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Direct structural characterization of the changes in the local environment of Mg occurring in the garnet structure as a function of the Ca content are determined by Mg K-edge X-ray absorption fine structure on synthetic samples along the pyrope-grossular join. With increasing Ca content, the short Mg-O2 distance of the dodecahedron slightly decreases, while the long Mg-O4 distance tends to increase, so that the dodecahedron is more distorted in grossular-rich garnets than in end-member pyrope. This quantitative direct description of the changes in the local environment of Mg in the pyrope-grossular solid solution confirms and better defines previous experimental and recent computational results. Keywords: Pyrope-grossular garnets, EXAFS, magnesium, local environment

The iron sulfate FeOHS[O.sub.4] studied was obtained as a dehydration product of metahohmannite [Fe.sub.2][([H.sub.2]O).sub.4][O[(S[O.sub.4]).sub.2]] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHS[O.sub.4] has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A ([a.sub.j], [b.sub.j], and [c.sub.j] are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 [degrees]C using the structural model from the literature yielded a poor fit of the observed data and a final [R.sub.p] value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHS[O.sub.4] belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators [2.sub.1/2] and [2.sub.-1/2], and yields an orthorhombic structure with space group Pnma and lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A. MDO2 results from the [2.sub.1/2][12.sub.1/2] [12.sub.1/2] ... sequence of symmetry operators and yields a monoclinic structure with space group P[2.sub.1]/c,[a.sub.M] = 7.33, [b.sub.M] = 7.14, [c.sub.M] = 7.39 A, and [beta] = 119.7[degrees]. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit ([R.sub.p] = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

Microbes play a fundamental role in the precipitation of silicate biominerals, thereby affecting the Si geochemical cycle. The fine mechanisms ruling biomineralization are not yet fully understood, and their microscopic structures can offer deep insight into their processes of formation, reactivity and stability. In this study, a Zn silicate biomineral, extracellularly produced by cyanobacterium Leptolingbya frigida, was investigated combining nuclear magnetic resonance (NMR), Zn K-edge X-ray absorption spectroscopy (XAS) and other complementary techniques. 29Si magic angle spinning and 29Si/1H cross polarization magic angle spinning analysis, Fourier transform infrared spectroscopy (FTIR) and XAS analysis revealed a poorly crystalline phase closely resembling hemimorphite [Zn4Si2O7(OH)2·H2O]. Zn K-edge extended X-ray absorption fine structure (EXAFS) provided further structural details, revealing that the Zn-O-Si interatomic distances were 7–8% shorter than the abiotic mineral. 13C NMR spectra analysis was conducted to investigate the composition of the Zn silicate biomineral organic matrix, and results revealed that C atoms occurred in several functional groups such as carbonyl carbons, C rings, O-aliphatic chains, N-aliphatic chains, and aliphatic chains. Under slightly alkaline conditions, bacterial cell walls exhibited fundamental control on the biomineralization process by binding Zn ions and forming Zn–O–Si bonds. In this way, L. frigida cell walls served as a reactive surface for the precipitation of this Zn sorosilicate, hindering the condensation of silicon dimers. Moreover, we found a 29Si NMR band at 85 ppm that could be attributed to a (C3H6O3)2Si complex. This complex could play a role in the control of silicon polymerization, with implications for Si biomineralization processes. [ABSTRACT FROM AUTHOR]

We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25-850 °C). Single-crystal synchrotron X-ray diffraction experiments were performed in a variable pressure range (0-100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, a0 = 4.10(7) 10-5 K-1, to magnesite, MgCO3, a0 = 7.04(2) 10-5 K-1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of a0 = 6.44(4) 10-5 K-1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures. [ABSTRACT FROM AUTHOR]

The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2½ and 2-1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 2½|2½|2½...sequence of symmetry operators and yields a monoclinic structure... [ABSTRACT FROM AUTHOR]

We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25-850 °C). Single-crystal synchrotron X-ray diffraction experiments were performed in a variable pressure range (0-100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, a0 = 4.10(7) 10-5 K-1, to magnesite, MgCO3, a0 = 7.04(2) 10-5 K-1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of a0 = 6.44(4) 10-5 K-1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures. [ABSTRACT FROM AUTHOR]

The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2½ and 2-1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 2½|2½|2½...sequence of symmetry operators and yields a monoclinic structure... [ABSTRACT FROM AUTHOR]