Kyanite

Kyanite is a metamorphic mineral that is fairly common component of sand.

Kyanite crystals are typically elongated and rectangular or with step-like features which is a typical sign of good cleavage in several directions. In addition to cleavage there is also a good basal parting. The lines running almost perpendicular (85°) to the longer edges are parting cracks. Parting is somewhat similar to cleavage but not every crystal has it and they are created by external stress.

Kyanite has an alternative name also — disthene. Both of these names have a meaning relating to the properties of the mineral. “Kyanos” is blue in Greek and disthene (also from Greek) means that its hardness varies. It is considerably weaker along the crystal (5 in Mohs scale like apatite) and stronger across it (7 like quartz).

Kyanite is typically light blue but not always. Among the crystals shown here only one has slight bluish tinge. Kyanite is often white, yellow, or gray. But even black, pink, and green colors are possible. Hence it is not very wise to identify kyanite by its color although it is a common perception that kyanite is a blue mineral. Beautiful blue kyanite crystals may be used as gemstones.

Large kyanite crystal (length 50 mm) in sericite (muscovite) schist from Pizzo Forno, Ticino, Switzerland. The sample belongs to the Museum of Geology of the University of Tartu.

Kyanite has several polymorphs. These are minerals that have the same composition but differ in the crystal structure. These are sillimanite and andalusite. Their composition is quite simple: Al₂SiO₅ and one may change to another as the metamorphism progresses. Typical order is andalusite → kyanite → sillimanite, but not always because kyanite tolerates very high pressure but not too high temperature. So it may avoid the sillimanite phase if the temperature is not rising as fast as the pressure does. That may happen in subduction zones for example. Kyanite is sometimes found in eclogites which are metamorphic rocks of very high pressure. The crystals on the picture below also come from kyanite-bearing eclogite.

Kyanite is almost always a metamorphic mineral (it is sometimes also found in pegmatites, kimberlites and veins). The rocks containing kyanite were most likely once muddy seabed or something like that. Eclogite, however, is usually metamorphosed basalt or similar rocks. Rocks that most typically contain kyanite are schist and gneiss. Kyanite is one of the index minerals of metamorphism. It means that kyanite is used to roughly estimate the conditions (temperature, pressure) that prevailed when these minerals formed. It often occurs together with staurolite, sillimanite, garnet, andalusite, and other metamorphic minerals. Kyanite is quite common constituent in sand because its resistance to weathering is good and it is a common constituent of several rock types.

Kyanite crystal picked from a beach sand of Thassos Island in Greece. The width of the view is 12 mm.

Kyanite crystals shown above are picked from this very interestingly versatile beach sand. It contains lots of kyanite, quartz, feldspar, epidote, staurolite, and almandine. Can you spot a grain of eclogite in the middle? Yes, the one with green pyroxene and red garnet. Metalia beach, Thassos Island, Greece. The width of the view is 9 mm.

Kyanite and biotite (brown) in quartzite. The width of the view is 12 cm. The specimen belongs to the Museum of Geology of the University of Tartu.