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Llanite is aporphyriticrhyolite with distinctivephenocrysts of blue quartz (a rare quartz color) andperthitic feldspar (light grayish-orangeish). The brown, fine-grained groundmass consists of very small quartz, feldspar, and biotite mica crystals.
Llanite comes from ahypabyssal porphyritic rhyolitedike that intrudes Precambrian metamorphics in theLlano Uplift of central Texas. Published radiometric dating on this llanite indicates that it is 1.106 billion years old (lateMesoproterozoic).
The quartz crystals found in llanite are blue hexagonalbipyramids. The unusual blue coloration of the quartz is thought to be due toilmenite inclusions.
It is named afterLlano County, Texas, the only place where it is found.[1]
The geology of northeast Africa is very similar to that of Texas, and many of the two regions' minerals and fossils are only found in these two locations.[2] A dike of llanite crops out onTexas State Highway 16 about nine miles north of the town of Llano.[3]
Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in2 or 26,577,180 kg/m2.[4] The mineral is also very similar in appearance topietersite.[citation needed]
Modal mineralogy of llanite:[5]
Heinrich, P. V., 2014,Llanite and the Blue Quartz of Texas The Backbender's Gazette. vol. XLV, no. 5, pp. 5–12. (Houston Gem and Mineral Society, Houston, Texas).
