An anisotropic garnet was found in skarn ores occurring in the Yamansu deposit, east part of Xinjiang, China. Optical birefringence of the anisotropic garnet varies from 0.001 to 0.004. The chemical composition of the garnet essentially belongs to the grossular (Grs)-andradite (And) solid solution ranging from Grs35.3And60.1 to Grs55.8And38.5, with minor (<8%) uvarovite, pyrope, almandine and spessartine constituent. Negative correlation was slightly observed between Al and Fe in the garnet. The absence of OH stretching vibration in IR spectrum suggests no substitution of [SiO4] with tetrahedral [(OH)4] in the garnet. A refinement of the crystal structure, I1 (R = 0.0435), showed no significant drifts of atoms from the ideal positions corresponding to the atomic positions in the Ia3d cubic symmetry (R = 0.0544). The Al-Fe ordering was observed in the octahedral M site, whereas the Ca-Fe ordering in the dodecahedral site was negligible. The Al-Fe ordering showed no symmetric distribution among the 8 octahedral M sites, indicating the true symmetry of this garnet to be triclinic I1.
Garnet-chloritoid schists occur in the Chandman district of the Lake Zone, SW Mongolia, which is located within the central part of the Central Asian Orogenic Belt. The Chandman district is composed of Neoproterozoic ophiolites of the Khantaishir Formation, eclogite-bearing orthogneisses and micaschists of the Alag Khadny metamorphic complex, marbles of the Maykhan Tsakhir Formation, and the basement block of the Zamtyn Nuruu Formation. Garnet-chloritoid schists occur as lenses or layers within marbles of the Maykhan Tsakhir Formation, which lie in contact with eclogite bodies. They consist mainly of garnet, chloritoid, phengitic muscovite, chlorite and quartz, with minor amounts of kyanite, rutile, ilmenite, zircon and carbonaceous matter. The texture and mineral chemistry of the garnet-chloritoid schists record prepeak and peak stages of high-pressure intermediate type metamorphism. The pre-peak stage of epidote-amphibolite facies conditions (T = 500-510 °C and P = 7-8 kbar) is deduced from cores of garnet containing mineral inclusions of chloritoid, Fe-rich chlorite, phengitic muscovite, and quartz. The peak mineral assemblage of the garnet-chloritoid schists defined by garnet rims containing chloritoid, phengitic muscovite, Mg-rich chlorite, kyanite and quartz as inclusions indicates T = 560-590 °C and P = 10-11 kbar, falling in transitional conditions between the epidote-amphibolite and the eclogites facies. The pressure conditions of the garnet-chloritoid schists are distinctly lower than those of the eclogites (T = 590-610 °C and P = 20-25 kbar), whereas temperature conditions are similar. The eclogites are considered to have formed by subduction of oceanic crust. In contrast, the metamorphism of the garnet-chloritoid schists was probably related to collisional tectonic event in the Central Asian Orogenic Belt.
The structural changes and amorphization of alkali feldspar by ball milling were investigated using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), thermal analysis, particle size analysis, and specific surface area measurements. The Or77Ab23 alkali feldspar crystal has an Ab-rich lamella texture in Or-rich host crystals. The mean particle size decreased from ∼ 67 μm to almost 3 μm after 50 h of ball milling. After 600 h of ball milling, the apparent mean particle size calculated from the observed particle size distribution increased prominently because of the agglomeration of small particles. It reached ∼ 38 μm after 1200 h of ball milling. While the density variation was not consistent with that of the particle size, it was consistent with that of the crystallinity estimated from X-ray diffraction data. The FTIR and Raman spectra indicated breaking of the Si-O-Si or Al-O-Si bonds and the formation of defects and distortion of the TO4 (T = Si or Al) tetrahedra as a result of the milling. An increase in the intensity of luminescence with milling supported these results. XRD analysis revealed the sample that was ball milled for 1200-h was amorphous. However, the structure and properties of this amorphous phase were different from those of fused feldspar glass and had weak crystalline features. These amorphous materials were similar to those in the fault zone that were formed naturally and experimentally without melting. The Si-OH (or Al-OH) shoulders ν = 890 cm-1) in the FTIR spectra were formed by the chemical reaction between the broken Si-O-Si (or Al-O-Si) bonds and atmospheric water surrounding the 300-h milled samples. Because of this reaction, the surface and interior structure of the milled particles depended on the agglomeration and formation of defects formation resulting from the long milling time. XRD results of the milled samples indicated that grains with a larger amount of Ab-rich lamella structures were downsized more rapidly than the Or-rich host.
The Kasuga High Sulfidation gold deposit is hosted in the silicified Late Miocene andesite of Nansatsu Volcanic Rocks in the southern part of the Satsuma Peninsula, Southwest Japan. Optical microscopic observations and X-ray fluorescence spectrometry (Horiba XGT-5000) analysis of two silicified andesite samples from the Kasuga ore body revealed that the rocks underwent different degrees of hydrothermal alteration and no tectonic deformation afterward. Crystallographic orientations of quartz composing the silicified andesite samples studied using Electron Backscatter Diffraction, EBSD (EDAX GE-1, TSL-OIM) displayed different trends in the c-axis orientations between the samples. The c-axes of quartz grains are mainly oriented sub-horizontal or plunging at low angles in the less-silicified andesite. The highly silicified sample has larger quartz grains with c-axes plunging at high angles. The difference in c-axis orientations is provably related to the silicification processes in the earlier and later stages of hydrothermal alteration. The results of this study suggest that quartz c-axis orientations of rocks deformed under low strain regimes are potentially inherited from initial undeformed state, for example crystallization from hydrothermal solutions.
The local structure of iron in tektites from six strewn fields, and impact- and non-impact-related glass were studied using the Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, in order to obtain quantitative data on Fe-O bond length and Fe coordination number. X-ray absorption fine structure (XAFS) spectra and Fe-O bonds in standard minerals such as hematite, fayalite, and magnetite were compared. The degree of oxidation was measured based on the valencies of iron in the samples. Tektites contain a greater proportion of ferrous than ferric iron [0.04(1)-0.13(1) Fe3+/ΣFe]. The ferric ratios of impact-related glass [0.18(1)-0.52(1) Fe3+/ΣFe], and volcanic glass [0.26(1)-0.30(1) Fe3+/ΣFe] are higher than that in tektites. Based on the measured Fe-O distance, it was inferred that 4- and 5-coordinated Fe exist in tektites, whereas volcanic glass contains 5- and 6-coordinated Fe. Impact-related glass possesses various local structures caused by the combination of 4-, 5-, and 6-coordinated Fe. During formation, tektites experience high temperatures and a reducing atmosphere when they were ejected into the outer space. In contrast, the impact-related glass, which was ejected into the atmosphere or which remained close to the crater, experienced a more complex environment, with air pressure, density, and temperature varying across the atmospheric layers. Thus, impact-related glass presents more complicated oxidation states and structure compared to tektites. Volcanic glass, on the other hand, has a relatively stable redox condition; and thus, it undergoes only a small change in the degree of oxidation. This study indicates that the local structure and oxidation state of Fe may change due to the environment that the glass experienced during its formation. These different kinds of natural glass can be distinguished from each other using the study of the local structure.
A high-pressure X-ray diffraction study of manganite (γ-MnOOH) was performed using 10.1 GPa at room temperature. The aim of this study was to determine the compression behavior of an oxyhydroxide with an InOOH-related structure. The pressure-volume data were fitted by a third-order Birch-Murnaghan equation of state with the following parameters: V0 = 270.47(9) Å3, K0 = 91(3) GPa, and K0' = 7(1). Analysis of axial compressibility showed that the a-and b-axes were more compressible than the c-axis.