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Classification
Crystal System: Triclinic
Class: Silicate
Sub Class: Tectosilicate (framework-silicate)
Group: Feldspar
Subgroup: Plagioclase Feldspar
Series: Albite-Anorthite (Na end member)
Structure and Composition
Composition: NaAlSi3O8
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 purple sodium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: White to gray or colorless, uncommonly blue tinted or rarely green or red tinted, while much included albite may be strongly colored
Luster: Vitreous, Sub-Vitreous
Note: Slightly pearly luster on cleavage planes
Hardness: 6 - 6.5
Streak: White
Specific gravity: 2.6 - 2.65
Common Habit: Tabular or blocky crystals
Cleavage: Perfect on [001], good on [010], imperfect on {110} (two planes at 90 degrees)
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 white (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 feldspars, like albite, is very common in a great variety of rocks, including most volcanic and plutonic igneous rocks and basic and metamorphic rocks derived from those igneous rocks. Comparing albite and anorthite (the calcium end member of the plagioclase group), albite is more common in felsic igneous rocks.
Significance
Albite is commonly used in ceramics to improve the hardness, durability, and smoothness of products like porcelain, tiles, and glass. Albite is also a useful mineral for geologists to understand the formation conditions of rocks.
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