Astron. Astrophys. 339, 904-916 (1998)

2. Structure of crystalline silicates

The silicates studied in this paper belong to the mineral groups of olivines and pyroxenes. The olivine group consists of silicates with the general sum formula . A and B are divalent cations, the most abundant of which are Mg, Fe, Mn, Co and Zn. In this paper we concentrate on the olivines in which the cations Mg and Fe dominate. They replace each other in the crystal: that means, these olivines can be considered as solid solutions of Mg₂SiO₄ and Fe₂SiO₄; the mixture ratio can be expressed by the subscript x with in the general formula . The end members of the isomorphous series, Mg₂SiO₄ (x=1) and Fe₂SiO₄ (x=0), have the mineral names forsterite and fayalite, respectively. The mineral "olivine" contains 8 to 20 mass percent FeO. Minerals with higher FeO contents are called hortonolite and ferrohortonolite. The olivines are neso-silicates (island silicates) because they consist of isolated SiO₄ tetrahedra, the basic structural unit of all types of silicates, which are linked by divalent cations. The metal ions are coordinated by six oxygens. There are two nonequivalent six-coordinate positions in the olivine structure which are both distorted (Burns 1970). According to the lattice symmetry, the olivines belong to the rhombic crystal system. In the solid solution series of olivines, the increasing Fe incorporation leads to larger metal-oxygen distances in the crystal.

The pyroxenes form a large group of minerals belonging to the inosilicates (chain silicates). The two SiO₄ tetrahedra form chains, i.e. each tetrahedron shares two of its oxygens with the neighbours. However, in chemical sum formula often the simple expression MeSiO₃ is used (Me means metal ion). The direction of the chains is the crystallographic c-axis. Like in the case of the olivines, we focus our interest on pyroxenes with the cations Mg and Fe and use the symbolic formula . There are two types of SiO₄ tetrahedral chains with different Si-O distances. The metal ions in the Mg-Fe-pyroxenes linking the SiO₄ chains are coordinated by six oxygens. There are also two different types of distorted MeO₆ octahedra. Like in the case of the olivines the Me-O distances increase with growing FeO content. The pyroxenes also form solid solution series. The Fe-free and Mg-free end members are enstatite (x=1) and ferrosilite (x=0), respectively. In contrast to olivines, pyroxenes occur in two different main crystallographic systems. The lattice can be of rhombic or monoclinic structure. The rhombic crystal structure is produced by twinning of the unit cell of clinopyroxene by operation of a b-glide parallel to (100)-plane. Pyroxenes containing cations with radii considerably larger than that of Mg, e.g. Ca, belong to the monoclinic system and are called clinopyroxenes. Under extreme formation conditions, monoclinic Mg-Fe pyroxenes can also arise, e.g. clinoenstatite. Synthetic Mg pyroxenes produced via melting usually are of this type. However, Mg-Fe clinopyroxenes also occur in meteorites. The most common Mg-Fe pyroxenes, the so-called orthopyroxenes, belong to the rhombic system. Orthopyroxenes with an FeO content between 5 and 15 mass % have the mineral name bronzite, orthopyroxenes with larger FeO content are called hypersthene. Ferrosilite does not occur as a natural mineral.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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