View source for Llanite

{{Short description|Type of rock}}
{{Technical|date=April 2023}}
{{Use dmy dates|date=April 2023}}
[[Image:llanite.jpg|thumb|Llanite]]
[[Image:llanite2.jpg|thumb|Llanite from the [[dike (geology)|dike]] on [[Texas State Highway 16]]]]

'''Llanite''' is a [[porphyritic]] [[rhyolite]]{{dubious|Rhyolite|date=May 2025}} with distinctive [[phenocryst]]s of blue quartz (a rare quartz color) and [[Perthite|perthitic]] feldspar (light grayish-orangeish). The brown, fine-grained groundmass consists of very small quartz, feldspar, and biotite mica crystals.

Llanite comes from a [[Subvolcanic rock|hypabyssal]] porphyritic rhyolite [[dike (geology)|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 [[bipyramid]]s. 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.<ref>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.</ref>

[[File:Map of Texas highlighting Llano County.svg|thumb|upright|Location of Llano County]]

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.<ref name="Reednda">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.</ref> A dike of llanite crops out on [[Texas State Highway 16]] about nine miles north of the town of Llano.<ref>Roadside Geology of Texas, 1991, by Darwin Spearing</ref>

Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800&nbsp;lb/in<sup>2</sup> or 26,577,180&nbsp;kg/m<sup>2</sup>.<ref>Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide – South-Central Section, p. 361-368.</ref> The mineral is also very similar in appearance to [[pietersite]].{{Citation needed|date=April 2023}}

Modal mineralogy of llanite:<ref>Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.</ref>
* 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 (geology)|Porphyry]]

==References==
{{reflist}}

==External links==
Heinrich, P. V., 2014, [https://hgms.org/BBG_Archives/2014/HGMS-BBG-May2014.pdf ''Llanite and the Blue Quartz of Texas''] The Backbender's Gazette. vol. XLV, no. 5, pp.&nbsp;5–12. ([http://www.hgms.org/ Houston Gem and Mineral Society, Houston, Texas]).
{{Rock type}}

[[Category:Igneous rocks]]

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