Costibite

Chemical formula: Co<sup>3+</sup>(SbS)<sup>3-</sup>

A rare antimony cobalt sulfide, distinguished by its metallic luster and very high hardness.

## Characteristics Costibite is a rare mineral from the sulfosalt group, being an antimony cobalt sulfide. Its name refers to its chemical composition (Co - cobalt, Sb - antimony). It usually forms small, anhedral (irregularly shaped) grains and inclusions within other minerals, less often occurring as fine aggregates. It is characterized by a steel-gray to tin-white color, often with a purple or pinkish hue. It is an opaque mineral with a strong, metallic luster. ## Physical Properties One of the most unusual and diagnostic features of costibite is its very high hardness, exceeding 8 on the Mohs scale, which is exceptional for sulfides and sulfosalts. This mineral also has a high density of approximately 7.45 g/cm³. It exhibits perfect cleavage in three mutually perpendicular directions. ## Colors and Varieties This mineral is monocolored – its color is always steel-gray or tin-white. The occasionally observed purple or pinkish tarnish is a result of superficial oxidation or light play. No colored or commercial varieties are distinguished. ## History and Name Costibite was first described in 1970 by L.J. Cabri, D.C. Harris, and J.M. Stewart. The name, as mentioned, comes from the symbols of its constituent elements: cobalt (Co) and antimony (Stibium - Sb). The type locality, from which the first studied specimens originated, is the Consols Mine in Broken Hill, New South Wales, Australia. ## Applications Due to its rarity, costibite has no industrial applications. It is solely an object of scientific interest and a valued acquisition for advanced systematic and regional mineral collections.

Properties

Mohs hardness
> 8
Luster
Metallic
Streak
Black
Density
7.45
Cleavage
Perfect on {100}, {010} and {001}
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification The key diagnostic feature of costibite is its exceptionally high hardness (>8 on the Mohs scale), which distinguishes it from almost all other minerals with a metallic luster. Other important features include a steel-gray color with a purple hue, strong metallic luster, high density, and perfect cleavage in three directions. ## Distinguishing from Similar Minerals Costibite can be confused with other steel-gray ore minerals such as skutterudite, safflorite, arsenopyrite, or ullmannite. However, all these minerals are significantly softer (hardness in the range of 5-6 on the Mohs scale). A definitive distinction often requires chemical analysis (EDS). ## Crystal Forms It most commonly occurs as irregular grains and massive aggregates. Well-formed crystals are extremely rare, and if they do appear, they take the form of small cubes or tabular crystals, reaching sizes of fractions of a millimeter.

Geological environment

## Genesis Costibite is a hydrothermal mineral. It forms in ore veins rich in cobalt, nickel, arsenic, and antimony, which cut through metamorphic or sedimentary rocks. ## Mineral Associations It most often co-occurs with other cobalt, nickel, and antimony minerals. Its typical associations include: skutterudite, safflorite, ullmannite, willyamite, native antimony, as well as calcite and quartz. ## Localities The most important and historically first locality for costibite is the Broken Hill deposits in Australia. Other significant localities include the Bou Azzer district in Morocco, known for its abundance of cobalt and nickel minerals, as well as some occurrences in Saxony, Germany.

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of costibite is determined by its rarity. The most highly prized specimens are those where the mineral forms rich, visible-to-the-naked-eye aggregates on a contrasting rock matrix. Specimens with rare, microscopic, but well-formed crystals are exceptionally valuable. Co-occurrence with other rare minerals typical of a given locality is also an important asset. ## Popular Localities Specimens from the type locality – Broken Hill in Australia – are considered classic and most desirable. Material from Morocco (Bou Azzer district), which provides rich samples of this mineral, is more commonly found on the collector's market.

Care and storage

## Cleaning Dry cleaning with a soft brush is recommended to remove dust. Ultrasonic cleaners should be avoided. If necessary, distilled water can be used, but the specimen should be thoroughly dried immediately to prevent oxidation. ## What to Avoid The mineral is sensitive to humid air and acidic environments, which can cause slow oxidation and tarnishing. Contact with chemicals and acids should be avoided. Despite its high hardness, due to its perfect cleavage, it is brittle and prone to fracturing upon impact. ## Storage Costibite specimens are best stored in a dry and stable environment. Sealed membrane boxes or display cases are recommended to protect against dust and limit air circulation, slowing down oxidation processes.

Sources

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