Llanite
Image: James St. John (CC BY 2.0)
Mineral NameLlanite
Chemical FormulaPrimarily SiO2 (quartz), (K,Na)AlSi3O8 (perthitic feldspar), and K(Mg,Fe)3AlSi3O10(F,OH)2 (biotite)
Mohs Hardness6
Streak ColorWhite to light brown
CleavageGood in feldspar, perfect in biotite, absent in quartz
FractureUneven to conchoidal
Specific Gravity2.5
Colorsbrown, blue, orange, gray
TransparencyOpaque (with translucent to transparent phenocrysts)
Type LocalityLlano, Texas, USA
Mineral GroupIgneous rock (felsic, porphyritic rhyolite)
Usescollector, building material, aggregate
Associated Mineralsquartz, perthitic feldspar, biotite mica
Rarityuncommon
Description
Llanite is a porphyritic rhyolite characterized by distinctive dark bluish quartz phenocrysts and light grayish-orangish perthitic feldspar phenocrysts set in a finely crystalline brown groundmass.
Geological Context
This Mesoproterozoic (1.106 Ga) hypabyssal igneous rock formed during magmatic events in the Llano Uplift of Texas, representing a unique variety of rhyolite.
