Chain Silicate

Minerals composed of chains of silicon tetrahedra. These minerals are divided into single chain and double chain silicates. The former include pyroxenes and pyroxenoidsSingle-chain silicates like pyroxene, but the tetrahedra composing chains are rotated and twisted. Octahedrally coordinated cations occur between chains as in pyroxenes. The interval of repetition is different for each pyroxenoid (below). The twisting results in lower symmetry than pyroxenes (all pyroxenoids are triclinic) and a splintery cleavage and sometimes and consist of linked (SiO₄)^4– tetrahedra each sharing 2 oxygens with its neighbors. Double chain silicates comprise the amphibole groupComplex family of hydrous double-chain silicate minerals. Amphiboles are common in terrestrial metamorphic and igneous rocks, forming in the presence of water. Except for kaersutite amphibole that is commonly found in Martian shergottites, amphiboles in general are not often found in extraterrestrial materials. For a detailed discussion, click here. and consist of two tetrahedral chains cross-linked by bridging oxygens. The cleavages are different, reflecting differences in underlying chain structures.

Generally, chemically similar pyroxenes and amphiboles have similar color, luster and hardness. However, amphiboles have (OH)^– groups in their structures, which are lacking in pyroxenes, resulting in lower densitiesMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much and refractive indices than pyroxenes with similar cationPositively charged ion. proportions.