HR: 16:50h
 AN: MR44A-03    [Abstracts]
 TI: Syntheses, X-ray Diffraction and Raman Spectroscopic Characterization of Clinopyroxenes With 6-coordinated Si in the Na(Mg0.5Si0.5)Si2O6- CaMgSi2O6 join
 AU: Konzett, J
EM: Juergen.Konzett@uibk.ac.at
 AF: Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52, Innsbruck, A-6020, Austria
AU: Lu, R
EM: renlu@email.arizona.edu
 AF: Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States
AU: Frost, D J
 EM: dan.frost@uni-bayreuth.de
 AF: Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstrasse 30, Bayreuth, D- 95447, Germany
AU: * Yang, H
EM: hyang@u.arizona.edu
 AF: Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States
AU: Downs, R T
 EM: rdowns@u.arizona.edu
 AF: Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States
AB: Clinopyroxene capable of accomodating 6-coordinated silicon is of fundamental importance towards our understanding of the crystal chemistry of high-pressure minerals in general and chain silicates in particular, both as a reservoir of Si, and because of the effect on the geophysical properties of the mantle. The present study was initiated to understand how the introduction of the Na(Mg0.5Si0.5)Si2O6 component (NaPx) into diopside (CaMgSi2O6) influences the P2/n-C2/c structural stability and to systematically characterize the crystal chemical effects of [6]Si-substitution in clinopyroxenes. Six clinopyroxenes with different compositions within the NaPx-CaMgSi2O6 join were synthesized with a 1000t-multi anvil device at P-T conditions of 15 GPa and 1500 or 1600 °C:
Run # Chemical composition Symmetry
JKB2002-2: Na(Mg0.5Si0.5)Si2O6P2/n
JKB2005-18: (Ca0.10Na0.88Mg0.01)(Mg0.56Si0.44)Si2O6 P2/n
JKB2006-6: (Ca0.21Na0.78Mg0.01)(Mg0.64Si0.37)Si2O6 P2/n
JKB2004-2: (Ca0.36Na0.56Mg0.08)(Mg0.73Si0.27)Si2O6 C2/c
JKB2005-15: (Ca0.65Na0.30Mg0.05)(Mg0.90Si0.13)Si2O6 C2/c
JKB2005-11: (Ca0.86Na0.08Mg0.06)(Mg0.98Si0.03)Si2O6 C2/c
All samples were analyzed by electron microprobe to determine the chemical compositions, and by X-ray single- crystal diffraction and Raman spectroscopy to obtain detailed structural information. The results show that the the phase transformation between the C2/c and P2/n structures takes place when the Si content is between 27% and 37% of the M1 site occupancy, due to the ordering of Mg and Si into two symmetrically-distinct octahedral sites. Within experimental uncertainties, all unit-cell parameters appear to change continously with increasing Si content in the M1 site, suggesting that the C2/c-to-P2/n transition may be tricritical or second order in nature. Many observable Raman bands in the C2/c structure split into doublets in the P2/n structures, and such splitting is most pronounced in the end-member NaPx phase. The Raman band assignments and variations with chemical compositions are discussed.
DE: 1749 Volcanology, geochemistry, and petrology
 DE: 3600 MINERALOGY AND PETROLOGY
 DE: 3900 MINERAL PHYSICS
 DE: 3934 Optical, infrared, and Raman spectroscopy
 DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
 SC: Mineral and Rock Physics [MR]
 MN: 2007 Joint Assembly