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Opal has long fascinated scientists. It is one of the few minerals with an amorphous structure, and yet, compared to silica glass, it is highly organized on the mesoscale. By means of inelastic neutron scattering (INS), we could document that in four samples of opal at low temperature an ice-like structure of water is present, with details depending on microstructural characteristics. While FTIR spectra for all samples are nearly identical and thus not very informative, INS shows clear differences, highlighting the significance of microstructures. Neutron diffraction at 100 K on one of the opal samples provides evidence for crystalline cubic ice.

In this study, we investigated four different ferroan dolomite samples using in situ Raman spectroscopy and powder X-ray diffraction (XRD) at pressures up to 48 GPa and room temperature. Our results show that the transition from Dolomite-I (Dol-I) to Dolomite-II (Dol-II) occurs above 13–16 GPa, and the transition pressure depends on the composition of the solid solution. Compression above 32–35 GPa results in the appearance of Dolomite-IIIc (Dol-IIIc) or Dolomite-IIIb (Dol-IIIb). In the high-pressure XRD study, we found that the XRD patterns of Ca0.97(Mg0.77Fe0.23Mn0.03)(CO3)2 (xFe = 0.23, Ank23) can be indexed as Dol-IIIc at 44 GPa, while the rhombohedral Dol-IIIb structure matches better with the XRD patterns of the xFe = 0.40 (Ank40) and 0.64 (Ank64) solid solutions. Additionally, in the Raman spectra of the Fe-richest sample (Ank64), we observed an abrupt frequency downshift of the CO3-stretching vibrations between 40–42 GPa, which may reflect a pressure-induced Fe2+ spin transition. We further investigated two samples with xFe = 0.19 (Ank19) and 0.23 (Ank23) at high pressure and high temperatures, up to at least 2600 K. The experiments revealed that the unquenchable Dol-IIIc structure could be a stable high-pressure/high-temperature polymorph in ferroan dolomite up to at least 2600 K. [ABSTRACT FROM AUTHOR]

Anisotropy of elastic properties in clay-rich sedimentary rocks has been of long-standing interest. These rocks are cap rocks of oil and gas reservoirs, as well as seals for carbon sequestration. Elasticity of shales has been approached by direct velocity measurements and by models based on microstructures. Here we are revisiting the classical Kimmeridge shale studied by Hornby (1998) by first quantifying microstructural features such as phase volume fractions, grain shapes and grain orientations, and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties. It is shown that by application of a self-consistent algorithm based on Eshelby's (1957) model of inclusions in a homogeneous medium, it is possible to explain most experimental elastic constants, though some discrepancies remain which may be due to the interpretation of experimental data. Using a differential effective medium approach, an almost perfect agreement with experimental stiffness coefficients can be obtained, though the physical basis of this method may be questionable. The influence of single crystal elastic properties, grain shapes, preferred orientation, and volume and shapes of pores on elastic properties of shale is explored. © 2013. American Geophysical Union. All Rights Reserved.

New petrographic and microanalytical studies of mineral inclusions in the Purang ophiolitic chromitites (SW Tibet) are used to scrutinize the evolution of the associated Cretaceous sub-oceanic lithospheric mantle section. Silicate inclusions in the chromite grains include composite and single-phase orthopyroxene, clinopyroxene, amphibole, and uvarovite. Most inclusions are sub-rounded or globular, whereas a few inclusions exhibit cubic/octahedral crystal morphologies. The latter are randomly distributed in the large chromite grains, though discrete aggregates are consistently confined to the grain centers. Abundant micrometer-scale, clinopyroxene inclusions are topotaxially aligned along crystallographic planes. Less-abundant sulfide, wüstite, apatite, and uvarovite inclusions are observed in some samples. The trace element geochemistry of the Purang chromitite evoke parental MORB- and boninitelike melts, consistent with the supra-subduction zone setting. The δ26Mg values of the high-Cr and high-Al chromitites range from –0.25 to –0.29‰ and –0.05 to –0.32‰, respectively. The associated harzburgite has nearly overlapping δ26Mg values of –0.13 to –0.37‰, but pyroxenite sills show distinct δ26Mg values (–0.61 to –0.67‰). The variable Mg isotope signatures, combined with abundant exotic, ultrahigh-pressure and super reduced (UHP-SuR) mineral inclusions in the chromite grains, suggest that recycling and recrystallization under different mantle conditions played an important role in the genesis and evolution of these rocks. Furthermore, discrete silicate, sulfide, and metal alloy inclusions in the Purang chromitites are comparable to those reported in other Tethyan ophiolites, and collectively suggest a common geodynamic evolution. [ABSTRACT FROM AUTHOR]

In order to characterize the [micro]m-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica in both 2D and 3D and compare our evidences with theoretical dissolution precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz-white mica and quartz-K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes, and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite. Amorphous leached layers with kaolinite composition were observed at the boundaries of K-feldspar-quartz and K-feldspar-white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution. We suggest that the nanoscale observations of the quartz-white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stockhert (Tectonophysics 608:1-29, 2013 (See CR35)), while K-feldspar-quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution reprecipitation as described by Hellmann et al. (Chem Geol 294-295:203-216, 2012 (See CR13)). Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1186-8) contains supplementary material, which is available to authorized users., Author(s): Zita Bukovska[sup.1] [sup.2] , Richard Wirth[sup.3] , Luiz F. G. Morales[sup.4] Author Affiliations: (1) Faculty of Science, Institute of Petrology and Structural Geology, Charles University in Prague, Albertov 6, [...]

The transition zone of the Earth's upper mantle is characterized by three discontinuities in seismic wave velocity profiles. One of these at about a depth of 520 km is assigned to the transformation of wadsleyite (β-) to ringwoodite [γ-(Mg, Fe).sub.2SiO.sub.4] (e.g., Shearer in J Geophys Res 101:3053-3066, 1996 (See CR32)). The exact location, width, and other properties of that discontinuity are affected by a multitude of parameters. The present study specifically focuses on the effect of water, iron content, and redox environment on the depth of the phase transition. We performed high-pressure experiments in a multi-anvil apparatus at 1200 °C with variation in Mg-Fe compositions (0.10 < x.sub.Fe < 0.24), water contents (0 < [Formula omitted] < 2 wt%), and the redox environment [using different buffers: Fe/FeO (reducing), Re/ReO.sub.2 (oxidizing)]. Run products were investigated using electron microprobe and Fourier transform infrared spectroscopy to obtain the composition including the hydroxyl concentration of coexisting phases. Mossbauer (MB) spectroscopy and electron energy loss spectroscopy as well as single-crystal X-ray diffraction were applied to gain insight into the Fe.sup.3+ content and incorporation mechanisms. Under hydrous and reducing conditions, the wadsleyite-ringwoodite boundary shifts by 0.5 GPa to higher pressures accompanied by a broadening of the region of coexisting wadsleyite and ringwoodite. In contrast, under hydrous and oxidizing conditions, the two-phase field gets narrower and the shift of the two-phase field to higher pressure is amplified. Thus, the stability field of wadsleyite is extended to higher pressure, most likely due to the higher water and Fe.sup.3+ content in the wadsleyite structure compared to ringwoodite. Based on results from MB spectroscopy and single-crystal X-ray diffraction, we infer that Fe.sup.3+ in wadsleyite is incorporated as a spinelloid component and stabilizes wadsleyite to higher pressures. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1163-2) contains supplementary material, which is available to authorized users., Author(s): Maria Mrosko[sup.1] , Monika Koch-Müller[sup.1] , Catherine McCammon[sup.2] , Dieter Rhede[sup.3] , Joseph R. Smyth[sup.4] , Richard Wirth[sup.1] Author Affiliations: (1) Sektion 3.3, Chemie und Physik der Geomaterialien, Deutsches [...]

The new minerals wenjiite, Ti10(Si,P,D)7 (IMA2019-107c) and kangjinlaite, Ti„(Si,P)10 (IMA2019-112b) occur with badengzhuite, zhiqinite, and a K-bearing dmisteinbergite-like mineral in a spheroid 20 ^m across enclosed in corundum from the Cr-11 podiform chromitite orebody near the Kangjinla, Luobusa ophiolite, Tibet, China. In addition, wenjiite occurs with deltalumite, jingsuiite, osbornite-khambaraevite, and the K-bearing dmisteinbergite-like mineral in a lamellar intergrowth 100 ^m long, also enclosed in corundum from the same locality. The new minerals were characterized by energy-dispersive spectroscopy and three-dimensional electron diffraction, which enabled us to obtain an ab initio structure solution and dynamical refinement from grains a few micrometers across hosted in a FIB lamella. Four analyses of wenjiite from the spheroid gave in wt% Si 21.67, P 6.24, Ti 66.39, V 1.37, Cr 2.20, Mn 0.97, and Fe 1.17 (normalized to 100), which corresponds to (Ti093Cr0.03Mn0.01Fe0.01V0.02)10 (Si0.79P0.21)6.51 on the basis of 10 cations excluding Si and P. The simplified formula is Ti10(Si,P)65, or more generally Ti10SixPy, where x > y and 6 < (x + y) < 7, i.e., Ti10(Si,P,D)7. Wenjiite has hexagonal symmetry, space group: P63/mcm (no. 193), with a = 7.30(10) A, c = 5.09(10) A, V = 235(6) A3, Z = 1, and is isostructural with xifengite, mavlyanovite, synthetic Ti5Si3, and synthetic Ti5P315. Four analyses of kangjinlaite gave in wt% Si 25.56, P 9.68, Ti 62.35, V 0.21, Cr 0.83, Mn 0.42, and Fe 0.95 (normalized to 100), which corresponds to (Ti10.65V0.03Cr0.13Mn0.06Fe0.14)m.01(Si7.43P2.55)S9.99. The simplified formula is Tin(Si,P)10. Kangjinlaite is tetragonal, with space group: I4/mmm (no. 139), a = 9.4(2) A, c = 13.5(3) A, V = 1210(50) A3, Z = 4, and is isostructural with synthetic compounds of the HonGe10 type, being the most compact of these phases. Despite there now being over 70 compounds containing 38 elements isostructural with HonGe10, synthesis of an analog of kangjinlaite has not been previously reported in either the Ti-P or Ti-Si binary systems or in a multicomponent system. The previously deduced crystallization sequence with decreasing temperature of the four minerals in the spheroid is wenjiite ^ kangjinlaite ^ zhiqinite + badengzhuite. This sequence is consistent with relationships reported in 9 binary systems containing intermetallic compounds of Ge and Sn isostructural with Mn5Si3 and HouGe10. In eight of these systems the Mn5Si3 analog melts congruently, whereas the HonGe10 analog never does. Instead, the HonGe10 analog melts peritectically, generally to an Mn5Si3 analog and less commonly to compounds with 5:4 stoichiometry. Final crystallization of the spheroid to zhiqinite + badengzhuite is expected to be well below the temperature of 1500 °C for the congruent melting of zhiqinite in the Ti-Si system, i.e., in the range of ~1100-1300 °C. [ABSTRACT FROM AUTHOR]

We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt or describes cones with opening angle 27.6° around Grt. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with micro-chemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as 'open-system precipitation' (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of [Fe.sup.2+] to [Fe.sup.3+] and transfer from the dodecahedral to the octahedral site just vacated by [Ti.sup.4+] : 6 [M.sup.2+.sub.3]TiAl[Al[Si.sub.2]][O.sub.12] + 6 [M.sup.2+.sub.2,5] TiAl[Si.sub.3][O.sub.12] = 10 [M.sup.2+.sub.3.0][Al.sub.1.8][Fe.sub.0.2][Si.sub.3][O.sub.12] + [M.sup.2+] + [2.sup.e-] + 12Ti[O.sub.2]. OSP is likely to occur at conditions where the transition of natural systems to open-system behaviour becomes apparent, as in the granulite and high-temperature eclogite facies. Keywords Exsolution * Rutile needles * Open-system precipitation * Ultrahigh-pressure * Rhodope, Introduction This paper intends to clarify the genesis of rutile needles in garnet that show a clear and very strict shape preferred orientation (SPO) in 3-4 easily observable directions controlled [...]

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤ 10 µm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-µm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone ([less than or equal to] 90 nm--diagenetic sample, 200-300 nm--hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution-re-precipitation processes at the zircon rim along with precipitation from the respective fluid. Keywords Zircon * Xenotime * Post-depositional processes * Interface, Introduction Several studies have shown that overgrowths of xenotime on detrital zircon can be used to successfully date post-depositional processes, such as diagenesis, hydrothermal overprint, metamorphism or deformation in (meta-)sedimentary [...]