Foyaite

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Foyaite rock from Brazil ( Azul Bahia )

Foyaites are a group of plutonites that are close to the syenites and have a differentiated mineral composition. Individual varieties are highly decorative and are used for special design purposes.

Rock description and mineral inventory

It is a rare group of rocks that contain feldspar representatives in their mineral inventory. They do not contain quartz . They carry biotite , to a small extent plagioclase feldspars, amphibole , epidote , zircon , ilmenite , arfvedsonite , melanite and aegirine . The lack of alkali feldspar is related to the fact that a SiO 2 deficit prevails during the formation of these rocks and therefore the low-silica feldspar representatives were formed from the available alkali metals . The feldspar representatives in the foyaites are sodalite , nosean , leucite , analcime , nepheline and hauyn . Furthermore occur zeolites , Cancrinit and Natrolith on.

Depending on the mineral composition, subtypes are distinguished. Examples of these are Agpait , Lujavrit , Shonkinit , Borolanit and Särnait .

Feldspars in the foyaite take on unusually idiomorphic forms for a rock (elongated or radial).

history

It was named after investigations on Mount Fóia and Picota in the Serra de Monchique on the northwestern edge of the Algarve in Portugal . The first scientific description comes from Reinhard Blum and was published in 1861. In 1890 Waldemar Christofer Brøgger defined the foyaite rocks as nepheline syenites with a trachytic structure.

Occurrence

Representatives of this rock group are known from Brazil , Zambia , Greenland , southern Norway , Östergötland in Sweden, northwestern Scotland and the Kola peninsula .

use

The blue colored Foyaites are very expensive due to their low occurrence and have a daily price like gold. These rocks can only be installed with expert advice and knowledge of the risk of discoloration.

These rocks react to the use of acids, alkalis and hot water (especially sodalite, which colors the foyaite blue). They show a spectacular decor and are therefore often used for exclusive design purposes.

Natural stone types

literature

  • Roland Vinx: Rock determination in the field . Munich (Elsevier) 2005 ISBN 3-8274-1513-6
  • Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Stuttgart (Enke) 1985 ISBN 3-432-94671-6

Individual evidence

  1. ↑ R. Blum: Foyaite, a new rock from southern Portugal. New Yearbook, p. 426, 1861
  2. ^ FJ Loewinson-Lessing / EA Struve: Petrografitscheski Slowar . Moskwa 1937, p. 362
  3. ↑ W. Brögger: The minerals of the syenite pegmatite dikes of the southern Norwegian augite and nepheline syenites. Journal of Crystallography, Vol.XVI, p. 28, 1890