Feldspar

Lore & Background

The name feldspar derives from the German Feldspat, a compound of Feld ('field') and Spat ('flake'). The change from Spat to -spar was influenced by the English word spar, meaning a non-opaque mineral with good cleavage. The alternative spelling felspar has fallen out of use. The term 'felsic' is an acronymic word derived from feldspar and silica. The feldspar group consists of tectosilicates with a three-dimensional network of aluminosilicate tetrahedra. Compositions are expressed in terms of three endmembers: orthoclase (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8). Solid solutions between orthoclase and albite are called alkali feldspar; those between albite and anorthite are called plagioclase. Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, defining the continuous Bowen's reaction series, while K-feldspar is the last to crystallize. Alkali feldspars include orthoclase, sanidine, microcline, and anorthoclase, while plagioclase feldspars range from albite to anorthite. Barium feldspars such as celsian and hyalophane also occur.

Reader's Guide

Feldspar is of fundamental significance as the most abundant mineral group in the Earth's crust, comprising about 60% of the crust by weight and 41% of the continental crust. Its widespread occurrence in igneous, metamorphic, and sedimentary rocks makes it a key component in understanding geological processes. The ratio of alkali feldspar to plagioclase feldspar, together with quartz proportion, forms the basis for the QAPF classification of igneous rocks. Feldspars are also important in the study of weathering, as they hydrolyze to form clay minerals such as illite, smectite, and kaolinite, which dominate sedimentary rocks and soils. The presence of abundant feldspar in sedimentary rocks indicates minimal chemical weathering and rapid burial. Economically, feldspar is a major raw material for glassmaking (providing fluxing agents and stabilizers) and ceramics (acting as a flux to promote vitrification). In the US, about 66% of feldspar is consumed in glassmaking, with the remainder used in ceramics and as fillers. Its low iron content, low cost per unit of Al2O3, and lack of volatiles make it valuable in industrial applications.

Did You Know?

• Feldspar makes up about 60% of the Earth's crust and 41% of the Earth's continental crust by weight.
• The name feldspar derives from the German Feldspat, a compound of Feld ('field') and Spat ('flake').
• Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, while K-feldspar is the last.
• Chemical weathering of feldspars by hydrolysis produces clay minerals such as illite, smectite, and kaolinite.