Return to Alkaline Rocks
Feldspathoids (or foids) are framework-structured aluminosilicates containing essential alkalis (usually Na and/or K) or Ca. Like structurally similar feldspars, feldspathoids are typically colorless (but may be "tinted" green, gray or red by tiny inclusions of other minerals), have moderate hardness values (5-7, with the exception of some zeolite-group minerals), and are fairly susceptible to alteration (both deuteric and surficial). The crystal morphology, optics and other physical properties of these minerals vary a great deal depending on their structure and chemistry. The nomenclature of many alkaline igneous rocks is based on the relative volumetric proportion and mode of occurrence of feldspathoid minerals. The two most common foids are nepheline (Na3KAl4Si4O16) and leucite (KAlSi2O6) found, respectively, in sodic and potassic alkaline rocks. Both are readily recognizable in hand-specimen and, especially, under the petrographic microscope. Nepheline crystallizes as hexagonal prismatic crystals, which appear either six-sided or rectangular in thin section (upper right) and have a very low birefringence and straight extinction. Leucite is isometric above ~400 oC; hence, its crystals appear roundish and isotropic in thin section (lower right). Volcanic rocks containing essential modal or normative nepheline (but little or no feldspar) are termed nephelinite and those with essential modal or normative leucite are referred to as... you guessed it - leucitite!