Feldspar – raw material for porcelain and artificial teeth
Feldspar is by far the most common mineral group in the earth’s crust. They are aluminum silicates which are mainly divided into alkali feldspar and plagioclase. Alkali feldspar can either be rich in potassium (K-feldspar) or sodium (albite), while plagioclase forms a continuous chemical series from sodium-rich (albite) to calcium-rich (anorthite) feldspar. The most common K-feldspars are microcline and orthoclase. Albite can be considered to belong to both main groups of feldspar. Alkali feldspar is most common in light-coloured rocks such as granites and gneisses, much of which is found in the Precambrian bedrock in southern Norway, while calcium-rich plagioclase is most common in rocks such as anorthosite and gabbro, and can also occur in rocks from the mantle (peridotite).
In Agder, it is particularly pegmatites with large crystals of microcline that have provided the basis for extensive mining. The first feldspar mine was Narestø near Arendal, which was opened in 1792. The largest mine that has been in operation in Agder is the Lie mine in Evje. In the period 1956-1995, approx. 150,000 tonnes of feldspar was mined here. On Iveland, Ertveit was the largest mine. In Kristiansand, Sømsgruva is the most famous mine. The last mine where there was major operation was Glamsland at Lillesand (1968-2011). The feldspar from Agder was largely exported, e.g. to Germany, and potassium feldspar was used for porcelain, while albite was better suited for glass production. Both albite and potassium feldspar have later been used to make artificial teeth. A greater proportion of feldspar (60-80%) is used in dental porcelain than in porcelain that has other uses.