Silica Minerals that Are Fibrous

Si02 is one of the most abundant compounds in nature. It forms a number of minerals and several varieties whose names are quite familiar agate, car-nelian, sard, amethyst, chalcedony, flint, and chert. All are composed of Si02 with only small or trace amounts of other elements or compounds included during crystallization. In many cases it is the additional components that impart the peculiar color, optical, or physical properties to these minerals. The names are familiar because since ancient times these minerals were used or commonly set in jewelry. All of the names are varieties of the mineral quartz, the stable form of SiOj, formed at ordinary temperatures and pressures. Si02 also forms several polymorphs, which are mentioned below. 

Chalcedony is a variety of quartz composed of fibrils that are not separable. The fibrils may have variable thickness, and lengths, from a few to hundreds of microns, and may be twisted (Fig. 2.19C). The fiber axis, usually perpendicular to the free or growing surface, is easily detected in the crystalline material consisting of tightly packed, parallel or subparallel fibrils. The range of color, and the mechanisms of inclusion of impurities, and especially of water and (OH) in chalcedony, have been described and discussed by many authors, most recently by Frondel (1978, 1982). 

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

The common gray, brown, or black flint result from admixed iron oxides or organic compounds. Pure quartz is usually colorless and transparent. The conchoidal fracture typical of flint was a boon to primitive humans, who desired clean, smooth, sharp edges that were not susceptible to uniform wear. This physical attribute and the densities of flint reflect the patterns of aggregation of its fibrous constituents. 

Tiger s eye is a yellow and brown-banded gemstone, and Hawk s eye is similar but with blue bands. Both come from South Africa and result from silicification of cross-fiber veins of crocidolite. Cut en cabachon, and parallel to the fiber length. Tiger s eye and Hawk s eye exhibit variations in sheen and color that arise from the not-quite-parallel arrangement of the intimately associated asbestos and quartz. 

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