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American Mineralogist; December 1978; v. 63; no. 11-12; p. 1107-1115
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Omphacite studies; II, Moessbauer spectra of C2/c and P2/n omphacites

L. P. Aldridge, G. M. Bancroft, M. E. Fleet, and C. T. Herzberg

Univ. West. Ont., Dep. Chem. and Geol., London, Ont., Canada

Mossbauer spectra of two C2/c, seven P2/n, and three P2/n omphacites annealed at 300 degrees to 1000 degrees C and 15 to 18 kbar, have been recorded at 295 K and in some cases at 110 K. The Mossbauer spectra of both C2/c and P2/n omphacites contain three ferrous doublets which have been assigned, on the basis of Mossbauer and compositional evidence, to Fe (super 2+) and in the M1 position. The three M1 doublets arise from next-nearest-neighbour effects caused by different combinations of Ca (super 2+) , Na (super +) cations in the M2, M2(1) positions. The doublets are characterized by Q.S. values of approximately 2.8 mm/sec, approximately 2.2 mm/sec and approximately 1.8 mm/sec, and arise respectively from the Ca (super 2+) , Na (super +) combinations: 1Ca 2Na, 2Ca 1 Na, and 3Ca. The trend in the relative ferrous peak areas with Ca/(Na+Ca) ratios is in agreement with theoretical predictions, and the observed and theoretical relative ferrous peak areas are in good agreement, particularly for C2/c samples. The Mossbauer spectra of samples of P2/n omphacites annealed below and above the P2/n-C2/c transition temperature are very similar. Thus, the nearest-neighbor environment of Fe (super 2+) in omphacite is independent of (Mg,Fe (super 2+) ), Al (super 3+) and Ca (super 2+) , Na (super +) long-range order. The M1 quadrupole splittings in pyroxenes appear to increase greatly with increasing distortion of the M1 site from octahedral symmetry.

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