The Crystal Chemistry of Mineral Materials

Many mineral species have the same or similar chemical basic units within their atomic structure. All common silicate minerals, for example, are characterized by the association of four large oxygen ions (0 ) bonded to a small silicon ion (Si ). The shape of the complex ion is a tetrahedral unit, with the composition (Si04) . The two- and the three-dimensional expressions of the silicate ion are presented in Fig. 2.1, parts A and B, respectively. The three-dimensional figures emphasize the potential variations in orientation between the ions as they have been observed in minerals. 

Polymerization into (Si40,i) units gives rise to minerals characterized as having double chains. The configuration depicted in Fig. 2. IF is common and typical of the amphiboles, a mineral group of special interest because five amphiboles have been mined as asbestos. 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

When all four oxygens of the tetrahedra are shared (Si 0 = 1 2), a fully polymerized ion results, and a three-dimensional framework is formed. Quartz, an example of this type of silicate array, is discusssed in the section on silica minerals. 

Some of the diversity that characterizes the properties and compositions of the silicate minerals stems from the ability of the aluminum ion (Al ) to substitute for silicon in the tetrahedral unit. When silicate tetrahedra in a mineral are replaced by aluminum-containing tetrahedra, concomitant changes occur in the size of the tetrahedron (usual Si—O bond length = 0.160 nm. A1—O bond length = 0.178 nm) and in the cations or protons that balance the tetrahedral unit charge. Regular substitutions with distinct chemistries and structures lead to the formation of groups of discrete minerals called aluminosilicates. 

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