Llanite is a

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

with distinctive

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s 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 Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic ...

). The quartz crystals found in llanite are blue hexagonal

bipyramid In geometry, a bipyramid, dipyramid, or double pyramid is a polyhedron formed by fusing two Pyramid (geometry), pyramids together base (geometry), base-to-base. The polygonal base of each pyramid must therefore be the same, and unless otherwise ...

s. The unusual blue coloration of the quartz is thought to be due to

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

inclusions. It is named after Llano County, Texas, the only place where it is found. Map of Texas highlighting 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.Reed, R. nd
''Llanite in Africa''''Rob's Granite Page''
, The University of Texas at Austin, Austin, Texas. A dike of llanite crops out on Texas State Highway 16 about nine miles north of the town of Llano. Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in² or 26,577,180 kg/m². The mineral is also very similar in appearance to pietersite. Modal mineralogy of llanite:Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231. * quartz – 34.6% * microcline – 27.8% * plagioclase – 27.9% * biotite – 8.6% * fluorite – 1.1% * apatite – 0.14% * magnetite – trace * ilmenite – trace * zircon – trace

References

External links

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
. Igneous rocks {{petrology-stub

See also

References

External links