Anorthoclase
Image: James St. John (CC BY 2.0)
Mineral NameAnorthoclase
Chemical Formula(Na,K)AlSi3O8
Mohs Hardness6
Crystal Systemmonoclinic
Lustervitreous
Streak Colorwhite
CleavagePerfect on {001}, good on {010}, intersecting at nearly 90 degrees
Fractureuneven to conchoidal
Specific Gravity2.59
Colorswhite, gray, yellowish, pinkish
Transparencytransparent to translucent
Type LocalityMount Erebus (Antarctica), Kenya, Italy, various oceanic islands
Mineral GroupSilicates (Tectosilicates, Feldspar Group)
Usescollector, scientific research
Associated Mineralspyroxene, amphibole, olivine, magnetite
Rarityuncommon
Description
Anorthoclase typically appears as white, gray, or yellowish blocky crystals with a vitreous luster, often showing twinning. The specimens in the image are light-colored, somewhat translucent, and exhibit characteristic feldspar cleavage.
Geological Context
Anorthoclase is a high-temperature alkali feldspar that forms in rapidly cooled volcanic rocks, particularly in silica-undersaturated to saturated lavas and pyroclastic rocks. It is characteristic of certain alkaline igneous environments.
