Llanite
Llanite is a porphyritic rhyolite with distinctive phenocrysts of blue quartz (a rare quartz color) and perthitic feldspar (light grayish-orangeish). 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 is 1.106 billion years old (late Mesoproterozoic). The quartz crystals found in llanite are blue hexagonal bipyramids. The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions. It is named after Llano County, Texas, the only place where it is found. 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. A dike of llanite crops out on Texas State Highway 16 about nine miles north of the town of Llano. Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in2 or 26,577,180 kg/m2. The mineral is also very similar in appearance to pietersite. Modal mineralogy of llanite:
 quartz – 34.6%
 microcline – 27.8%
 plagioclase – 27.9%
 biotite – 8.6%
 fluorite – 1.1%
 apatite – 0.14%
 magnetite – trace
 ilmenite – trace
 zircon – trace

See also
 Porphyry

References

External links
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). Igneous rocks