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Llanite

Llanite
Llanite from the dike on Texas State Highway 16

Llanite is a porphyritic rhyolite with distinctive blue quartz and reddish microcline feldspar phenocrysts found in the Llano Uplift north of Llano, Texas, and dated at 1.1 bya. The purplish-blue color of the quartz is caused by inclusions of ilmenite.[1][better source needed][2] It is named after Llano County, Texas.[3]

According to geologist and historian Richard Gibson, referring to the phenocrysts in the aphanitic matrix, "...implies that there were two periods of cooling, one at deeper depths where it took the larger crystals a longer time to cool (and grow), followed by a later, quicker period of cooling, so the matrix crystallized so fast the grains are very small, but the larger, older grains are still there within the matrix".[excessive quote] This cooling took place during a time of mountain-building called the Grenville orogeny. The llanite intruded into older rocks as dikes.[1][better source needed]

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.[4] A dike of llanite crops out on Texas State Highway 16 about nine miles north of the town of Llano.[5]

Llanite has a crushing strength of 37,800 lb/in2 or 26,577,180 kg/m2,[6] similar to quartz, due to a mineral content of quartz (35%), microcline (28%), plagioclase (28%), and biotite (9%).[7]

See also

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References

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  1. ^ a b Gibson, Richard (2025). "Llanite". The Geologic Column. Retrieved August 6, 2025.
  2. ^ Heinrich, Paul (2014). "Llanite and the Blue Quartz of Texas" (PDF). Houston Gem and Mineral Society. pp. 5–12. Retrieved August 6, 2025.
  3. ^ 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.
  4. ^ Reed, R. nd, Llanite in Africa Archived March 19, 2012, at the Wayback MachineLlanite, Archived April 28, 2014, at the Wayback Machine Rob's Granite Page Archived April 4, 2011, at the Wayback Machine, The University of Texas at Austin, Austin, Texas.
  5. ^ Roadside Geology of Texas, 1991, by Darwin Spearing
  6. ^ Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide – South-Central Section, p. 361–368.
  7. ^ Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225–231.
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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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