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»Llanite Rhyolite«

»Llanite Rhyolite«, Madagaskar

»Llanite Rhyolite«, Madagaskar

Foto: K. Sieber, www.makrogalerie.de

A colorful rock with roundish to oval domains of shimmering bluish colors, is marketed as »Llanite«, »Llianit« or »Llanoithe« or simply as "Porphyrite" and "Volcanite". The latter term gives an idea about its formation ("it is from volcanic origin") but it does not specify what kind of rock it is - there are many different volcanic rocks out there. The term "Porphyrite" is an outdated name for a class of volcanic rocks, called "rhyolite" in modern time. The term »Llanite« is based on the place of discovery in Llano County, Texas, USA. Known occurrences of rhyolithic rocks with this specific look are Brazil, Madagascar and the USA.

The samples we have examined in the EPI-Lab, were reported to come from Madagascar. We identified a slightly metamorphosed (alkalifeldspar-) rhyolite with exceptionally large crystals, embedded in a fine grained groundmass. Orthoclase feldspar crystals up to 10mm / 0.4in in size and large quartz and plagioclase feldspar crystals up to 5mm / 0.2in are visible to the naked eye. The analyses of various thin sections revealed a well crystallized groundmass. It consists of quartz, mica and feldspar very rich in inclusions, with no glassy component.

Due to a beginning metamorphosis the original groundmass has been recrystallized and developed bigger and aligned minerals. In contrast, the larger feldspar and quartz phenocrysts are nearly unaffected. The elevated pressure during metamorphosis has led to surface corrosion and plastically deformation of the quartz crystals. This deformation could be the reason for the bluish color of the quartz phenocrysts in this stone.

According to a paper of Zolensky et al., 1988, the blue color of "Llanite" rhyolite from Texas is attributed to finely distributed ilmenite inclusions. In the examined rhyolite from Madagascar Ilmenite was not detected.

Lit.:
Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyro liparose-alaskose) 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.