STRUCTURAL MECHANISMS OF SOLID-SOLUTION AND CATION ORDERING IN AUGITE-JADEITE PYROXENES - I - A MACROSCOPIC PERSPECTIVE

Single-crystal and powder X-ray diffraction data were collected to cha racterize the macroscopic solid-solution and cation-ordering behavior in the system augite-jadeite (low ac-mite content). We examined 28 nat ural pyroxenes with compositions on the join augite-jadeite and with d ifferent degrees of order. Annealing experiments were carried out to o btain crystals with different degrees of order (P2/n) and complete dis order (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(M 2) of the M1 and M2 sites were obtained by a minimization procedure co mbining single-crystal X-ray diffraction data and chemical analyses. F or both C2/c and P2/n pyroxenes, the a, b, c lattice parameters and un it-cell volume, as well as tetrahedral and octahedral mean bond distan ces depend linearly on composition. Only the angle beta of ordered omp hacites slightly deviates from the linear trend of the C2/c samples. T he out-of-plane tilting of the basal face of tetrahedra is sensitive t o the different degrees of order.