High-Pressure Phase Transition in CaSnOSiO₄ Malayaite

Stephanie Rath (rath@kristall.erdw.ethz.ch), Ronald Miletich & Martin Kunz

Laoratorium für Kristallographie, Sonneggstr. 5, ETH Zürich, Switzerland

The mineral malayaite (CaSnOSiO₄) is isostructural to the monoclinic A2/a aristotype of titanite (CaTiSiO₅). While titanite undergoes several phase transitions as a function of P and T, only a thermal anomaly [2,4,5] is known for Malayaite so far. No pressure dependent data are available as yet. Here we report a pressure-induced phase transition.

A monoclinic-to-triclinic phase transition was observed at a pressure of about 5.0 GPa by means of single crystal X-ray diffraction in a diamond anvil cell. The angles alpha and gamma change from 90° to 89.078(7)° and 91.230(7)°, respectively when increasing pressure from 4.95 GPa (monoclinic) to 7.39 GPa (triclinic). The beta angle decreases linearly between room pressure and 7.39 GPa from 113.352° to 112.745°. At 7.39 GPa the unit cell metric is a = 6.9958(4), b= 8.8080(9) and c= 6.4968(4). Although the alpha and beta angles clearly deviate from 90°, there is no first or second order discontinuity recognizable for volume and lattice vectors.

Up to now we collected 23 data points from room pressure to the maximum pressure of 7.39 GPa. They can be fitted with a 3^rd Birch-Murnaghan Equation of State using a K₀ of 123.1(2) and a V₀ of 389.718(2). K' = 4.0 was fixed at 4.0.

The results of the single crystal diffraction studies, which include calculation on spontaneous strain and high-pressure investigation, will be discussed in detail.

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Order-Disorder in Aluminous Orthopyroxene

Simon A. T. Redfern (satr@cam.ac.uk)¹, Hugh St. C. O'Neill (hugh.oneill@anu.edu.au)² & Kevin S. Knight (ksk@isise.rl.ac.uk)³

¹ Dept Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom

² Research School of Earth Sciences, ANU, Canberra, ACT 0200, Australia

³ ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom

Orthopyroxene has two tetrahedral sites (A, B) and two octahedral sites (M1, M2) over which Al may substitute for Si and Mg respectively. Previous X-ray studies of Al partitioning into enstatite have relied on bond-length differences to infer site occupancies and degrees of order over these sites, since the scattering factors of Mg, Si, and Al are so alike. The neutron scattering lengths of Mg and Al, as well as of Al and Si, are of sufficient contrast to allow site occupancies to be obtained directly from diffraction data. We have exploited the unique characteristics of the HRPD instrument at ISIS to refine the crystal structures, including site occupancies, of synthetic aluminous orthopyroxenes. Three samples have been synthesised as approximately 5 g ceramic slugs (15 kbar, 1400^oC, single synthesis for each composition) at ANU, with compositions of 90:10, 91:9, and 94:6 wt% MgSiO₃:Al₂O₃. Neutron powder diffraction data were collected using the high-resolution time of flight instrument, HRPD, at the ISIS neutron spallation source, Rutherford Appleton Laboratory. Structures were refined by the Rietveld method using chemical constraints, to R-factors of around 3%. The results for M-site partitioning show that in both the 9 and 10 wt% Al₂O₃ samples Al is around 88% ordered onto M1, while for the 6% sample Al is 85% ordered onto M1. Both the refined T-site occupancies and the <T-O> bond lengths indicate that Al is almost completely ordered onto B. These results are in fair agreement with the model proposed by Wood and Banno (1973), and contrast with alternative suggestions that Al disorders significantly over the M-sites. They provide the first direct evidence of the degree of octahedral ordering of Al in orthopyroxene, and may now be used to constrain thermodynamic models for the free energy of this important mantle phase.

Wood BJ & Banno S, Contrib. Mineral. Petrol., 42, 109-124, (1973).

Intracrystalline Partitioning of Mn²⁺ in Amphiboles

James J. Reece (jree99@esc.cam.ac.uk)¹, Mark D. Welch (m.welch@nhm.ac.uk)², Simon A. T. Redfern (satr@cam.ac.uk)¹ & Michael Henderson (michael.henderson@man.ac.uk)³

¹ Department of Earth Sciences, Cambridge University, Downing Street, Cambridge., CB2 3EQ, England

² Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, England

³ Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, England

Intracrystalline partitioning of Mn²⁺ has been studied in three related amphiboles: two dannemorites D1 and D2 from the Swedish Bergslagen orefield, with compositions Ca_0.1Mn_1.9Mg_1.25Fe²⁺_3.56Fe³⁺_0.38Si_7.81O₂₂(OH)₂ and Ca_0.24Mn_1.57Mg_2.27Fe²⁺_2.57Fe³⁺_0.32Si_7.84O₂₂(OH)₂, respectively, and a synthetic tirodite Mn_2.3Mg_4.7Si₈O₂₂(OH)₂. These amphiboles allow the effects of Mg:Mn²⁺:Fe²⁺ upon site partitioning of Mn²⁺ to be examined.

The room-temperature infrared spectrum of the tirodite suggests that octahedrally-coordinated Mn²⁺ is distributed statistically over M(1), M(2) ± M(3) sites. The IR spectra of the dannemorites in the OH stretching region comprise three (D1) or four (D2) peaks corresponding to the O(3) cation triplets MgMgMg (D2 only), MgMgFe²⁺, MgFe²⁺Fe²⁺ and Fe²⁺Fe²⁺Fe²⁺ with relative intensities that indicate a statistical Mg/Fe²⁺ distribution over all the octahedral sites, and the presence of a MgMgMn²⁺ contribution to the MgMgFe²⁺ peak implies Mn²⁺ at M(1) and/or M(3). There is no Fe³⁺ at M(1) or M(3).

The composition of D1 is consistent with the amphibole crystallizing during low-T metamorphism of the Mn-ore, with Mn fully ordered at M(4). However, our neutron powder-diffraction studies of D1 and D2 indicate that there is considerable Mn²⁺-Fe²⁺ exchange between M(4) and the octahedral sites in both dannemorites. The neutron results point to Mn-Fe²⁺ disordering during a second metamorphic episode.