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A synthetic pyroxene with composition LiFe[Ge.sub.2][O.sub.6] and space group P[2.sub.1]/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, [beta], and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P[2.sub.1]/c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P[2.sub.1]/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130 [degrees]). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms. Keywords: Single-crystal structure analysis, germanates, high-pressure phase transformations, X-ray diffraction

Aluminocerite-(Ce), ideally [(Ce,Ca).sub.9]Al[(Si[O.sub.4]).sub.3] [[Si[O.sub.3](OH)].sub.4][(OH).sub.3], is isostructural with cerite-(Ce) and cerite-(La). The holotype was found at the Ratti quarry, near Baveno, Italy, in millimeter-sized secondary cavities hosted in aplite-pegmatite veins and pods within pink granite. Aluminocerite-(Ce) forms aggregates of pseudo-octahedral to rhombohedral crystals flattened on the c axis. The cotype of aluminocerite-(Ce) was discovered at the Locatelly quarry, also near Baveno, where it occurs in centimeter-sized miarolitic cavities in pink granite. The mineral is pale pink to pink-reddish, with a white streak, and is translucent with a vitreous luster. Aluminocerite-(Ce) is non-fluorescent. The hardness based on the Mohs scale is 5, and the tenacity is brittle. Neither cleavage, fracture, or twinning were observed. Calculated density is 4.675 g/[cm.sup.3]. It is uniaxial, optically positive, with [n.sub.o] = 1.810-1.816 and [n.sub.e] = 1.812-1.822 ([lambda], = 589 nm) and non-pleochroic. The average of 15 electron microprobe analyses for the holotype gave (wt%): [Ce.sub.2][O.sub.3] 23.37; [Nd.sub.2][O.sub.3] 15.59; [La.sub.2][O.sub.3] 7.43; [Sm.sub.2][O.sub.3] 4.38; [Pr.sub.2][O.sub.3] 3.54; [Gd.sub.2][O.sub.3] 3.12; [Y.sub.2][O.sub.3] 1.68; [Dy.sub.2][O.sub.3] 0.46; [Yb.sub.2][O.sub.3] 0.07; CaO 8.31; [Fe.sub.2][O.sub.3] 0.47; [Al.sub.2][O.sub.3] 2.47; Si[O.sub.2] 24.01; and [H.sub.2]O 3.63 (calculated from crystal-chemical constraints), total 98.53 wt%, corresponding to the empirical formula [([Ca.sub.2.60][Ce.sub.2.49][Nd.sub.1.62][La.sub.0.80][Sm.sub.0.44] [Pr.sub.0.38][Gd.sub.0.30][Y.sub.0.26][Dy.sub.0.04][Yb.sub.0.01]).sub.[SIGMA] 8.94][([Al.sup.3+.sub.0.85][Fe.sup.3+.sub.0.10]).sub.[SIGMA]0.95] [(Si[O.sub.4]).sub.3][[Si[O.sub.3](OH)].sub.4] [(OH).sub.3.06], calculated on the basis of Si = 7. Aluminocerite-(Ce) is trigonal, space group R3c, with a =10.645(1), c = 38.019(5) [Angstrom], V = 3731 [[Angstrom].sup.3]. The strongest eight lines in the X-ray powder diffraction pattern are [d in [Angstrom] (I)(hkl)]: 3.405(27)(122), 3.250(26)(124), 2.914(100)(02,10), 2.647(58)(220), 2.198(40)(03,12), 1.923 (34) (238), 1.826(24)(051), and 1.732(46)(03,18). The crystal structure has been refined to R1 = 0.056 for 745 observed reflections. The name is an allusion to the fact that it is the Al-dominant analog of cerite-(Ce). Keywords: Aluminocerite-(Ce), single-crystal XRD, EMP analyses, new mineral, Raman spectroscopy

Crowningshieldite is the natural analog of the synthetic compound α-NiS. It has a NiAs-type structure and is the high-temperature polymorph relative to millerite (β-NiS), with an inversion temperature of 379 °C. Crowningshieldite is hexagonal, space group P63/mmc, with a = 3.44(1) Å, c = 5.36(1) Å, V = 55.0(2) Å3, and Z = 2. It has an empirical formula (Ni0.90Fe0.10)S and dcalc = 5.47(1) g/cm3. The five strongest lines in the powder X‑ray diffraction data are [dmeas in angstroms (I) (hkl)]: 1.992 (100) (102), 1.718 (55) (110), 2.978 (53) (100), 2.608 (35) (101), and 1.304 (17) (202). Crowningshieldite was found as part of a multiphase inclusion in a gem-quality, colorless, type IIa (containing less than ~5 ppm N) diamond from the Letseng mine, Lesotho. The inclusion contains crowningshieldite along with magnetite-magnesioferrite, hematite, and graphite. A fracture was observed that extended from the inclusion to the diamond exterior, meaning that fluids, possibly kimberlite-related, could have penetrated into this fracture and altered the inclusion. Originally, the inclusion might have been a more reduced, metallic Fe-Ni-C-S mixture made up of cohenite, Fe-Ni alloy, and pyrrhotite, akin to the other fracture-free, pristine inclusions within the same diamond. Such metallic Fe-Ni-C-S primary inclusions are a notable recurring feature of similar type IIa diamonds from Letseng and elsewhere that have been shown to originate from the sublithospheric mantle. The discovery of crowningshieldite confirms that the α-NiS polymorph occurs in nature. In this case, the reason for its preservation is unclear, but the relatively iron-rich composition [Fe/(Fe+Ni) = 0.1] or the confining pressure of the diamond host are potential factors impeding its transformation to millerite. The new mineral name honors G. Robert Crowningshield (1919–2006) (IMA2018-072). [ABSTRACT FROM AUTHOR]

Structural refinements from single-crystal X-ray diffraction data are reported for olivine with a composition of Fo100 (forsterite Mg2SiO4, synthetic), Fo80 and Fo62 (~Mg1.6Fe0.4SiO4 and ~Mg1.24Fe0.76SiO4, both natural) at room temperature and high pressure to ~8 GPa. The new results, along with data from the literature on Fo0 (fayalite Fe2SiO4), were used to investigate the previously reported structural mechanisms which caused small variations of olivine bulk modulus with increasing Fe content. For all the investigated compositions, the M2 crystallographic site, with its bonding configuration and its larger polyhedral volume, was observed to control the compression mechanisms in olivine. From Fo100 to Fo0, the compression rates for M2–O and M1–O bond lengths were observed to control the relative polyhedral volumes, resulting in a less-compressible M1O6 polyhedral volume, likely causing the slight increase in bulk modulus with increasing Fe content. [ABSTRACT FROM AUTHOR]

At Arvogno, Vigezzo valley in the Central Alps, Italy, pegmatite dikes are unique in the scenario of a tertiary alpine pegmatite field because they show marked geochemical and mineralogical niobium–yttrium–fluorine features. These pegmatites contain AB2O6 aeschynite group minerals and ABX2O8 euxenite group minerals as typical accessory minerals including aeschynite-(Y), polycrase-(Y), and samarskite-(Y). They are associated with additional typical minerals such as fluorite, Y-dominant silicates, and xenotime-(Y). The Y–Nb–Ti–Ta AB2O6 and ABX2O8 oxides at the Arvogno pegmatites did not exhibit any textural and compositional features of oxidation or weathering. They are characterized by low self-radiation-induced structural damage, leading to the acquisition of unit-cell data for aeschynite-(Y), polycrase-(Y), and samarskite-(Y) by single-crystal X-ray diffraction. Aeschynite-(Y) and polycrase-(Y) crystals allowed for both to provide space groups whereas samarskite-(Y) was the first crystal from pegmatites for which cell-data were obtained at room temperature but did not allow for the accurate determination of the space group. According to the chemical compositions defined by Ti-dominant content at the B-site, the cell parameters, respectively, corresponded to polycrase-(Y), aeschynite-(Y), and the monoclinic cell of samarskite-(Y). Emplacement of Alpine pegmatites can be related to the progressive regional metamorphic rejuvenation from east to west in the Central Alps, considering the progressive cooling of the thermal Lepontine Barrovian metamorphic dome. Previous studies considered magmatic pulses that led to emplace the pegmatite field in the Central Alps. As an example, the pegmatites that intruded the Bergell massif were aged at 28–25 millions of years or younger, around 20–22 m.y. [ABSTRACT FROM AUTHOR]

Abstract: The equation of state for γ-Fe2SiO4 was determined by single-crystal X-ray diffraction up to 10.2GPa at room temperature. The pressure–volume data, measured at 10 different pressures, do not show any evidence of a phase transformation and were fit by a third-order Birch–Murnaghan equation of state. The following coefficients were refined simultaneously: V 0 =559.44(6)Å3, K T0 =187.3(1.7)GPa, and K′=5.5(4). This result implies that the Mg/Fe substitution along the γ-Fo–Fa join does not significantly affect the bulk modulus. The sample shows higher K′ than other compositions along the Fo–Fa join, which generally have K′∼4. Based on our results we can calculate the sound bulk velocity of γ-Fe2SiO4 up to the pressures of the Transition Zone. Due to the larger K′ of γ-Fe2SiO4, the difference in bulk sound velocity between the two end-members decreases with increasing pressure. [ABSTRACT FROM AUTHOR]