Anorthite

Anorthite is a feldspar mineral made of calcium, aluminium, silicon and oxygen (CaAl₂Si₂O₈). It is the calcium-rich end of the plagioclase series, whose other end is sodium-rich albite. It occurs in dark rocks such as basalt and gabbro, but pure anorthite with no sodium is rare on Earth. Because it is the main mineral of the anorthosite that makes up the Moon’s pale highlands, it matters a great deal to space science.

Anorthite has no important known industrial use; its value is scientific: Understanding the Moon: Anorthosite, the dominant rock of the Moon’s highlands, is made of anorthite-rich plagioclase. These rocks shed light on the early history of the lunar crust; the “Genesis Rock”, sample 15415, is about 4.1 billion years old. A rock-forming mineral: It is a main mineral of rocks such as gabbro and basalt; geologists use the anorthite content of plagioclase to classify rocks and to understand the conditions in which they formed.

It has a hardness of 6–6.5 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.72–2.76 g/cm³.

Its best-known localities: Monte Somma, Vesuvius, Campania, Italy; Val di Fassa (Mount Monzoni), Trentino-Alto Adige, Italy; Miyakejima, Tokyo, Japan; Karpinsk (formerly Bogoslovsk) area, Urals, Russia; Lohja area, Uusimaa, Finland.

It is usually white, greyish or reddish, with a glassy lustre. Crystals are short prisms, though it is more often seen as coarse grains or in masses. It scratches glass (hardness 6–6.5) and, like all plagioclase, shows fine parallel lines (multiple twinning) on its cleavage surfaces. It occurs with olivine, pyroxene and corundum, in dark rocks such as gabbro and in cavities of some volcanic rocks. Anorthite is most often confused with other feldspars such as labradorite, albite and orthoclase. The differences are very hard to see by eye; a laboratory is needed for a sure identification.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Anorthite is mainly a rock-forming mineral and has no price of its own. Collectors look for sharp, clear, undamaged crystals and specimens from well-known places such as Monte Somma or Miyake Island; cut stones are very rare. Its value in science comes from what it tells us: the anorthite in Moon and meteorite samples records the history of the Solar System and of the lunar crust.

Crystal-therapy websites claim that anorthite strengthens bones and the skeleton and helps with calcium balance in the blood, pain and high blood pressure. The same sites say anorthite raises mental clarity, focus and confidence and is linked to the “third-eye chakra”. No study supports these claims. Anorthite is a silicate that holds its calcium tightly in its crystal structure; touching or carrying a stone does not pass calcium into the body. Someone with a bone, pain or blood-pressure problem should not leave treatment to a stone but see a doctor. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.