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Llanite from the dike on Texas State Highway 16

Llanite is a hypabyssally intruded variety of rhyolite with phenocrysts of microcline and euhedral quartz. The quartz crystals are blue hexagonal bipyramids; the unusual blue coloration of the quartz is thought to be due to ilmenite inclusions.[1] It is named after Llano County, Texas, the only place where it is found (geologists have identified other locations where similar types of rock may be found).[2] A dike of llanite crops out on Texas State Highway 16 about 9 miles north of the town of Llano.[3]

Llanite is very strong, with a crushing strength of 37,800 lb/in2 or 26,577,180 kg/m2.[4]

Modal mineralogy of llanite mode:[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[edit]


  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 2012-03-19 at the Wayback MachineLlanite, Archived 2014-04-28 at the Wayback Machine Rob's Granite Page Archived 2011-04-04 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.

External links[edit]

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).