Minerals and Rocks by J. Wilson

By J. Wilson

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They appear to be orthorhombic) crystals. Cruciform twins are a characteristic feature of staurolite (Fig. 2). It is named after the Greek word stauros meaning cross, referring to its cruciform twins. Fig. 4: Formation of staurolite and cordierite relative to the Al2SiO5 polymorphs during metamorphism. These minerals are very useful in the determination of the pressure-temperature conditions during the metamorphism of clay-rich sediments (metapelites). 3. 7 Topaz Topaz (Al2[SiO4](OH,F)2) is an orthorhombic mineral that is often used as a gem stone.

Silicate minerals are dominated by the [SiO4]4- silicate tetrahedron. [SiO4]4- tetrahedra exist independently in some minerals but can share one, two, three or all four oxygen anions in other minerals. This possibility gives a range of silicate structures (Fig. 1). 41 Systematic Mineralogy Minerals and Rocks Fig. 1: Types of SiO4 linkage in silicate minerals The most simple silicate structure involves individual SiO4-units (nesosilicates). The structure becomes more complex as the number of oxygens shared by adjacent SiO4-units increases.

Three of the four oxygens in the SiO4-units are shared to form a sheet structure in phyllosilicates, and all four are shared to form a complex three-dimensional structure in the tectosilicates. g. cations Na and K ; (OH) anions). 1 Olivine group The olivine group consists of two end members: forsterite (Fo) Mg2[SiO4] and fayalite (Fa) Fe2[SiO4] 42 Systematic Mineralogy Minerals and Rocks in which Mg2+ and Fe2+ readily substitute for each other. All intermediate compositions exist. The composition of the olivine group can be written as (Mg,Fe)2[SiO4].

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