|Nesosilicates (Greek, nesos, island) are silicates
where the SiO4 tetrahedra form isolated units. The silica tetrahedra are
radicals just like the carbonates or silicates.
Sorosilicates (Greek soros, urn) contain units made up of two tetrahedra that share an oxygen. The resulting formula for this unit is Si2O7.
Nesosilicate units are shown at left, sorosilicate at right. The top diagram shows the atomic arrangement, the bottom diagram is a polyhedral representation, and the middle view shows how the two are related.
|Just like any other mineral containing radicals, the negatively-charged radicals are held together by intervening cations (in red, at left).|
Because the radicals are small and surrounded by cations, nesosilicates and sorosilicates tend to have fairly uniform properties in all directions. Many have no or weak cleavage, and those with good cleavage tend to have only one direction. Crystals are fairly equidimensional or not overly elongated. Hexagonal symmetry is conspicuously absent, and two of the most common tetragonal minerals, zircon and vesuvianite, are nesosilicates. Another important tetragonal mineral, melilite, is a sorosilicate.
You might wonder why, with only isolated silica tetrahedra in the nesosilicates, they aren't mostly isometric. One answer is that in many of them, octahedrally coordinated aluminum cations link to neighbors to form sheets or chains of aluminum octahedra, which are then cross-linked by the silica units. The aluminum structures impart directional properties to the minerals and tend to dominate the structure.
The most common sorosilicates are the epidote group of minerals, which actually contain both SiO4 and Si2O7 radicals. Others include lawsonite, pumpellyite, and melilite.
Created 30 November 2001, Last Update 14 December 2009
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