In situ high temperature single crystal X-ray diffraction study of a natural omphacite

Abstract

In situ high temperature single crystal X-ray diffraction (XRD) experiments have been performed on a chemically quasi-ideal omphacite sample [(Ca 0.49 Na 0.51 ) S1 (Mg 0.46 Al 0.48 Fe 3+ 0.06 ) S1 (Si 1.97 Al 0.03 ) S2 O 6 ], up to 10008C. The lattice parameters were studied as a function of temperature, and their thermal expansion coefficients determined. The b and c cell edges show discontinuities as a function of temperature which are interpreted here in terms of intracrystalline cation diffusion processes. Structure refinements have been carried out using data collected at room temperature, at 8008C and at ambient conditions after cooling. The structural behaviour as a function of temperature of chemically quasiideal omphacites is compared with those of jadeite and diopside.

The effect of cation order on the elasticity of omphacite from atomistic calculations

“…As expected, since the elastic constants depend on 301 the second derivative of the internal energy a n d a r e c o n s e q u e n t l y m o r e s e n s i t i v e t o m o d e l 302 deficiencies, some of the elastic constants calculated in GULP (Table 2) deviate significantly from 303 the DFT calculations presented in Table 1. One of the most obvious differences is the anomalously 304 high values of C 13 and C 33 found for all three minerals, Given that the stiffness of the c-axis is 305 controlled by chains of SiO 4 tetrahedra (Pavese et al 2000), this suggests that the charge of the Si 306 atoms (+3.4) may be somewhat too large. Conversely, the diagonal shear constants C 44 , C 55 , and C 66 307 are all somewhat underestimated, particularly for jadeite and P2/n omphacite.…”

“…In particular, the fact that C 11 , C 12 , and C 66 depend on the order parameter of the M1 site, 391 but not that of the M2 site is likely due to the moderate sensitivity of the a cell parameter to the 392 occupancy of the M1 sites (Pavese et al 2000). Conversely, since the stiffness of the c axis is 393 controlled by the chains of SiO 4 tetrahedra (Pavese et al 2000), C 33 should have little direct 394 dependence on the occupancies of either the M1 or M2 cation sites. This is in accord with the 395 results presented in Table 2 and Fig.…”

New Chemical Data on the Clinopyroxene-Garnet Pair in the Alpe Arami Eclogite, Central Alps, Switzerland

“…The first author (Bocchio 1975, 1977, Bocchio et al 1978 focussed on bulk composition and mineral chemistry, and Rossi et al (1978) and Pavese et al (2000Pavese et al ( , 2001 investigated the crystallography of the clinopyroxene. In the present work, the rare-earth elements (REE) and selected trace elements in the bulk rock were investigated by the ICP-MS technique at CRPG-CNRS (Vandoeuvre-lès-Nancy, France).…”

“…Handpicked separate grains of garnet and clinopyroxene, embedded in epoxy resin, were analyzed in situ for major and minor elements with an ARL EMX-SEMQ electron microprobe (EMP) at CNR-IDPA (Milan), and for trace constituents (REE, Sc, Ti, V, Cr, Sr, Y, Zr) with a Cameca IMS 4f ion microprobe installed at CNR-IGG (Pavia). The analytical methods are described in Bocchio et al (2000).…”

“…The concentration of the first transition-series trace elements Co, Ni, and Cu have been determined on purified concentrates of the two minerals by atomic absorption spectrometry using the FAA and GFAA techniques, at the University of Milan; see Bocchio et al (2000) for details. Additional information was derived from O isotope analyses performed on the bulk rock and separate grains of the two minerals with the BrF 5 method using a MAT-251 EM mass spectrometer at the Institute of Mineral Resources of Beijing (China).…”