At lake Gaupåsvatnet. The white patch a little to the left in the picture shows a fresh section of anorthosite. In the hillside behind an anorthosite quarry can be seen, which belongs to the Arna stone-crushing plant (Svein Nord).
At Gaupås, anorthosite is broken and crushed to make gravel and pebbles and foundations forgardens. In Indre Arna, Rådalskrysset and in the Sandsli- Kokstad area, this light rock type is easily visible from the road. It is related to the anorthosite massifs in the Gudvangen- Mjølfjell area, where anorthosite is taken out in larger quantities. Anorthosite has many possible applications, for example, as an abrasive in washing powder and toothpaste, and for use in rockwool insulation. It is unusually rich in aluminium. If one in the future can develop a good method for releasing aluminium from this rock type, it would be possible to use it asa source of raw material for aluminium production.
Koronitt, or anorthosite gabbro, which is the geological name, is a beautiful and unusual rock type. Even on a world scale it is rare. The rock type is found in several zones in the Bergen arc, together with the more common anorthosite. Anorthosite gabbro is a white rock type, with black lenses or bands. To expose such a special stone is not done at the turn of a hand. The process started a billion years ago, when the magma solidified to rock deep beneath the earth's crust. The new rock type consisted of plagioclase, with layers or clumps of the heavy, iron-rich mineral olivine. It was good and warm to begin with, around 1200 °C. When the temperature gradually sank to around 350°C, new minerals formed at the expense of the old. The olivine weathered away and got replaced by pyroxene that was nearly black, with a beautiful rim of red-brown garnet, and at the same time the plagioclase got its white colour. The rim structure is called a "corona structure", and the name "koronitt" is therefore used for this rock type. Koronitt is so special and nice to look at that it has been named as Hordaland's county stone.
Anorthosite and anorthosite gabbro
Anorthosite is an intrusive rock type that contains at least 90% of the aluminium-rich mineral plagioclase feldspar. In cases where the minerasl pyroxene, amphibolite and garnet appear as clumps or bands, the rock takes on a more gabbro-rich mineralology and is called anorthosite gabbro.
- Austrheim, H.; Griffin, W. L. 1985. Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs. Chemical Geology 1985:267–281.