A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often interlayered with white, but is also commonly composed of various shades of gray and white. Since agate is relatively porous, it can be dyed permanently in red, green, blue, and a variety of other colors. The difference in porosity of the adjacent layers permits the dye to penetrate unevenly and preserves marked differences in appearance between layers. The properties of agate are those of chalcedony: refractive indices 1.535 and 1.539, hardness 6.5 to 7, and specific gravity about 2.60.

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab₁₀₀An₀ to Ab₉₀An₁₀, where Ab (= albite) is NaAlSi₃O₈ and An (= anorthite) is CaAl₂Si₂O₈ (see illustration). Albite occurs in crustal igneous rocks as a major component of pegmatites and granites, in association with quartz, mica (usually muscovite), and potassium feldspar (orthoclase or microcline). Sodium and potassium feldspars usually occur as distinct mineral grains, sizes varying from millimeter to meter scale. However, they are frequently intergrown, having exsolved from a single phase at high temperatures. If the intergrowth is visually observable in a hand specimen, the composite material is known as macroperthite; if visible only in a microscope, microperthite; and if submicroscopic in scale, cryptoperthite. In metamorphic rocks albite is found in granitic gneisses, and it may be the principal component of arkose, a feldspar-dominant, sedimentary rock. Cleavelandite, a platy variety, is sometimes found in lithium-rich pegmatites. See also: Arkose; Gneiss; Perthite

A mineral of composition KAl₃(SO₄)₂(OH)₆. Alunite occurs in white to gray rhombohedral crystals or in fine-grained, compact masses. It has a hardness of 4 and a specific gravity of 2.6–2.8.

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin to amber (Fig. 1) is in the millions of years.

A lithium aluminum phosphate mineral of basic formula LiAl(PO₄)(F). The structure of amblygonite consists of phosphate (PO₄) groups of tetrahedra and AlO₆ groups of octahedra. Each PO₄ tetrahedron is connected to an AlO₆ octahedron. Corner-sharing octahedra form zig-zag chains along the b axis. Lithium (Li) is in fivefold coordination, and lies between the PO₄ tetrahedra and nearest AlO₆ octahedra. The dominant substitution in this mineral structure is hydroxyl (OH) for fluorine (F). This substitution gives rise to the amblygonite-montebrasite [LiAlPO₄(OH)] solid solution series. When OH is greater than F, the mineral is known as montebrasite. Appreciable amounts of sodium substitute for lithium in the five-coordinated polyhedra. The sodium-rich varieties such as natromontebrasite and hedronite are rare.

The transparent purple to violet variety of the mineral quartz. Although quartz is perhaps the commonest gem mineral known, amethyst (see illustration) is rare in the deep colors that characterize fine quality. Amethyst is usually colored unevenly and is often heated slightly in an effort to distribute the color more evenly. Heating at higher temperatures usually changes it to yellow or brown (rarely green), and further heating removes all color. The principal sources are Brazil, Arizona, Uruguay, and Russia. Amethyst is often cut in step or brilliant shapes, and drilled or carved for beads. Carvings are made both from transparent and nontransparent material. See also: Gem; Quartz

A supergroup of silicate minerals that have similar chemical compositions and atomic arrangements and occur as rock-forming or accessory constituents in a wide variety of rocks. The crystal structure of amphiboles (Fig. 1) is very chemically compliant and, as a result, the amphiboles show a larger range of chemical composition than any other group of minerals. The structural and chemical complexity of the amphiboles contains considerable information on the geologic processes that have affected the rocks in which they occur. See also: Mineral; Silicate mineral

A class of metamorphic rocks with one of the amphibole minerals as the dominant constituent. Most of the amphibolites are dark green to black crystalline rocks that occur as extensive layers widely distributed in mountain belts and deeply eroded shield areas of the continental crust. Amphibolite is the main country rock that has been intruded by the large granite masses found in most mountain ranges, with small and large masses of amphibolite present also as inclusions in granites. See also: Amphibole; Metamorphic rock; Mineral

A mineral with a framework structure in which all the aluminosilicate tetrahedral vertices are linked, thus allying it to the feldspars, feldspathoids, and zeolites. The structure is cubic, space group Ia3d, a = 1.371 mm. Its formula is Na(H₂O)[AlSi₂O₆]; in this sense it is a tectosilicate.

A nesosilicate mineral, composition Al₂SiO₅, crystallizing in the orthorhombic system. Andalusite (Fig. 1) occurs commonly in large, nearly square prismatic crystals. The variety chiastolite has inclusions of dark-colored carbonaceous material arranged in a regular manner. When these crystals are cut at right angles to the c axis, the inclusions form a cruciform pattern. There is poor prismatic cleavage; the luster is vitreous and the color red, reddish-brown, olive-green, or bluish. Transparent crystals may show strong dichroism, appearing red in one direction and green in another in transmitted light. The specific gravity is 3.1–3.2; hardness is 7.5 on Mohs scale, but may be less on the surface because of alteration.