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 usually not found in rocks containing primary quartz. A notable exception where feldspathoids and quartz-bearing rocks are found together is the Red Hill Syenite.[1]

Foid, a 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 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 – Tectosilicate mineral
  • Analcime – Tectosilicate mineral
  • Cancrinite – Feldspathoid mineral
  • Kalsilite – Vitreous white to grey feldspathoidal mineral
  • Leucite – Potassium and aluminium tectosilicate mineral
  • Nepheline – Silica-undersaturated aluminosilicate mineral
  • Petalite – Silicate mineral, used in ceramic glazing
  • Sodalite Group
    • Hauyne – Silicate mineral
    • Lazurite – Alumino-silicate mineral whose blue colour is due to a sulfide species and not copper
    • Nosean – tectosilicate mineral
    • Sodalite – Blue tectosilicate mineral
  • Tugtupite – tectosilicate mineral

References

  1. ↑ "The Feldspathoid Group of Minerals". Amethyst Galleries' Mineral Gallery. Amethyst Galleries, Inc. Retrieved 6 Jul 2015.
  2. ↑ Allaby, Ailsa; Allaby, Michael (1999). A Dictionary of Earth Sciences. Oxford University Press.


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