Andesite
Image: Photographer: <a href="//commons.wikimedia.org/wiki/User:Siim" title="User:Siim">Siim Sepp</a>, 2005 (CC BY-SA 3.0)
Mineral NameAndesite
Chemical FormulaIntermediate composition, primarily plagioclase feldspar (oligoclase-andesine) with pyroxene and/or amphibole
Mohs Hardness6
Streak ColorWhite to light gray
CleavageNot applicable for a rock; constituent minerals exhibit cleavage (e.g., plagioclase)
FractureUneven to conchoidal
Specific Gravity2.5
Colorsdark gray, black, brown, reddish
TransparencyOpaque
Type LocalityAndes Mountains (type locality), common in volcanic arcs worldwide
Mineral GroupIgneous rock (intermediate volcanic rock)
Usesconstruction aggregate, road building, dimension stone
Associated Mineralsplagioclase feldspar, pyroxene, amphibole, biotite, zeolite
Raritycommon
Description
The image displays a dark gray, fine-grained andesite rock with numerous lighter-colored, irregular to rounded amygdules, which are vesicles filled with white to yellowish zeolite minerals.
Geological Context
Andesite is an extrusive igneous rock formed from the rapid cooling of intermediate-composition lava, typically found in volcanic arcs above subduction zones. The amygdaloidal structure indicates gas bubbles in the lava that were later filled by secondary minerals like zeolite.
