Over 50% of the earth’s crust is made up by a large group of rock-forming silicate minerals that we call feldspar.
The name was derived from the German word ‘feldspat’ meaning ‘field rock’. Feldspar has many uses from ornamental stone carvings, granite counter tops and fine jewellery pieces to its use in the production of glass and ceramics as well as a component in some paints, plastics and rubber. In 2010 alone, about 20 million tonnes of feldspar was produced.
The feldspar group of minerals can crystallize in either the monoclinic or triclinic crystal systems. The group is a solid solution between three end members potassium feldspar, albite(sodium) and anorthite(calcium). The solutions between potassium and sodium are known as ‘alkali feldspars’ and include orthoclase(monoclinic), sanidine(monoclinic), microcline(triclinic) and anorthoclase(triclinic). The solutions between sodium and calcium are known as plagioclase feldspars and include albite, oligoclase, andesine, labradorite, bytownite, and anorthite.
All the plagioclase feldspars are triclinic.
Feldspar can exhibit a range of different phenomena from adularescence in moonstone, labradorescence in labradorite to aventurescence in sunstone and even chatoyancy can rarely be seen in feldspar.
This is a complicated group, and even though together they are the most common mineral in the earth’s crust their varied chemistry and diversity of colours and phenomena makes feldspar an interesting family of minerals.