Description
The mineral andalusite is an aluminum silicate with the chemical formula Al2SiO5. It belongs to the nesosilicate group. This dark green, gray, brown, or red mineral has a pronounced cleavage along the [110] crystal plane and poor cleavage along the [010] crystal plane, a white streak color, and a vitreous luster. The crystal system is orthorhombic, the average density is 3.15, and the hardness is 6.5 to 7. Andalusite is neither magnetic nor radioactive. The chiastolite variety commonly contains dark inclusions of carbon or clay minerals that form an X or a cross. Andalusite is named after the region where it was first described, Andalusia, Spain in 1789. The name of the chiastolite variety is derived from the Greek letter chi, which is written in the shape of a "cross." Andalusite is a very common mineral found in many rocks. It occurs, among other places, in metamorphosed sedimentary rocks. It is also, along with the other aluminum silicates sillimanite and kyanite, an indicator mineral for the depth and temperature at which a rock has undergone metamorphism. Andalusite is generally stable at temperatures below 500°C and at pressures below 0.3 GPa (gigaPascals). Pressure and temperature are related, meaning that at high temperatures (800°C), a lower pressure (0.1 GPa) is required to form sillimanite from andalusite. At temperatures below 600°C and pressures above 0.3 GPa, kyanite is formed from andalusite. Andalusite occurs in the sand fraction of Dutch Quaternary river sediments. In the heavy mineral analysis conducted in the Netherlands by the National Geological Survey during the second half of the twentieth century, the mineral is classified in the so-called stable group.
