Llanite

Llanite is a hypabyssally intruded 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 (Zolensky, et al. 1988). It is named after Llano County, Texas, the only place where it is found.

Llanite is very strong with a crushing strength of 37,800 lb. per sq. inch or 26,577,180 kg per sq. m (Barnes, 1988).

Llanite Mode (Iddings, 1904)
*quartz - 34.6%
*microcline - 27.8%
*plagioclase - 27.9%
*biotite - 8.6%
*fluorite - 1.0%
*apatite - 0.13%
*magnetite - trace
*ilmenite - trace
*zircon - trace

The largest piece of polished Llanite in the world can be found at the Badu House, a historic bed and breakfast in Llano, Texas.

References

Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide-- South-Central Section, p. 361-368.

Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyroliparosealaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.

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.