Llanite
Llanite from the dike on Texas State Highway 16

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.[1]

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.[2] A dike of llanite crops out on Texas 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 to pietersite.

Modal mineralogy of llanite:[5]

  • 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

References

  1. ↑ 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.
  2. ↑ Reed, R. nd, Llanite in Africa Archived 19 March 2012 at the Wayback MachineLlanite, Archived 28 April 2014 at the Wayback Machine Rob's Granite Page Archived 4 April 2011 at the Wayback Machine, The University of Texas at Austin, Austin, Texas.
  3. ↑ Roadside Geology of Texas, 1991, by Darwin Spearing
  4. ↑ Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide – South-Central Section, p. 361-368.
  5. ↑ Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.

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).

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