a widely distributed group of rock-forming minerals that belongs, based on the crystal chemical structure, to the framework aluminosilicates—the feldspar group. According to chemical composition, plagioclases form a continuous isomorphous series of sodium-calcium aluminosilicates—albite Na[AlSi3O8] and anorthite Ca[Al2si2O8] unlimited mis-cibility. Plagioclases sometimes contain admixtures of K2O (up to several percent), BaO, SrO, FeO, and Fe2O3. In accordance with E. S. Fedorov’s proposal, plagioclase composition is designated by numbers that represent the percent content of the anorthite component. For example, plagioclase no. 72 is an isomorphous mixture containing 72 percent anorthite and 28 percent albite. Plagioclases are also assigned specific names corresponding to the numbers: albite (0–10), oligoclase (10–30), andesine (30–50), labradorite (50–70), bytownite (70–90), and anorthite (90–100). The relative silicic acid content decreases with an increase in the anorthite component content in the isomorphous plagioclase series. Accordingly, plagioclases nos. 0–30 are called acid, nos. 30–50 neutral, and nos. 50–100 basic.
Plagioclases crystallize in the triclinic system, and the crystal grains usually form complex twins. The properties of these minerals vary considerably, depending on the composition and the degree of ordering of Al-Si in the structure. Proceeding from pure albite to pure anorthite, the density increases from 2,620 to 2,760 kg/m3; the hardness on Mohs’ scale, from 6 to 6.5; the refractive index, from 1.53 to 1.58; and the melting point, from 1100° to 1550°C. The plagioclase number— that is, its composition—is determined, using a polarizing microscope with a Fedo-rov, or universal, stage, by studying the refractive indexes, the optical axis angle, the position of optical indicatrices, the laws of twinning, and other optical properties. Then, special diagrams showing the relationship between the properties and composition of plagioclases help determine the number.
Most plagioclases are formed during the crystallization of magma. Plagioclases are the principal rock-forming minerals found in magmatic rocks. They also occur in contact-metamorphic formations (scarns, hornfels) and in hydrothermal veins (albite). Weathering causes plagioclases to alter to hydromica, minerals of the epidote group, and clay minerals, for example, kaolinite and montmorillonite. Oligoclases with an iridescent play of color, for example, azure, blue, or gold (called moonstones), and labradorite are gem minerals.