Geerite

Chemical formula: Cu<sub>8</sub>S<sub>5</sub>

Geerite is a rare copper sulfide with a metallic luster, occurring as fine, black grains and masses.

## Characteristics Geerite is a copper sulfide that typically forms fine-grained, earthy, or massive aggregates. It is rarely observed as small, poorly formed crystals, approximately cubic or octahedral in shape, not exceeding 0.2 mm. Its color is black, often with a brownish or purplish tint, and on fresh surfaces, it exhibits a metallic luster that dulls upon exposure to air. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 3. It is opaque, and its density ranges from 5.35-5.65 g/cm³. It has a black streak with a brownish tint. It is brittle and has an uneven fracture. ## History and Name Geerite was first described in 1980 by R.H. Geer, after whom it is named. It was found in the supergene zone of the De Lamar deposit in Owyhee County, Idaho (USA).

Properties

Mohs hardness
3
Luster
Metallic
Streak
Black
Density
5.35-5.65
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Cubic

Diagnostic features

## Identification Geerite is identified by its black color, metallic luster on fresh surfaces, black streak, and co-occurrence with other copper sulfides. Its identification is difficult and often requires advanced research methods, such as X-ray diffraction (XRD). ## Distinguishing from similar minerals It can be confused with other black, massive copper sulfides, such as digenite, djurleite, or anilite. Distinguishing these minerals based on macroscopic features is practically impossible and requires specialized laboratory analyses. ## Crystal forms It most often occurs as very fine, anhedral (unformed) grains forming massive or earthy aggregates. It rarely forms poorly developed, pseudocubic crystals.

Geological environment

## Genesis Geerite is a secondary mineral, formed in the supergene enrichment zones of copper sulfide deposits. It forms under low-temperature conditions as a result of the alteration of other primary copper sulfides, such as chalcopyrite or bornite. ## Mineral associations It often co-occurs with other copper sulfides, such as spionkopite, digenite, djurleite, anilite, chalcocite, covellite, as well as pyrite and tenorite. ## Localities The most important geerite localities worldwide include the De Lamar deposit in Idaho (USA), where it was discovered, as well as the Magma Mine in Arizona (USA) and Tsumeb in Namibia. It also occurs in many other locations worldwide where supergene enrichment zones of copper deposits are present.

Rarity

Very rare

For collectors

## Quality criteria As a rare mineral typically occurring in microscopic grains, geerite is not a typical collector's item for hobbyists. The most valuable specimens for specialized collections and scientific institutions are those where the presence of geerite has been analytically confirmed, especially those from the type locality (De Lamar, Idaho). The value of a specimen is related to its scientific significance and the richness of its mineral associations, rather than its aesthetics.

Care and storage

## Cleaning Geerite specimens should only be cleaned mechanically, using a soft brush to remove dust. Contact with water and any chemical agents should be avoided. ## What to avoid Geerite is sensitive to moisture and atmospheric oxygen, which cause it to dull and may lead to further alterations. It should be protected from contact with acids and other chemicals. It should not be heated. ## Storage It is recommended to store specimens in dry conditions, preferably in tightly sealed containers or display cases with a desiccant, to limit the process of oxidation and surface dulling.

External references

Sources

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