This is the oddest rhyolite rock I've come across.  This is llanite (sometimes spelled “llanoite”).  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.  It’s 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.




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