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Classification
Crystal System: Triclinic
Class: Silicate
Sub Class: Tectosilicate (framework-silicate)
Group: Feldspar
Subgroup: Plagioclase Feldspar
Series: Albite-Anorthite (Ca end member)
Structure and Composition
Composition: CaAlSi3O8
Structure: The core building block of orthoclase is the silicate tetrahedron (SiO4), which in its framework each oxygen in the tetrahedron is shared with another adjacent tetrahedron. Aluminum substitutes for silicon in some of the tetrahedrons, leading to sodium cations being included in the structure to balance the overall charge of the mineral.
In the structure on the right, the red spheres are oxygen, the grey aluminum, and the green calcium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Colorless, reddish grey, white
Luster: Vitreous
Hardness: 6-6.5
Streak: White
Specific gravity: 2.74 - 2.76
Common Habit: Tabular crystals
Cleavage: Perfect on {001}, less so on {010}, imperfect on {110}
Identification In Thin Section
PPL Properties
Relief: Low-moderate negative/positive (Albite-Anorthite series)
Habit/Form: Plagioclase minerals commonly occur both as euhedral and anhedral grains. Crystals are usually tabular parallel to (010) and elongate parallel to the c or a axis. Cross sections are more or less rectangular.
Color: Colorless, frequently cloudy due to incipient alteration
Cleavage: Sometimes after 1 or 2 directions; but is difficult to see in thin section due to very low relief; perfect on {001} and good/distinct on {010}, intersecting essentially at right angles.
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray to light yellow (see bar below)
Extinction angle: The extinction is inclined in almost all orientations.
Twins: Lamellar twinning that resembles alternating white and black bars (see image right)
Uniaxial/Biaxial: Biaxial (+/-)
Lamellar twinning, photo by me.
Geologic Occurrence
Plagioclase, including anorthite, is widespread and common in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks. Comparing anorthite and albite (the sodium end member of the plagioclase group0, anorthite is more common in mafic igneous rocks.
Significance
Anorthite is used in ceramics and glass. Anorthite is also a useful mineral for geologists to understand the formation conditions of igneous rocks.
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