Cafeosite

Chemical formula: Ca₄Fe₂+₃Fe₃+₂ &#₉₇₄₄;O₆S₂-₄

An extremely rare calcium iron oxide sulfide, occurring as microscopic, black grains in pyrometamorphic rocks.

## Characteristics Cafeosite is a mineral with a metallic luster and black color, occurring as very small, irregular (anhedral) grains, typically not exceeding 100 micrometers (0.1 mm) in size. It is opaque. Due to its microscopic size, its visual characteristics can only be assessed under high magnification. It occurs as inclusions in other minerals, mainly spurrite. ## Physical Properties This mineral is characterized by a metallic luster and a black streak. It is brittle. Its density, calculated based on its chemical composition and unit cell parameters, is 4.20 g/cm³. Hardness and cleavage have not yet been determined due to the very small size of the grains. ## Colors and Varieties Cafeosite is monochromatic – it occurs exclusively in black. No color or commercial varieties are known. ## History and Name The name cafeosite is derived from its chemical composition, containing calcium (**Ca**), iron (**Fe**), oxygen (**O**), and sulfur (**S**). The mineral was officially approved by the International Mineralogical Association (IMA) in 2017. Its discovery was made in the Hatrurim Formation in the Negev Desert, Israel, which is also its type locality.

Properties

Luster: Metallic
Streak: Black
Density: 4.20
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Amateur identification of cafeosite is impossible. Its recognition requires advanced analytical techniques, such as electron microscopy with chemical composition analysis (EDS/WDS) and X-ray diffraction (XRD). A key diagnostic clue is its occurrence in the specific geological environment of the Hatrurim Formation, in association with a characteristic mineral assemblage. ## Differentiation from Similar Minerals Cafeosite can be confused with other black, metallic minerals found in the Hatrurim Formation, such as magnetite, hematite, or other complex sulfides. Definitive differentiation is only possible based on precise chemical and structural analysis. ## Crystal Forms This mineral forms exclusively anhedral, i.e., irregular, lacking crystal faces, grains. They usually occur as small inclusions within other minerals, mainly spurrite.

Geological environment

## Genesis Cafeosite is a product of pyrometamorphism – a process of rock alteration occurring under very high temperatures (above 900°C) and low pressure. It forms in carbonate rocks (spurrite marbles) within the so-called contact metamorphic zone, characteristic of the Hatrurim complex. ## Mineral Associations This mineral occurs in close association with other minerals typical of the Hatrurim Formation. These most commonly include: spurrite, calcite, brownmillerite, hematite, and magnesioferrite. ## Localities The only confirmed locality of cafeosite in the world is its type locality – the Hatrurim Formation, located in the Negev Desert, Israel. This is a unique geological environment, known for the occurrence of many rare minerals formed under conditions of natural pyrogenic metamorphism.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality and value of cafeosite specimens are solely scientific and collectible within systematic collections. The attractiveness is determined by the abundance of microscopic grains on the rock matrix and the analytically confirmed presence of the mineral. Specimens where cafeosite co-occurs with other rare minerals from this locality are particularly valued. ## Popular Localities The only source of specimens is the Hatrurim Formation in Israel. Material from this locality is very difficult to obtain and appears on the collector's market sporadically, mainly through scientific institutions or specialized dealers.

Care and storage

## Cleaning Specimens containing cafeosite should be handled with utmost care. Due to the microscopic size of the grains and potential reactivity, wet cleaning is not recommended. Any contaminants can only be removed using a gentle stream of compressed air or a very soft, dry brush. ## What to Avoid This mineral, as a sulfide containing iron in a lower oxidation state (Fe²⁺), is potentially susceptible to oxidation in a humid environment. Contact with water, acids, and other chemicals should be strictly avoided. Heating the specimen is also not advisable. ## Storage It is recommended to store specimens under stable, dry conditions. The best solution is to place them in a sealed, airtight box (perky box) with a desiccant (e.g., silica gel) to minimize the risk of chemical degradation.

Sources

IMA Mineral List (CNMNC)