Plagioclase


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Related to Plagioclase: Potassium feldspar, pyroxene, biotite

plagioclase

[′plā·jē·ə‚klās]
(mineralogy)
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.

Plagioclase

 

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.

REFERENCE

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.
References in periodicals archive ?
The Black Polished sherd Samall revealed very different fabric elements such as abundant coarse-grained granite and granodiorite fragments associated with K-feldspar, perthite, plagioclase and abundant quartz.
In a microscopic study of the sample ZMT--375 that is removed from this zone plagioclases of the rock became sosority and changed to the set of epidote and albite.
The rocks correspond to actinolite schists and chlorite schists (Figure 3a and 3b) with mineral assemblages as follows: actinolite + chlorite + quartz + plagioclase (albite) + epidote minerals [+ or -] muscovite [+ or -] calcite.
CHEMIN RESULTS REVEALED: NASA's Curiosity rover gets its first taste of Mars and finds minerals: plagioclase feldspar, pyroxene and olivine.
These clasts are essentially ophiolitic consisting of serpentine, olivine, pyroxenes, plagioclase and amphiboles with minor of cherty made up of microquartz and mega quartz.
This endorsed the magma ocean hypothesis, which states that a large amount of energy liberated during the Moon's formation caused its surface to become completely molten, letting the plagioclase rise to the surface because of its lighter density.
It was established that quartz, plagioclase and pyroxene made the most positive influence on the resistance of clay roofing tiles to frost.
These organisms create minute bore holes heading toward olivine microcrystals while avoiding plagioclase feldspar, as Roger Buick (2008) puts it, "like the plague" (p.
The sills contain small pseudomorphs of olivine, plus plagioclase and clinopyroxene phenocrysts.
The gneiss consists of alternating amphibole + plagioclase and epidote + quartz layers.
For less-altered saprolite, the kaolin formed from plagioclase fills fractures and voids between and within residual alkali feldspar and quartz grains (Kew et al.
Just like the rocks on Earth, Moon's have lots of basalt and olivine, pyroxene, and mafic breccias, along with plagioclase feldspar, also known as anorthosite.