LLANITE
This is the oddest rhyolite rock I've come
across. This is llanite (sometimes spelled ÒllanoiteÓ). It's
a porphyritic rhyolite with distinctive phenocrysts of blue quartz (a rare
quartz color) and perthitic feldspar (light grayish-orangish). The brown,
fine-grained groundmass consists of very small quartz, feldspar, and biotite
mica crystals.
Llanite comes from a hypabyssal porphyritic rhyolite
dike that intrudes Precambrian metamorphics in the Llano Uplift of central
Texas. Published radiometric dating on this llanite indicates that it's
1.106 billion years old (late Mesoproterozoic).
Locality:
roadcut ~9 miles north of the town of Llano, Llano County, central Texas, USA.
Llanite
(9.5 cm across) from the Precambrian of Texas. ItÕs a porphyritic rhyolite with
phenocrysts of quartz (dark blue) and perthitic feldspar (light
grayish-orangish). The brown material surrounding the phenocrysts is very
finely crystalline quartz-feldspar-biotite.
Llanite
(field of view 1.6 cm across) with nice dark bluish-gray quartz
phenocrysts. Blue is a rare color in quartz.