively.Menchettiite is monoclinic, space group P21/n, with unit-cell parameters: a = 19.233(2), b = 12.633(3), c = 8.476(2) Å, ß = 90.08(2)°, V = 2059.4(8) Å3, a: b: c 1.522:1:0.671, Z = 2, and it is twinned on {100}. The crystal structure was refined to R = 0.0903 for 2365 reflections with Fo 〉 4s(Fo) and it resulted to be topologically identical to those of ramdohrite, uchucchacuaite, and fizélyite. The six strongest X-ray powder-diffraction lines [d in Å (I/I0) (hkl)] are: 3.4066 (39) (3¯12), 3.4025 (39) (312), 3.2853 (100) (520), 2.8535 (50) (2¯32), 2.8519 (47) (232), and 2.1190 (33) (004). Electron-microprobe analyses gave the chemical formula Ag1.95Cu0.01Pb4.81Mn3.20Fe0.02Zn0.01Sb6.09As3.94Bi0.01S23.95Se0.01, on the basis of 44 atoms and according to the structure refinement results. Menchettiite can be classified among the Sb-rich members of the lillianite homeotypic series, which are described with the general formula AgxPb3-2xSb2+xS6. Besides the heterovalent substitution 2Pb2+ ? Ag+ + Sb3+ taken into consideration by the above formula, two isovalent substitutions relate menchettiite to the other lillianite homeotypes, i.e., Mn2+ ? Pb2+ and As3+ ? Sb3+. The name is after Silvio Menchetti (1937–), Professor of Mineralogy and Crystallography at the University of Florence. The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2011–009).