Abstract

Single-crystal and powder X-ray diffraction data were collected to characterize the macroscopic solid-solution and cation-ordering behavior in the system augite-jadeite (low acmite content). We examined 28 natural pyroxenes with compositions on the join augite-jadeite and with different degrees of order. Annealing experiments were carried out to obtain crystals with different degrees of order (P2/n) and complete disorder (C2/c) at compositions between 35 and 60% Jd. Three synthetic C2/c pyroxenes with composition Di 80 Jd 20 , Di 60 Jd 40 , and Di 50 Jd 50 were also examined. The long-range order parameters Q M1 and Q M2 of the M1 and M2 sites were obtained by a minimization procedure combining single-crystal X-ray diffraction data and chemical analyses. For both C2/c and P2/n pyroxenes, the a, b, c lattice parameters and unit-cell volume, as well as tetrahedral and octahedral mean bond distances depend linearly on composition. Only the angle beta of ordered omphacites slightly deviates from the linear trend of the C2/c samples. The out-of-plane tilting of the basal face of tetrahedra is sensitive to the different degrees of order.

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