Oldhamite

Chemical formula: CaS<sup>₂-</sup>

An extremely rare calcium sulfide mineral found almost exclusively in meteorites, characterized by high sensitivity to moisture.

## Characteristics Oldhamite is a mineral with the chemical composition of calcium sulfide, known almost exclusively from its occurrence in certain types of meteorites. It forms small, usually microscopic, grains or aggregates. When fresh, it ranges in color from pale to chestnut brown, sometimes with a yellowish or reddish tint. Its most characteristic feature is its instability in humid air – it rapidly alters to gypsum and calcite, becoming covered with a yellow, and then white, powdery coating. ## Physical Properties Oldhamite crystals, though rarely observed in well-formed shapes, exhibit perfect, three-directional cleavage, typical of the isometric system. The mineral's hardness on the Mohs scale is 4, and its density is approximately 2.58 g/cm³. It has a vitreous luster and is transparent to translucent. ## Colors and Varieties This mineral does not form color varieties or commercial grades. Its coloration is usually uniform, in shades of brown. Color changes to yellow and white are a result of its chemical degradation, not a natural characteristic. ## History and Name Oldhamite was first described in 1862 by Nevil Story-Maskelyne. The mineral is named in honor of Thomas Oldham, then director of the Geological Survey of India. Type specimens come from the Bustee meteorite, which fell in 1852 in Uttar Pradesh, India. ## Applications Due to its extreme rarity and instability, oldhamite has no industrial applications. It is solely an object of scientific research, providing information about conditions in the early Solar System, and is a prized acquisition in specialized collections of meteoritic minerals.

Properties

Mohs hardness: 4
Luster: Vitreous
Streak: White
Density: 2.58
Cleavage: Perfect on {100}, {010}, and {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Cubic

Diagnostic features

## Identification The key diagnostic feature of oldhamite is its occurrence environment – it is an almost exclusively meteoritic mineral. In collecting conditions, it is most easily identified by its rapid reaction to moisture: brown grains become covered with a characteristic, yellowish, and then white coating. It also exhibits phosphorescent properties. ## Distinguishing from Similar Minerals In the specific environment of enstatite chondrites, it is difficult to confuse it with other minerals. Accompanying troilite has a brown color and metallic luster. Niningerite, with which it forms a series, is usually darker, closer to black, and difficult to distinguish without advanced analysis. ## Crystal Forms Oldhamite crystallizes in the isometric system. It most commonly occurs as irregular grains or poorly formed, microscopic cubes and octahedra, forming aggregates within the meteorite matrix.

Geological environment

## Genesis Oldhamite is a primary mineral that condensed under highly reducing (anaerobic) conditions from the presolar nebula. Its presence is characteristic of a rare group of stony meteorites – enstatite chondrites (EH and EL types) and enstatite achondrites (aubrites). ## Mineral Associations Minerals typically associated with oldhamite in meteorites include enstatite, niningerite, troilite, sinoite, perryite, and iron-nickel alloys (kamacite and taenite). ## Localities As an extraterrestrial mineral, its localities are the fall sites of specific meteorites. The most important include: Bustee (India – type locality), Abee (Alberta, Canada), Indarch (Azerbaijan), Jajh deh Kot Lalu (Pakistan), Norton County (Kansas, USA), and numerous enstatite chondrites found in Antarctica.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector appeal of oldhamite is inextricably linked to the quality and type of meteorite in which it occurs. Most valued are specimens containing possibly large, well-visible, and unaltered (not coated) grains or aggregates of oldhamite. The richness of association with other rare meteoritic minerals is also of great importance. ## Popular Localities The most known and valued specimens by collectors come from classic falls of enstatite chondrites, such as Abee, Indarch, or Bustee. Specimens from these localities have historical and scientific value.

Care and storage

## Cleaning Oldhamite is extremely sensitive to moisture and must not be cleaned with water or any liquids. Any contact with water leads to its irreversible decomposition. Only very careful, mechanical removal of dust with a soft, dry brush or a stream of compressed gas (from a distance to avoid thermal shock) is permissible. ## What to Avoid Absolute avoidance of contact with water, water vapor, and humid air is necessary. Even moisture from human breath can initiate its degradation process. Specimens should not be touched with bare hands. It must be protected from all chemicals. ## Storage The only way to preserve oldhamite is to completely isolate it from moisture. Specimens must be stored in airtight containers (e.g., perky box type) with a desiccant (e.g., silica gel). Advanced collectors store it in an inert gas atmosphere (e.g., argon) or immersed in anhydrous, inert mineral oil.