The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are not found in rocks containing primary quartz.[1]
Foid, contraction of the term feldspathoid, is applied to any igneous rock containing up to 60% modal feldspathoid minerals. For example, a syenite with significant nepheline present can be termed a '(nepheline)-bearing syenite', or a '(nepheline)-syenite', with the term (nepheline) replaceable by any 'foid' mineral. Such terminology is used in the Streckeisen (QAPF) classification of igneous rocks.[2]
Feldspathoid minerals
- Afghanite
- Analcime – A zeolite mineral
- Cancrinite – Feldspathoid mineral
- Kalsilite – A vitreous white to grey feldspathoidal mineral
- Leucite – Potassium and aluminium tectosilicate mineral
- Nepheline – A silica-undersaturated aluminosilicate mineral
- Petalite – Silicate mineral, used in ceramic glazing
- Sodalite Group
- Tugtupite
References
- ^ Amethyst Galleries, Inc. "The Feldspathoid Group of Minerals". Amethyst Galleries' Mineral Gallery. Retrieved 6 Jul 2015.
- ^ A Dictionary of Earth Sciences, Ailsa Allaby and Michael Allaby, Oxford University Press 1999