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Classification
Crystal System: Monoclinic
Class: Silicate
Sub Class: Tectosilicate (framework-silicate)
Group: Feldspar
Subgroup: Alkali Feldspar
Series: Celsian-Orthoclase
Structure and Composition
Composition: KAlSi3O8
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 potassium 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 potassium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Colorless to white, greenish-white, grayish yellow, pale pink
Luster: Vitreous, Sub-Vitreous, Resinous
Note: Slightly pearly luster on cleavage planes
Hardness: 6
Streak: White
Specific gravity: 2.55 - 2.63
Common Habit: Prismatic or blocky crystals
Cleavage: Two cleavage planes that intersect at 90°. Perfect on {001}, good on {010}.
Identification In Thin Section
PPL Properties
Relief: Low negative
Habit/Form: Common as anhedral and euhedral grains in igneous rocks. Crystals are commonly elongated parallel to c or a, and roughly tabular parallel to (010). Some orthoclase is perthitic, and much of it is cryptoperthitic.
Color: Colorless, frequently cloudy due to incipient alteration (kaolinization)
Cleavage: Sometimes after 1 or 2 directions; but are difficult to see in thin section due to very low relief; perfect on {001} and good/distinct on {010}, intersecting essentially at right angles; {110} cleavage is poor.
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray to white (bar below)
Extinction angle: On (001) parallel, on (010) from 5° to 12°, increasing with the Na content (to 20°).
Twins: Carlsbad twinning with two individuals is common (simple lamellar twinning). Manebach and Braveno types are less common. (See image right)
Uniaxial/Biaxial: Biaxial (-)
Microcline (above), the high-temperature form of orthoclase, possesses the same structure and many of the same properties of orthoclase except it lacks Carlsbad twinning, instead displaying tartan twinning. Photo by me
Carlsbad twinning(above), photo by me
Geologic Occurrence
Orthoclase is the dominant alkali feldspar in most granite, granitic rocks, plutonic rocks, and other felsic rocks with an intermediate cooling regime. Orthoclase is also found in arkoses and certain contact and regional metamorphic rocks. Usually found with quartz and muscovite.
Significance
On top of being an important rock-forming mineral, orthoclase is also used in the manufacturing of ceramics and glass.
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