Cation disordering in ankerite as a function of Fe content

Order–disorder transitions in minerals are of significance for technological applications and for the development of models that aid the understanding of the dynamics and composition of the Earth's interior. The present study investigates the effect of Fe content in ankerite, Ca(FexMg1−x)(CO3)2 (0 ≤x≥0.7, R3¯ space group), on the distribution of cations in its crystal structure as a function of temperature. This investigation was conducted using ex situ experiments in a piston cylinder apparatus performed at 2–3 GPa and variable-temperature conditions (450–1000 °C). Crystal structure refinements, using single-crystal X-ray diffraction data, indicate that the temperature of the order–disorder phase transition in ankerite, when the space group changes from R3¯ to R3¯c, is significantly influenced by the amount of Fe in the mineral's crystal structure, being full disordering conditions attained at 1000 and 800 °C in ankerites with x=0.3 and x=0.7, respectively. Prior to undergoing the order–disorder phase transition, it is shown that Fe exhibits a greater aptitude than Mg to exchange in the place of Ca (and vice versa). Mg, conversely, has a tendency to be bound at the M2 site or to exchange in smaller quantities than Fe. Furthermore, the significance of Fe as a parameter influencing the chemo-physical behavior of ankerite, as well as the temperature and character of the disordering process, is highlighted. This has the potential to significantly impact the mineral physics of ankerite under non-ambient conditions, particularly with regard to compressibility, phase stability, thermal and electric conductivity, and its role in the Earth's mantle geophysical modeling.</p

Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1070K

The effects of temperature on the crystal structure of a natural epidote [Ca1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10. 50(6) \uc5 and \u3b2 = 115.36(5)\ub0, Sp.Gr. P21/m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)\ub7\ub7\ub7O(2) and O(10)-H(1)\ub7\ub7\ub7O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)\ub7\ub7\ub7O(4) is energetically more favourable, as H(1)\ub7\ub7\ub7O(4) = 1.9731(28) \uc5, O(10)\ub7\ub7\ub7O(4) = 2.9318(22) \uc5 and O(10)-H(1)\ub7\ub7\ub7O4 = 166.7(2)\ub0, whereas H(1)\ub7\ub7\ub7O(2) = 2.5921(23) \uc5, O(10)\ub7\ub7\ub7O(2) = 2.8221(17)\uc5 and O(10)-H(1)\ub7\ub7\ub7O2 = 93.3(1)\ub0. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 \uc5. The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)1,070K = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)1,070K = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)\ub7\ub7\ub7O(4) and H(1)\ub7\ub7\ub7O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed

Improved calibtration curve for the Sm²⁺:BaFCl pressure sensor

The pressure sensor proposed by Shen, Gregorian &amp; Holzapfel [High Press. Res. (1991), 7, 73–75], based on the pressure-dependent shift of the luminescence line due to the 5 D 0−7 F 0 electronic transition of Sm2+ in a matrix of BaFCl, has been tested in a diamond-anvil cell and calibrated against the R 1−R 2 doublet shift of ruby and the known NaCl equation of state, in the pressure range between 0.0001 and 4.3 GPa. The parabolic dependence of the shift from the pressure can be approximated by the equation Δ(nm) = 1.46P − 0.047P 2, where the shift, Δ, is in nm and the pressure, P, in GPa. The estimated error in the pressure measurements is 5%. The Sm2+: BaFCl luminescence sensor can be advantageously used in the low to moderate pressure range (0.0001–5 GPa or more).</jats:p

New insights on high-pressure behaviour of microporous materials from X-ray single-crystal data

The main deformation mechanisms induced by pressure on different structural types of zeolites were analysed by comparing experimental data and theoretical models. Data of single-crystal X-ray diffraction obtained with the sample in a Merrill-Bassett diamond anvil cell on a four-circle diffractometer were collected at different pressures for samples of heulandite, scolecite and bikitaite, using non-penetrating pressure transmitting media (glycerol or silicon oil), up to 5 GPa. The results indicated that, at first approximation, the theoretical approach reproduces the structural evolution of zeolites under pressure. However, the flexibility possessed by framework microporous silicates resulted more complex than that which can be modelled by undeformable "rigid-unit modes", being completely flexible in the oxygen hinges. Moreover, the compressibility of the zeolites under study does not appear to be directly related to the microporosity represented by the framework density (FD): The bulk moduli (simply defined as the inverse of volume compressibility coefficients) of heulandite (27.5(2) GPa) and scolecite (54.6(3) GPa) were different even though their FD's were quite similar. Single crystal data have shown that the structural evolution of the open-framework silicates, is strongly controlled by the framework, whereas the role of the extra-framework content was less important. In all three zeolites the position of the extra-framework water molecules and cations was maintained approximately and their coordination numbers remained unchanged within the pressure range investigated