Omphacite studies; II, Moessbauer spectra of C2/c and P2/n omphacites

Mössbauer spectra of two C2/c, seven P2/n, and three P2/n omphacites annealed at 300° to 1000°C and 15 to 18 kbar, have been recorded at 295 K and in some cases at 110 K. The Mössbauer spectra of both C2/c and P2/n omphacites contain three ferrous doublets which have been assigned, on the basis of Mössbauer and compositional evidence, to Fe²⁺ in the M1 position. The three M1 doublets arise from next-nearest-neighbor effects caused by different combinations of Ca²⁺,Na⁺ cations in the M2, M2(1) positions. The doublets are characterized by Q.S. values of ∼2.8 mm/sec, ∼2.2 mm/sec and ∼1.8 mm/sec, and arise respectively from the Ca²⁺, Na⁺ combinations: 1Ca 2Na, 2Ca 1Na, 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 Mössbauer 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 Fe²⁺ in omphacite is independent of (Mg,Fe²⁺), Al³⁺ and Ca²⁺,Na⁺ long-range order. The M1 quadrupole splittings in pyroxenes appear to increase greatly with increasing distortion of the M1 site from octahedral symmetry.