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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.

A synthetic CaCO 3 – SrCO 3 solid solution with composition Ca 0.82 Sr 0.18 CO 3 was investigated by single-crystal X-ray diffraction in the pressure range between 0 and 22 GPa using different pressure-transmitting media. The samples were compressed in DACs using Ne up to ∼ 9 GPa and Ar up to ∼ 22 GPa. At ambient conditions, Ca 0.82 Sr 0.18 CO 3 crystallizes in a monoclinic structure, isostructural to CaCO 3 -II, Sr-calcite-II (Sr-CC-II), with space group P 2 1 / c , 4 formula units per unit cell, Z, a = 6.4237 (7) Å, b = 5.0176 (1) Å, c = 8.1129 (1) Å, β = 108.064 (1) ∘ and V = 248.60 (1) Å 3 (where the number in parenthesis is 1 σ uncertainties on the last digit). At 1.72(5) GPa, a structural phase transition is observed to a new monoclinic structure, Sr-calcite-IIIc (Sr-CC-IIIc), with space group P 2 1 / m and Z = 8 ( a = 6.2683 (2) Å, b = 9.9220 (5) Å, c = 7.6574 (6) Å, β = 103.856 (6) ∘ and V = 462.39 (5) Å 3 ), different from any pure CaCO 3 polymorph. At 12 GPa, the sample transformed to a triclinic structure, Sr-calcite-IIIb (Sr-CC-IIIb), with space group P 1 ¯ and Z = 4 ( a = 6.059 (5) Å, b = 6.280 (2) Å, c = 6.331 (2) Å, α = 95.20 (3) ∘ , β = 108.89 (5) ∘ , γ = 110.52 (5) ∘ and V = 207.7 (2) Å 3 ), isostructural to end-member CaCO 3 -IIIb. Finally, at 17 GPa, a transition is observed to Sr-calcite-VI (Sr-CC-VI), with space group P 1 ¯ and Z = 2 ( a = 3.444 (3) Å, b = 4.985 (4) Å, c = 5.761 (5) Å, α = 77.05 (7) ∘ , β = 84.92 (7) ∘ , γ = 89.00 (7) ∘ and V = 96.0 (1) Å 3 ), isostructural to end-member CaCO 3 -VI, which is preserved up to the maximum investigated pressure of 22 GPa. The results of this study show the effect of Sr/Ca cationic substitution on the high-pressure behavior and physical properties of a CaCO 3 – SrCO 3 solid solution. The phase evolution of Ca 0.82 Sr 0.18 CO 3 and the crystallization of a new phase, Sr-CC-IIIc, different from the high-pressure polymorphs of end-member CaCO 3 , point to the importance of extending the study of carbonates to more complex systems than pure end-member compositions. [ABSTRACT FROM AUTHOR]