Feldspar is by far the most abundant group of minerals in the earth’s crust, forming about 60% of terrestrial rocks. Most deposits offer sodium feldspar as well as potassium feldspar and mixed feldspars. Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements.
Most of the products we use on a daily basis are made with feldspar: glass for drinking, glass for protection, fiberglass for insulation, the floor tiles and shower basins in our bathrooms, and the tableware from which we eat. Feldspar is part of our daily life.
Feldspar minerals are essential components in igneous, metamorphic and sedimentary rocks, to such an extent that the classification of a number of rocks is based on feldspar content. The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase, Albite and Anorthite. Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements. The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars. The latter category is of particular interest in terms of industrial use of feldspars. Amongst the numerous rocks in which they are present, feldspars are particularly abundant in igneous rocks like granite, which contains up to 50 or 70% of alkaline feldspar.
Granite is however rarely used for its feldspatic content. Rather a whole range of rocks geologically connected to granite is used. Most often, commercial feldspar is mined from pegmatite or feldspatic sand deposits. Aplite, which is a fine-grained igneous rock with the same mineralogical composition as granite is also frequently mined for its feldspar content. Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content. On those elements we can distinguish three families: Feldspatic sand, Pegmatite and Feldspar.