Crystal chemical and structural variations in orthopyroxenes from different petrogenetic environments
Bertolo, Sara; Nimis, Paοlο
European Journal of Mineralogy Volume 5 Number 4 (1993), p. 707 - 720
published: Jul 22, 1993
manuscript accepted: Mar 8, 1993
manuscript received: May 8, 1992
ArtNo. ESP147050504010, Price: 29.00 €
Abstract Seventeen orthopyroxenes belonging to different petrogenetic environments (2 volcanic rocks, 13 granulites, 1 mantle xenolith, 1 synthetic pyrolite) were analysed by means of X-ray diffractometry and microprobe analysis to obtain accurate site occupancies and to define the relationships between structural and chemical variations over a wide range of compositions. The data are combined with earlier analyses of twenty-two orthopyroxenes from volcanic rocks (4), granulites (2), and mantle xenoliths (16). The data show that the geometry of each polyhedron is not only a function of polyhedron cations, but is also constrained by geometric variations in the adjacent sites. Octahedral cation-oxygen distances generally lengthen non-linearly with increasing Mg→Fe* (Fe* = Fe2+ + Mn) substitution. Geometric variations in the tetrahedral chains are ascribed mainly to chemical and geometric variations of the M2 polyhedron and, only for SiB, to the Si ↔ A1IV substitution. Ca in the M2-site induces a lengthening of all M2-O distances and a significant enlargement of the octahedral layer. The enlargement of the octahedral layer due to the Mg → (Fe*,Ca) substitution is positively correlated with the cell volume, and selectively influences the unit-cell parameters; while c is strictly related to M2, b is mainly influenced by M1, and a is affected to the same extent by M1 and M2. Orthopyroxenes from different petrogenetic environments are selectively affected by the different possible cation substitutions.