Light interacts with matter in several ways. A ray of light directed at an opaque substance, that is, having metallic luster, is reflected totally (see Ore Microscopy). Light incident upon nonopaque substances-gas, liquid, or solid-is reflected, refracted, and absorbed (see Color in Minerals).
Refractive Index
The velocity of light (v) while traveling through a substance is less than its velocity in vacuo (c = 3 × 1010 cm/sec). The refractive index (n) of a nonopaque material is the ratio of the velocity of light in vacuo to its velocity in a material (n = c/v). Most minerals have refractive indices (RI) between 1.4 and 2.5 (see Refractive Index). Isotropic materials (see Isotropism ), which include gases, most liquids, most glasses, and minerals with isometric symmetry (see Symmetry) are characterized by a single RI (Table 1). Because of variations in chemical composition, some minerals exhibit a range of RI.
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References
Hartshorne, N. H., and Stuart, A., 1969. Practical Optical Crystallography, 2nd ed. New York: American Elsevier, 326p.
Kerr, P. F., 1977. Optical Mineralogy, 4th ed. New York: McGraw-Hill, 492p.
Phillips, W. R., 1971. Mineral Optics: Principles and Techniques. San Francisco: Freeman, 249p.
Phillips, W. R., and Griffen, D. T., 1981. Optical Mineralogy: The Nonopaque Minerals. San Francisco: Freeman, 677p.
Shelley, D., 1975. Manual of Optical Mineralogy. Amsterdam: Elsevier, 239p.
Cross-references
Anisotropism; Dispersion, Optical; Crystallography, Morphological; Isotropism; Optical Mineralogy; Symmetry.
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© 1981 Hutchinson Ross Publishing Company
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Frye, K. (1981). Minerals, uniaxial and biaxial . In: Mineralogy. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30720-6_83
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