Ronald Miletich

Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn3+ hydrogarnet, henritermierite

Abstract Henritermierite, space group I41/acd, at 293 K a = 12.489(1), c = 11.909(1) Å, Z = 8, with close to end-member composition (Ca2.98Na0.01Mg0.01)VIII(Mn1.95Fe0.01Al0.04)VI[SiO4]2.07[H4O4]0.93 from the N’Chwaning II mine at the Kalahari manganese fields, Republic of South Africa, has been studied by single-crystal X-ray diffraction at 100 and 293 K at ambient pressure and up to 8.7 GPa in a diamond-anvil cell at 293 K. Polarized FTIR spectroscopy at 80 and 293 K was also performed. The Mn3+O6 octahedra display a tetragonally elongated type of Jahn-Teller distortion where the oxygen atoms of the elongated O-Mn-O axis (Mn-O: 2.2 Å) are moderately hydrogen bonded (O-H…O: 2.76 Å) to the H4O4 tetrahedra, which replace 1/3 of SiO4 tetrahedra in an ordered fashion. Thus Jahn-Teller distortion and H4O4 arrangement are coupled and both are responsible for the tetragonal bulk symmetry. The H4O4 tetrahedra have a center-to-O distance of 1.98 Å and the H atoms are slightly above the tetrahedral faces as similarly observed in the synthetic katoite end-member, Ca3Al2[H4O4]3. However, in henritermierite the O-H…O hydrogen bond is considerably bent (ca. 131°) and gives rise to an OH stretching mode at 3432(5) cm-1. Additional, though weak, IR absorptions at 3508(2) and 3553(2) cm-1 may be due to more remote hydrogen-bond acceptors (O-H…O: 3.29 Å) within the H4O4 tetrahedra. Compressibility data for a third-order Birch-Murnaghan equation of state yield a bulk modulus of K0 = 97.9(9) GPa with a pressure derivative of K = 5.3(3). The axial compressibilities indicate a pronounced compressional anisotropy which is explained by the orientation of the elongated axes of the Jahn-Teller distorted MnO6 octahedra along the slightly more compressible [100]tetr directions compared to the c-axis. The crystal structure was refined at a pressure of 8.6 GPa. The MnO6 octahedra were observed to show anisotropic compression towards a more isometric shape. Calculated spontaneous strain reveals a trend towards a weaker tetragonal distortion. If henritermierite is heated above 800 K in air it dehydrates and Mn3+ is partially oxidized to Mn4+. This topotactic transformation leads to a new garnet-like phase of Ia3d symmetry with a = 12.12 Å and of Ca3Mn2.26O2.32[SiO4]2.42 composition in which instead of H4O4 tetrahedra a new disordered octahedral site is occupied by Mn

Comparison between beryllium and diamond-backing plates in diamond-anvil cells: Application to single-crystal x-ray diffraction high-pressure data

A direct comparison between two complete intensity datasets, collected on the same sample loaded in two identical diamond-anvil pressure cells equipped, respectively, with beryllium and diamond-backing plates was performed. The results clearly demonstrate that the use of diamond-backing plates significantly improves the quality of crystal structure data. There is a decrease in the internal R factor for averaging, structure refinement agreement factors, and in the errors and uncertainties of the atomic coordinates, atomic displacement parameters, and individual bond lengths.

Suredaite, PbSnS3, a new mineral species, from the Pirquitas Ag-Sn deposit, NW-Argentina: mineralogy and crystal structure

Abstract Suredaite, ideally PbSnS3, is a new mineral species from the Pirquitas Ag-Sn deposit (Province Jujuy, NW-Argentina). It was observed in symmetrically banded veins in the Oploca district, and is associated with sphalerite, arsenopyrite, pyrite-marcasite, cassiterite, cylindrite, franckeite, hocartite, rhodostannite, and various Ag-Sb and Ag-Bi sulfosalts in minor amounts. Suredaite occurs in layers up to 1 cm in thickness as aggregates of radially arranged tabular-prismatic (single) crystals, has a metallic lustre, and a dark grey streak. VHN50 ranges between 18.2 and 20.6 (mean 19.6) GPa, the Mohs hardness is 2.5-3. It has perfect cleavages parallel to {001}, {101}, and {100}. The measured density varies between 5.54 and 5.88 g/cm3, Dx was determined to be 5.615 g/cm3. In reflected plane-polarised light, it is white and is not perceptibly bireflectant or pleochroic. It lacks internal reflections and is weakly anisotropic with metallic blue, mauve to brown rotation tints. Specular reflectance percentages in air and in oil are tabulated from 400 to 700 nm and compared graphically with those for the type specimen of teallite, PbSnS2. Electron microprobe analyses showed suredaite to be chemically inhomogeneous with respect to the compositional variations (in wt%): Pb 42.3- 48.5, Ag 0.3-1.1, Fe 0.3-1.0, As 0.2-2.1, Sn 27.7-30.2, S 23.1-24.7. The crystal structure determined from single-crystal X-ray diffraction data revealed orthorhombic symmetry [space group Pnma, Z = 4, a = 8.8221(3), b = 3.7728(3), c = 14.0076(3) Å; V = 466.23(4) Å3]. The atomic arrangement is isostructural to the NH4CdCl3 structure type which exists in a series of isotypic sulfides and selenide compounds. The suredaite structure, which is the natural analogue of synthetic PbSnS3, consists of columns of double-edge sharing octahedra running parallel to the b axis, which house the Sn atoms. These columns are linked by rods of eightfold-coordinated Pb atoms. On the basis of the structure determination, the empirically determined idealized formula follows a [8](Pb, As,Ag, Sn) [6](Sn,Fe)S3 stoichiometry. Crystalchemical arguments suggest Ag possibly to occupy interstitial sites according to the alternative formula [4](⃞,Ag) [8](Pb, As, Sn) [6](Sn,Fe) S3. The name of this new mineral species is in honor of R.J. Sureda Leston, head of the Department of Mineralogy and Economic Geology, University of Salta, Argentina.

Multipurpose high-pressure high-temperature diamond-anvil cell with a novel high-precision guiding system and a dual-mode pressurization device

A novel diamond-anvil cell (DAC) design has been constructed and tested for in situ applications at high-pressure (HP) operations and has proved to be suitable even for HP sample environments at non-ambient temperature conditions. The innovative high-precision guiding mechanism, comparable to a dog clutch, consists of perpendicular planar sliding-plane elements and is integrated directly into the base body of the cylindrically shaped DAC. The combination of two force-generating devices, i.e., mechanical screws and an inflatable gas membrane, allows the user to choose independently between, and to apply individually, two different forcing mechanisms for pressure generation. Both mechanisms are basically independent of each other, but can also be operated simultaneously. The modularity of the DAC design allows for an easy exchange of functional core-element groups optimized not only for various analytical in situ methods but also for HP operation with or without high-temperature (HT) application. For HP-HT experiments a liquid cooling circuit inside the specific inner modular groups has been implemented to obtain a controlled and limited heat distribution within the outer DAC body.

First evidence of P2{sub 1}/n to P2{sub 1}/c structural transformation in pyroxene-type LiAlGe{sub 2}O{sub 6} under high-pressure conditions

Abstract The high-pressure behavior of the pyroxene-type compound LiAlGe2O6, the unique representative of a P21/n-pyroxene, was investigated by in-situ X-ray diffraction and Raman spectroscopy on single-crystal samples hydrostatically pressurized in a diamond-anvil cell. The structure was found to undergo a first-order phase transition on compression, with a critical transition at 5.23±0.02xa0GPa. Together with a strong volume discontinuity of −ΔV/V0=−4.1% the transition shows a remarkable hysteresis loop over at least 0.70xa0GPa pressure interval. The bulk modulus of the low- and high-pressure polymorphs corresponds to K0=114(1) and 123(2)xa0GPa, respectively, as described by a 2nd order Birch–Murnaghan equation of state. Based on the systematic extinctions the transition was identified as a P21/n-to-P21/c transformation. The mechanism of structural transformation was identified to be controlled by the stereochemistry of the Li atoms at the M2 site, which changes its coordination number from 5 to 6. The formation of new Li–O bonds involves the co-operative folding of Ge2O6-chains, which explains the anisotropy of axial elasticities and the spontaneous strain across the transformation. Simultaneously the distortion correction of AlO6 units associated with the transition further explains the preference of the P21/c structure under pressure.