Item talk:Q43306

Mindat Description

Felsites (acid volcanic rocks) tentatively identified as rhyolite, with felsic minerals comprising >20% quartz and alkali feldspar/plagioclase 40-100%

Geoscience Ontology Description

fine_grained_igneous_rock consisting of quartz and alkali feldspar, with minor plagioclase and biotite, in a microcrystalline, cryptocrystalline or glassy groundmass. Flow texture is common. Includes rocks defined modally in QAPF fields 2 and 3 or chemically in TAS Field R as rhyolite. QAPF normative definition is based on modal mineralogy thus: less than 90 percent mafic minerals, between 20 and 60 percent quartz in the QAPF fraction, and ratio of plagioclse to total feldspar is less than 0.65. Note that technical definition, based on modal mineralogy plotted in a QAPF triangle may be applied qualitatively, based on phenocryst mineralogy when ground mass mineralogy can not be determined optically, or based on CIPW norm. Although TAS categories are defined based on chemical analyses, the correspondence with the QAPF defined categories is generally close enough that QAPF categories are commonly used interchangeably with TAS categories. It is important to note the basis for assignment of fine-grained igneous rocks to a specifice lithology category.