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Elastic behaviour and P-induced structure evolution of clintonite and B-mullite

G.D. Gatta1, M. Merlini1, H.P. Liermann2, and A. Rothkirch2

1 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy 2DESY, HASYLAB, PETRA III, Notkestr. 85, D-22607 Hamburg, Germany

Clintonite (often called xanthophyllite) is a rare trioctahedral brittle (Fig. 1) with ideal composition Ca(Mg2Al)(Al3Si)O10(OH)2, which forms as a result of thermal of Ca- and Al-rich, Si-poor rocks. It was found in chlorite schists, in metasomatically altered , or in siliceous in proximity to the near-contact metamorphic zones. A series of studies have been devoted to the crystal chemistry, phase stability of clintonite and petrological implications. From a crystallochemical point of view, an interesting feature of clintonite is in the violation of the Loewenstein's aluminum avoidance rule, due to the Al/Si (a.p.f.u.)>2. To the best of our knowledge, no experiment has so far been devoted to the high pressure (HP) and high temperature behavior of brittle , and so no thermo-elastic data are available for this class of materials. In this light, the elastic behavior of a natural clintonite-1M from the metamorphic complex of Lago della Vacca, Adamello, Italy, [with composition: Ca1.01(Mg2.29Al0.59Fe0.12)Σ3.00(Si1.20Al2.80)Σ4.00O10(OH)2] has been investigated up to 10 GPa (at room temperature) by means of in-situ synchrotron single-crystal diffraction at beamline P02.2 (Extreme Conditions Beamline) at DESY/PETRA III, using X-rays with an energy of 42.7 keV (0.29036 Å wavelength) and a focusing spot of ~ 2.0 (H) x 1.8 (V) μm² originating from a 320 mm KB mirror system. Sample detector distance of 401.34 mm was calibrated using a CeO2 standard (NIST 674a). A single crystal of clintonite (~ 50 x 50 x 15 μm) was loaded in a Boehler Almax type diamond anvil cell (DAC) with a 70° opening and equipped with diamonds of 400 μm culets size. A 250 μm thick rhenium gasket was indented to 40 μm and drilled with 196 μm hole that contained the sample, a mixture of methanol:ethanol = 4:1 as pressure transmitting medium and some calibrated ruby spheres for pressure determination. Pressure was increased mechanically through the Boehler Almax gear device and measured with the offline ruby/alignment system located in the laser lab of the Extreme Condition Science Infrastructure (ECSI) at PETRA III. Diffraction images were acquired on a MAR345 online image plate using an in house script for collecting step-scan diffraction images. MAR345 image files were collected with a pixel resolution of 150 x 150 microns to reduce data collection time. After collection, the images were converted with an in house software script to conform to the standard format of the program CrysAlis [1]. The diffraction data were first collected at ambient pressure, with the crystal in the DAC and without any P-medium. A pure ω-scan (-30 ≤ ω ≤ +30°), with a step size of 1° and a time of 1 s/frame was used to maximize the accessible reciprocal space portion. Intensities were integrated and corrected for Lorentz-polarization effects, using the CrysAlis package. The reflection conditions were consistent with those of the space group C2/m according to previous experimental findings [2]. The isotropic structural refinement was conducted using the SHELX-97 software. HP data collections at different pressures were performed, adopting the same experimental set-up, strategy and data treatment as those used at 0.0001 GPa. No evidence of phase transition, and no violation of the reflection conditions of the C2/m symmetry, was observed within the P-range investigated. P-V data fitted with an isothermal third-order Birch-Murnaghan Equation of State (BM-EoS) give: V0= 3 457.1(2)Å , KT0= 76(3)GPa and ∂KT0/∂P= 10.6(15). The evolution of the “Eulerian finite strain” vs “normalized stress” (i.e. Fe) shows a linear positive trend. The linear regression yields Fe(0) = 76(3) GPa as intercept value. The evolution of the lattice parameters with pressure is significantly -1 -1 anisotropic [β(a) = 1/3KT0(a) = 0.0023(1) GPa ; β(b) = 1/3KT0(b) = 0.0018(1) GPa ; β(c) = -1 1/KT0(c) = 0.0072(3) GPa ]. The β-angle increases in response to the applied P, with: βP = β0 + 0.033(4)P (P in GPa). The structure refinements of clintonite up to 10.1 GPa show that under hydrostatic pressure the structure rearranges by compressing mainly isotropically the inter-layer Ca-polyhedron. The bulk-modulus of the Ca-polyhedron, described using a second order-BM-EoS, is KT0(Ca-polyhedron) = 41(2) GPa. The compression of the bond distances between and the basal oxygens of the tetrahedral sheet leads, in turn, to an increase of the ditrigonal distortion of the tetrahedral ring, with ∂α/∂P≈ 0.1 °/GPa within the P-range investigated. The Mg-rich octahedra appear to compress in response of the applied pressure, whereas the tetrahedron appears to behave as a rigid unit. A manuscript with title “On the thermo-elastic behavior of clintonite up to 10 GPa and 1000 °C” by Gatta et al [3], based on the data of this experiment is in press. A second experiment aimed to extend a previous investigation of the HP elastic behavior and P- induced structure evolution of Al5BO9 (i.e. B-mullite). Al5BO9 is a ceramic material with outstanding technological features. Gatta et al. [4] investigated its elastic behavior and P-induced structure evolution up to 7 GPa on a conventional lab diffractometer. The materials was found to be significantly stiff (KT0 = 165(7) GPa), hindering a reliable description of the P-response on the basis of data collected in a conventional lab. Thus, we performed additional single-crystal X-ray diffraction at the beamline P02.2 at P>10 GPa to extend the compression study. A symmetric type DAC with a sloted WC seat on the downstream side and a cBN seat on the upstream side was used for these single crystal diffraction experiments. Quasi-hydrostatic condition was achieved by loading the gasket hole with the sample with Ne as P-transmitting medium using the Gas Loader of the ECSI. The experiment was conducted adopting the same protocol as that used for clintonite. The data collected up to 25 GPa show no evidence of phase transition. However, because of the limited access to reciprocal space in used DAC, only low-quality structure refinements at high- pressure were possible. We will perform additional high-pressure experiments in a symmetric DAC that was retrofitted with the Boehler Almax seeds or a Boehler Almax DAC (both have an opening of 70°) to gain more access to reciprocal space to improve the quality of the data refinements.

Figure 1: The of clintonite and a view of the ditrigonally distorted 6-membered rings of tetrahedra.

References

[1] Oxford Diffraction (2010) Oxford Diffraction Ltd., Xcalibur CCD system, CrysAlis Software system. [2] E. Alietti, M.F. Brigatti, and L. Poppi, Am. Mineral. 82, 936 (1997). [3] G.D. Gatta, M. Merlini, H.P. Liermann, A. Rothkirch, M. Gemmi, and A. Pavese, Phys. Chem. Minerals (in press, DOI: 10.1007/s00269-012-0493-0) 2012. [4] G.D. Gatta, N. Rotiroti, M. Fisch, and T. Armbruster, Phys. Chem. Minerals 37, 227 (2010).