Image: <a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a> (CC BY 2.0)
monoclinicuncommonSilicates (Tectosilicates, Feldspar Group)
Anorthoclase
(Na,K)AlSi3O8
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.
Anorthoclase Physical Properties
Mohs Hardness
6Crystal System
monoclinicLuster
vitreousStreak Color
whiteCleavage
Perfect on {001}, good on {010}, intersecting at nearly 90 degreesFracture
uneven to conchoidalSpecific Gravity
2.59Colors
white, gray, yellowish, pinkishTransparency
transparent to translucentType Locality
Mount Erebus (Antarctica), Kenya, Italy, various oceanic islandsMineral Group
Silicates (Tectosilicates, Feldspar Group)Uses
collector, scientific researchAssociated Minerals
pyroxene, amphibole, olivine, magnetiteRarity
uncommonGeological 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.
