Cooperite

Chemical formula: Pt<sup>₂+</sup>S<sup>₂-</sup>

Cooperite is a rare platinum sulfide, an important source of this metal in some deposits, characterized by a metallic luster and steel-gray color.

## Characteristics Cooperite is a platinum sulfide, belonging to the platinum group minerals (PGM). It occurs as fine, irregular grains or, less commonly, as poorly formed, tabular crystals. Its typical specimens are small, often visible only under a microscope in polished ore samples. It has a steel-gray color, which quickly tarnishes on a fresh surface, and may exhibit a creamy tint in reflected light. ## Physical Properties Cooperite is an opaque mineral with a strong, metallic luster. Its Mohs hardness is 4-5, making it relatively hard among sulfides. It is a heavy mineral, with a density of approximately 9.5 g/cm³, which is typical for platinum-bearing minerals. ## Colors and Varieties This mineral is homogeneous in color, always appearing in shades of steel-gray to white. It does not form color varieties and has no distinct trade names. ## History and Name The name cooperite was given in honor of Richard A. Cooper, an analyst from Johannesburg, who first described this mineral in 1928. It was discovered in platinum-bearing deposits of the Bushveld Complex in the Republic of South Africa. ## Uses Cooperite, along with other platinum group minerals, is an important component of platinum ores. Its main significance is industrial, as a source of this valuable metal. Cooperite specimens also have collector's value, especially those that are visible to the naked eye or form distinct crystals.

Properties

Mohs hardness: 4-5
Luster: Metallic
Streak: Black
Density: 9.5
Cleavage: Good on {110}, poor on {001}
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Field identification of cooperite is practically impossible due to its small size and resemblance to other sulfides. Positive identification requires specialized laboratory methods, such as reflected light analysis (ore microscopy) and chemical analysis (e.g., electron microprobe) to confirm its composition (platinum and sulfur). ## Distinguishing from Similar Minerals Cooperite can be confused with other sulfides with a metallic luster, such as braggite ((Pt,Pd,Ni)S), sperrylite (PtAs₂), or pyrite (FeS₂). It differs from braggite by its tetragonal crystal system (braggite is orthorhombic) and subtle differences in color in reflected light. It differs from sperrylite by its chemical composition (lack of arsenic) and crystal habit. Pyrite is brighter, has a brass-yellow hue, and is much more common. ## Crystal Forms Cooperite crystallizes in the tetragonal system. It most often forms irregular grains and aggregates intergrown with other minerals. Elongated or tabular crystals are rarely observed, usually of small size, not exceeding a few millimeters.

Geological environment

## Genesis Cooperite is a magmatic mineral, crystallizing at high temperatures. It forms as a result of sulfide segregation from mafic and ultramafic magma. It is a typical component of large, layered magmatic intrusions, where it concentrates in specific sulfide horizons. ## Mineral Associations This mineral occurs in association with other platinum group minerals (PGM) and base metal sulfides. It most commonly co-occurs with braggite, sperrylite, native platinum, as well as chalcopyrite, pyrrhotite, and pentlandite. ## Localities The most important and historical occurrence of cooperite is the Bushveld Igneous Complex in the Republic of South Africa, especially in the Merensky Reef horizon. Other significant localities include the Sudbury Complex in Canada, the Stillwater intrusion in Montana (USA), and deposits in the Norilsk region of Siberia, Russia.

Rarity

Rare

For collectors

## Quality Criteria The most valuable specimens for collectors are those with visible, well-formed cooperite crystals, which is extremely rare. Rich aggregates of this mineral in the host rock are also highly prized, especially if accompanied by other rare platinum group minerals. Due to its rarity, even microscopic but well-documented and identified grains have scientific and collector's value. ## Popular Localities By far the most prized specimens come from classic localities such as the Merensky Reef horizon in the Bushveld Complex (RSA) and the Stillwater intrusion in the USA. Specimens from these locations are considered reference material for this mineral species.

Care and storage

## Cleaning Cooperite specimens, especially those intergrown in the host rock, should be cleaned very carefully. It is best to use a soft brush to remove dust. Due to its rarity and often small size, intensive mechanical cleaning is not recommended. ## What to Avoid Avoid contact with acids and other aggressive chemicals that may react with sulfides. The mineral is stable, but should be protected from scratching by harder minerals. ## Storage Cooperite specimens are best stored in separate, padded display boxes to prevent abrasion and mechanical damage. Due to the small size of the crystals, they are often stored in special micromount boxes.

Sources

IMA Mineral List (CNMNC)