Roxbyite

Chemical formula: Cu<sub>1.78</sub>S

Roxbyite is a copper sulfide with a metallic luster, forming granular or massive aggregates, usually black or dark gray in color.

## Characteristics Roxbyite is a copper sulfide belonging to the chalcocite-digenite group. It typically occurs as fine-grained or massive aggregates, rarely forming visible crystals. Its surface is often covered with a black, dull layer of secondary copper minerals, which can mask its true appearance. A fresh fracture reveals a metallic luster and a dark gray to black color. ## Physical Properties This mineral is characterized by a Mohs hardness of 2.5-3, making it relatively soft. It has a metallic luster and is opaque. Its density is quite high, around 5.8 g/cm³. It is malleable and ductile, meaning it can be cut with a knife and does not crumble under impact. ## Colors and Varieties Roxbyite is usually black, dark gray, or bluish-black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery and description in 1986 – the Olympic Dam mine, located near the Roxby Downs pastoral station in South Australia. It was approved as a new mineral by the International Mineralogical Association (IMA) in the same year. ## Uses As one of the copper sulfides, roxbyite is a minor component of some copper deposits and can be locally exploited as an ore. Due to its inconspicuous appearance and rarity of occurrence in well-formed shapes, it has limited significance for collectors.

Properties

Mohs hardness
2.5-3
Luster
Metallic
Streak
Black
Density
5.79
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of roxbyite include its black or dark gray color, metallic luster on a fresh fracture, high density, and softness (it can be scratched with a copper coin). It is malleable and ductile. It often occurs in association with other copper sulfides. ## Distinguishing from Similar Minerals Roxbyite is difficult to distinguish from other dark copper sulfides, such as chalcocite, digenite, or djurleite, without advanced analytical methods (e.g., XRD). Chalcocite often exhibits a conchoidal fracture, while roxbyite has an uneven fracture. It is distinguished from sphalerite by its higher density and softness. ## Crystal Forms Roxbyite crystallizes in the monoclinic system. Well-formed crystals are extremely rare. It usually forms massive, fine-grained, or earthy aggregates, often as rims around other minerals or as a product of replacement of earlier sulfides.

Geological environment

## Genesis Roxbyite forms as a result of low- to medium-temperature hydrothermal processes. It is a mineral of the supergene enrichment zone in copper deposits, where it forms at the expense of primary sulfides such as chalcopyrite or bornite. It can also crystallize directly from hydrothermal solutions. ## Mineral Associations It most commonly co-occurs with other copper sulfides, such as chalcocite, digenite, djurleite, anilite, and covellite. It is also accompanied by bornite, chalcopyrite, pyrite, and native copper minerals. ## Localities The most important locality, which is also the type locality, is the Olympic Dam copper-uranium-gold deposit in South Australia. Other confirmed localities include the Magma mine in Arizona (USA), the Sudbury region in Ontario (Canada), and some copper deposits in Chile and Peru.

Rarity

Rare

For collectors

## Quality Criteria The collector appeal of roxbyite is limited due to the rarity of well-formed specimens. The most desirable samples are those in which the mineral forms distinct aggregates with a visible metallic luster, contrasting with other minerals. Due to identification difficulties, specimens with analytically confirmed identity from a reputable locality (especially Olympic Dam) are more valuable. ## Popular Localities By far the most prized and known specimens come from the type locality – the Olympic Dam deposit in Australia. Samples from other locations are mainly of scientific and systematic importance.

Care and storage

## Cleaning Roxbyite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. The use of water, especially with detergents, is inadvisable as it can accelerate oxidation processes and lead to the formation of undesirable tarnish. ## What to Avoid Avoid contact with chemicals, acids, and moisture. Roxbyite, like many sulfides, is susceptible to oxidation, which causes its surface to dull and change color. It should not be heated or exposed to prolonged periods of humid air. ## Storage It is recommended to store specimens in a dry place, preferably in closed, airtight containers or display boxes with a desiccant. This will slow down oxidation processes and preserve the mineral's natural appearance.

Sources

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