β-Quartz
Llano
Llano, Llano county, Texas state, USA
IMA approved name
Quartz pseudomorph after β-Quartz
Source for silicon
Width: 40 mm. Unusual blue quartz pseudomorph after hexagonal bipyramical β-quartz phenocryst of rhyolite. The rectangular grains are cross-sections of bipyramidal β-quartz crystals. Note that the center of crystal is blue whereas its edge is colorless. The brown grains are microcline. The polished surface is so beautiful that this rhyolite has been used as building stones by the name of Llanite.
Recovered from a rhyolite dike. Formed in the Stenian, Mesoproterozoic (1093 Ma).
Blue color had been considered as scattering by fine rutile inclusions, but recent studies revealed that fine ilmenite crystals cause this blue color.