Learn more about anorthosite

Feldspar is a very common mineral worldwide, making up more than 50% of the earth's crust, but a variety with as high aluminium-calcium content as the Gudvangen type is rare in large and homogenous quantities. Plagioclase occurs in nature as a mixture of the sodium-rich type albite and the calcium-rich type anorthite and all grades of mixture are possible. The Inner Sogn type consists of about 60-80% anorthite and is thus especially rich in aluminium.

Of special interest in industrial applications is the fact that the plagioclase feldspar has a variable solubility in mineral acids, dependent on the anorthite content. Plagioclases with 60-70% anorthite and above are soluble, while content below 50% results in a non-soluble plagioclase. This fact is vital when uses of anorthosite are considered in relation to processes involving acid leaching of the rock. However the main product at the present extractions by Gudvangen Stein is an altered anorthosite-type that is almost non-soluble in acids.