La llanite est une rhyolite porphyrique avec des phénocristaux distinctifs de quartz bleu et de feldspath perthitique. La masse souterraine brune à grains fins est constituée de très petits cristaux de quartz, de feldspath et de mica à biotite. La ville de Llano est le siège du comté de Llano, dans l’État du Texas, aux États-Unis. Elle comptait 3 232 habitants en 2010.


Fiche d'identité :


Roche volcanique

Origine : Llano, Texas, USA

10,82 carats environ

16 x 17 mm



English :

Llanite is a porphyritic rhyolite with distinctive phenocrysts of blue quartz (a rare quartz color) and perthitic feldspar (light grayish-orangish). 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's 1.106 billion years old (late Mesoproterozoic). The quartz crystals found in llanite are blue hexagonal bipyramids. The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions. It is named after Llano County, Texas, the only place where it is found.

However, the geology of North-East Africa is very similar to that of Texas. Many of the minerals and fossils found are only found in these two locations on the whole planet. (geologists have identified other locations where similar types of rock may be found).A dike of llanite crops out on Texas State Highway 16 about 9 miles north of the town of Llano.

Modal mineralogy of llanite mode:

  • quartz - 34.6%
  • microcline - 27.8%
  • plagioclase - 27.9%
  • biotite - 8.6%
  • fluorite - 1.1%
  • apatite - 0.14%
  • magnetite - trace
  • ilmenite - trace
  • zircon - trace

Llanite - 10,82 carats - Texas, USA

30,00 €Prix