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  • Introduction to feldspar mineralogy





  • Ores and feldspar production






    • Brief introduction to feldspar mineralogy:


    Feldspar is by far the most abundant group of minerals in the earth's crust, forming about 60% of terrestrial rocks. 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 crystalline structure of feldspars consists of an infinite network of SiO2 and AlO4 tetrahedra. They usually crystallise in the monoclinic or triclinic system.

    The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8).
    The minerals whose 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. This latter category is of particular interest in terms of industrial use of feldspars.



    Ores and feldspar production:


    Amongst the numerous rocks in which they are present, feldspars are particularly abundant in igneous rocks like granite, which contains from 50 to 70% of alkaline feldspar. Granite is however rarely used for its feldspatic content. However, a whole range of rocks geologically connected to granite are used. Most often, commercial feldspar are 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.



    Feldspars are either selectively mined or processed by flotation and/or magnetic separation, in order to remove the accessory minerals (e.g. quartz, mica, rutile, etc.) present in the ore. The feldspar may then undergo a milling step which allows to adapt the particle-size to the intended use. The degree of refining and possible milling is very dependant upon the final use of the product. For a number of uses, it is perfectly acceptable, and even advantageous, that the product retains some accessory minerals, e.g. quartz, while at the other extreme some applications require extremely pure and fine-grounded grades. Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content.




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