llanite

{{Short description|Type of rock}}

{{Technical|date=April 2023}}

{{Use dmy dates|date=April 2023}}

Image:llanite.jpg

Image:llanite2.jpg on Texas State Highway 16]]

Llanite is a porphyritic rhyolite{{dubious|Rhyolite|date=May 2025}} 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.Zolensky, M.E., Sylvester, P.J., and Paces, J.B. (1988) Origin and significance of blue coloration in quartz from Llano rhyolite (llanite), north-central Llano County, Texas. Am. Min. v. 73, p. 313-323.

File:Map of Texas highlighting Llano County.svg

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.Reed, R. nd, [http://uts.cc.utexas.edu/~rmr/llanite2.html Llanite in Africa] {{webarchive|url=https://web.archive.org/web/20120319162900/http://uts.cc.utexas.edu/~rmr/llanite2.html |date=19 March 2012 }}[http://uts.cc.utexas.edu/~rmr/llanite.html Llanite,] {{webarchive|url=https://web.archive.org/web/20140428115152/http://uts.cc.utexas.edu/~rmr/llanite.html |date=28 April 2014 }} [http://uts.cc.utexas.edu/~rmr/ Rob's Granite Page] {{webarchive|url=https://web.archive.org/web/20110404082153/http://uts.cc.utexas.edu/~rmr/ |date=4 April 2011 }}, The University of Texas at Austin, Austin, Texas. A dike of llanite crops out on Texas State Highway 16 about nine miles north of the town of Llano.Roadside Geology of Texas, 1991, by Darwin Spearing

Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in² or 26,577,180 kg/m².Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide – South-Central Section, p. 361-368. The mineral is also very similar in appearance to pietersite.{{Citation needed|date=April 2023}}

Modal mineralogy of llanite:Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.

• quartz – 34.6%
• microcline – 27.8%
• plagioclase – 27.9%
• biotite – 8.6%
• fluorite – 1.1%
• apatite – 0.14%
• magnetite – trace
• ilmenite – trace
• zircon – trace