Feldspathoid

Foidite are rocks of volcanic origin, which consist of minerals of Foidgruppe (at least 90 % of the bright components) and variable levels of feldspars and dark minerals. At lower and higher feldspar content Foid the Foidite go see st. to the phonolite Foiditen ( alkalifeldspatdominiert ) or tephritischen Foiditen ( plagioklasdominiert ) over. Foidite belong to the group of alkaline rocks.

Designation

Foidite be further subdivided based on the available Foidminerale. Only occurs in a significant amount Foid on, the rock is as nephelinite (only nepheline ), Leucitit (only leucite ) referred etc.. In the presence of several different Foide these are listed as suffix (eg leucite nosean - Foidit ). Other mineral name may optionally be preceded by just (eg Olivinnephelinit ).

Mineralogical and chemical composition

Most Foidminerale in leucite and nepheline Foiditen are, however, other Foide as analcime, nosean, Haüyn or sodalite occur in greater quantity. In addition to the Foiden to high temperature alkali feldspar found ( sanidine ) and plagioclase in variable ratio, but always in small amounts (up to 10 volume percent of the bright Gemengteile ). At mafic minerals are often in greater quantity augite, olivine, melilite, Melanite, biotite, hornblende or wollastonite to find. Minor constituents may apatite and various titanium minerals such as Be perovskite or titanite. In the classification diagram of Streckeisen the Foidite located in box 15

The chemical composition of Foidite reflects their modal mineral constituent. It is characterized by a high content of alkali metals ( sodium at nephelinite, potassium at Leucitit ) coupled with a low SiO2 content.

Occurrence

Foidite come in the form of pyroclastics and lavas in volcanic areas around the world before. Known occurrences for Leucitit example, Vesuvius and the Roman volcanic province in Italy. Nephelinite occurs in Europe, among others, in the volcanic Eifel, the Vogelsberg, the cat's back, the Emperor chair (all Germany ) and in the volcanic Massif Central in France.

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