Geerite is a copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 sulfide mineral

Sulfide mineral

The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...

 with the chemical formula Cu₈S₅. The mineral is named after the original collector, Adam Geer, of Utica, New York, USA.

Crystallography

Geerite is in the crystal class . This means that the crystal could be inverted and then rotated by 120 degrees to return to its original position. The optical class of Geerite is unknown. Geerite is anisotropic which means that it will show interference colors when it is rotated in cross polarized light and that the mineral has different properties in different directions

Discovery and occurrence

It was first described in 1980 for an occurrence as thin coatings or platelets replacing sphalerite

Sphalerite

Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...

 in the type locality

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

 in De Kalb Township, Saint Lawrence County, New York. The geologic setting that it occurs in is in the replacement of Sphalerite. It has also been used to study crystal structure and bonding in copper sulfides.