Feldspathoid

A feldspathoid is a group of minerals, also known as *foids*, that are tectosilicate minerals similar in composition to feldspars but notably deficient in silica (SiO2). They are typically found in silica-poor igneous rocks. Feldspathoids crystallize from magma that has insufficient silica to form feldspar minerals. This leads to their presence alongside other silica-undersaturated minerals like olivine and nepheline. They are typically aluminosilicates of sodium, potassium, calcium, and barium. The most common feldspathoids are leucite, nepheline, sodalite, cancrinite, and lazurite. Their formation can indicate specific geochemical conditions, particularly a lack of silica in the parental magma, offering valuable clues about the Earth's mantle processes and igneous rock genesis.

Feldspathoid meaning with examples

• Geologists analyzing a newly discovered volcanic rock found it contained significant amounts of nepheline and leucite. This indicated the magma from which it formed was silica-undersaturated, thereby classifying the rock's origin. This observation provides information about the mantle source composition and the volcanic environment.
• During the analysis of a pegmatite, the presence of cancrinite, a feldspathoid, immediately indicated that it was a silica-deficient environment. This allowed for a deeper assessment of the rock's chemical evolution, and it helped reveal the metasomatic processes.
• Petrographic studies of an intrusive rock showed the coexistence of olivine and sodalite. This combination of minerals demonstrated an unusual geochemical signature and confirms the formation of the feldspathoid in a silica-poor environment where typical feldspars were unable to crystallize.
• In the study of kimberlites, feldspathoids like lazurite can act as indicators of high-pressure, mantle-derived magmas. The presence or absence of feldspathoids and their mineral proportions contribute towards models of deep-Earth processes. The kimberlites hold information about the rock formation.
• Exploration of igneous terrains sometimes involves searching for ore deposits associated with silica-deficient rocks containing feldspathoids. Knowing the geological context in which feldspathoids occur can aid in the discovery of potential valuable deposits of metals.
• Furthermore, the crystal structure of feldspathoids and the specific mineral composition can reveal information about the surrounding magmatic environment, and any changes in the conditions when the rock formed.