Feldspathoïde

The feldspathoïdes are mineral anhydrous belonging to the family of the Tectosilicate S and chemically close to the Feldspath S. the structure is more open of that of feldspars and the cations (Sodium, Potassium, Calcium…) occupy of the cavities generally larger than those of feldspars, cavities which form channels in which ions can enter and leave and where small Molécule S can circulate without destroying the structure.

Some magmas are so rich in Silice which they form of the rocks rich in quartz (rock acids). Other magmas have right what it is necessary of silica to form of the Silicate S, mainly of the Feldspath S, but not enough to form quartz. Others have even less silica and cannot form Feldspath S: they then form feldspathoïdes, which contain only approximately 2/3 of the silica present one in alkaline feldspars. Obviously the coexistence feldspathoïdes - quartz in the Na₂O system - Al₂O₃ - SiO₂ is not possible: if there is enough silica to form very stable feldspars, minerals and of intermediate composition between the feldspathoïdes and quartz, one does not observe a feldspathoïdes.

Many a Roche S contains feldspathoïdes, but the principal ones are the nephelinic Syénite S and syenites with Leucite: the area where these rocks are most abundant is the peninsula of Kola, in the north-Western Russia.

The feldspathoïdes can be classified on the basis of their crystalline Structure:

* feldspathoïdes with structure derived from the quartz: β- Eucriptite LiAlSiO₄

* feldspathoïdes with structure derived from the Cristobalite: Leucite KAlSi₂O₆ Pollucite CsAlSi₂O₆·H₂O Analcime NaAlSi₂O₆·H₂O

* feldspathoïdes with structure derived from the Tridymite: Kalsilite KAlSiO₄ Nephelite Na₃KAl₄Si₄O₁₆ Carnégieite NaAlSiO₄

* Scapolite S

* Group of the Sodalite

* Petalite LiAlSi₄O₁₀

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