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Related to Plagioclase: Potassium feldspar, pyroxene, biotite


A type of triclinic feldspars having the general formula (Na,Ca)Al(Si,Al)Si2O8; they are common rock-forming minerals.
A series in the plagioclase group which can be divided into a number of varieties based on the relative proportion of the solid solution end members, albite and anorthite (An): albite (An 0-10) oligoclase (An 10-30), andesine (An 30-50), labradorite (An 50-70), bytownite (An 70-90), and anorthite (An 90-100). Also known as sodium-calcium feldspar.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


Deer, W. A., R. A. Howie, and J. Zussman. Porodoobrazuiushchie mineraly, vol. 4. Moscow, 1966. (Translated from English.)
Mafunin, A. S. Polevye shpaty—fazovye vzaimootnosheniia, opticheskie svoistva, geologicheskoe raspredelenie. Moscow, 1962.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
The granite consists of plagioclase, quartz and alkali feldspar (exclusively orthoclase) with minor amounts of biotite, muscovite, sericite, an opaque ore mineral and trace amounts of apatite.
Their main rock-forming minerals are plagioclase, hornblende, mica and quartz.
Figure 3(b) shows minerals as high-relief impurities with crystalline habits from anhedral to subhedral; these are represented by quartz, plagioclase, and K-Feld.
In situ Sr-isotope ratios were determined on plagioclase phenocrysts (Figures 7(c)-7(e)), from both domes and enclaves, which preserve evidence of the complex history of interaction between the mafic (i.e., enclaves) and felsic (i.e., rhyodacitic domes) magmas in their growing zones [8].
Feldspars (plagioclase and K-feldspar) and chlorite (clinochlore and daphnite) are predominantly dissolved (anorthite will be excluded from discussion because it was added to fit the low concentration of Ca).
Dygert said in the release that the results support a "crustal overturn" on the lunar surface where the old mixed crust was replaced with young, buoyant, hot deposits of pure plagioclase. The older crust could have also been eroded away by asteroids slamming into the Moon's surface.
The RM mineralogy, as shown by X-ray diffractometry, was dominated by plagioclases and pyroxens with ilmenite, magnetite, apatite and quartz as accessory minerals (Figure 1).
A prediction model was established by using segment wear as the evaluation criterion, and stone properties, including Si[O.sub.2] content, quartz content, orthoclase content, plagioclase content, shore hardness, density, uniaxial compressive strength, and bending strength as input parameters, and FAHP and TOPSIS techniques were employed to evaluate the sawability.
###6La (W)###Medium###Subangular###Moderately###Grain###Siliceous and###Quartz and Plagioclase. alkali
Plagioclase is ignored by the diagram (CaO is not plotted), but we can resolve biotite and garnet because they clearly have different compositions in the AFM plot.