HR: 17:00h
AN: MR54A-05    [Abstracts]
TI: Natural occurrence of a new mineral with an olivine structure and pyroxene composition in the Tenham L6 chondrite
AU: * xie, z
EM: zhidongx@nju.edu.cn
AF: Department of Earth Sciences, Nanjing University, Nanjing, 210093, China
AU: sharp, t
EM: tom.sharp@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
AU: Leinenweber, K
EM: kurtl@asu.edu
AF: Department of Chemistry, Arizona State University, Tempe, AZ 85287, United States
AU: DeCarli, p
EM: paul.decarli@sri.com
AF: SRI International, 333 Ravenswood Ave, Menlo Park, CA 94025, United States
AB: Here we report a new mineral with an olivine structure and a pyroxene composition, which occurs in shock- induced melt veins of the Tenham L6 chondrite. This new phase was identified with transmission electron microscopy (TEM) using SAED and EDS. It occurs in clusters of acicular crystals in a glassy matrix within shock melt veins. The crystals have a distinctive curvature and aspect ratios up to 25, with width ranging from 5 nm to 20 nm and length up to 500 nm. EDS analyses provide relative cation abundances that are consistent with a pyroxene stoichiometry: Na_0.06Ca_0.02Mg_0.71Fe_0.20Al_0.11Si_0.94O₃. These compositions are similar to that of majorite garnet from the vein center and to that of the vitrified perovskite from the vein edge. Single-crystal and polycrystalline SAED patterns are consistent with the olivine structure and space group (Pbnm). The refined cell parameters for this orthorhombic structure are: a = 0.4782 nm, b = 1.0119 nm, and c = 0.5946 nm. The new olivine-structured phase crystallized either at the rapidly quenched margins of large veins or within thin, less than 0.03 mm melt veins. Our hypothesis is that the extremely rapid quench in these samples led to significant under-cooling of the melt to temperatures below the metastable melting curve of olivine. This allowed the rapid crystallization of the new phase with high-entropy compositional features and unusual morphologies. Our diffraction and EDS data pose a problem of how to accommodate a pyroxene-like composition in an olivine structure. If we write an olivine chemical formula based our EDS data from new phase (Na_0.06Ca_0.04Mg_1.01Fe_0.29Al_0.10Si_0.24)_1.74()_0.26Si_1.00O ₄, the olivine structure requires about 0.25 formula units of Si⁴⁺ in octahedral M1 sites, about 0.25 formula units of vacant M sites (()), and 0.10 formula units of Na⁺ and Ca²⁺ in M2 octahedral sites. Our observations demonstrate that the olivine structure can accommodate significant non-stoicheometry at high pressure and temperature. If non-stoichiometry and excess silica occur in equilibrated olivines, one might expect them to also occur in hot regions of the deep upper mantle. We suggest that it would be useful to investigate the stability of this phase via ab initio calculations and high-pressure experiments.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3625 Petrography, microstructures, and textures
DE: 3662 Meteorite mineralogy and petrology (1028, 6240)
DE: 3924 High-pressure behavior
DE: 6022 Impact phenomena (5420, 8136)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting