Nesosilicates

Nesosilicates - II- NesosilicatesIsolated tetrahedra bound...

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Unformatted text preview: II- NesosilicatesIsolated tetrahedra bound by ionic bonds involving cations as Fe2+, Mg2+and Ca2+.Most nesosilicates occur as equidimensional crystalsCleavage is rare or absentAmount of Al substituting for Si in nesosilicates is minimal.A- Olivine GroupComposition and Structure: X2SiO4Where X could be Fe2+, Mg, Mn2+or Ca, all in octahedral coordination.The two sites occupied by X cations are known as the M1& M2sites. Although Fe and Mg substitute freely for one another in these 2 sites, the M1site is more distorted as a result of tetrahedra sharing edges (Fig. 1). In the mineral Monticellite, Ca, being larger than Fe and Mg prefers the larger less distorted M2site.End-members: (Fig. 2)Forsterite (Fo; Mg) Fayalite (Fa; Fe) Tephroite (Mn)Monticellite: CaMgSiO4Stability of Fo and Fa (Fig. 3)Optical properties: (Figs. 4, 5 & 6)Colorless in thin section, Cleavage: poor in {110}, high relief, n > b, strong birefringence, orthorhombic, 2V large, (82 90), most members biaxial negative, except Fo and Fo rich compositions (100 85% Fo) which are positive. Alter very easily to the Fo and Fo rich compositions (100 85% Fo) which are positive....
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Nesosilicates - II- NesosilicatesIsolated tetrahedra bound...

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