Coombsite
Chemical formula: KMn<sup>2+</sup><sub>13</sub>(Si,Al)<sub>18</sub>O<sub>42</sub>(OH)<sub>14</sub>
Coombsite is a very rare, brown silicate mineral, found as microscopic, lath-like crystals in metamorphic rocks.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous
- Streak
- Pale brown
- Density
- 3.06-3.12
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Identification of coombsite is only possible using advanced analytical methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS/WDS). In field conditions or in a typical collection, it is impossible to identify based on visual characteristics due to the microscopic size of the crystals. ## Differentiation from Similar Minerals It can be confused with other microscopic manganese silicates, such as zussmanite or bannisterite, with which it often co-occurs. Differentiation requires specialized laboratory analysis to determine its chemical composition and crystal structure. ## Crystal Forms It forms very small, lath-like or platy crystals, often grouped into felted or radial aggregates embedded in the host rock.
Geological environment
## Genesis Coombsite forms under low-grade metamorphic conditions in manganese-rich rocks. Its formation is associated with the zeolite and glaucophane facies, at temperatures and pressures that lead to the transformation of sedimentary manganese rocks. ## Mineral Associations This mineral co-occurs with other manganese silicates and minerals typical of metamorphic rocks. It is most commonly associated with quartz, calcite, hausmannite, tephroite, rhodonite, pyroxmangite, zussmanite, stilpnomelane, and bannisterite. ## Localities The most important and confirmed occurrences of coombsite are: - Hoskins Mine in Grenfell, New South Wales, Australia (type locality). - Wessels Mine in Hotazel, Kalahari Manganese Field, Republic of South Africa. - Cerchiara Mine in Liguria, Italy. - Otago Schist area in New Zealand.
Rarity
Very rare
For collectors
## Quality Criteria As a microscopic mineral, coombsite is not evaluated according to typical criteria such as size or color. The value of a specimen primarily depends on the abundance of crystals in the host rock and their association with other rare minerals. The most prized samples are those in which coombsite forms well-defined, though still microscopic, aggregates, identifiable under a microscope. The certainty of identification confirmed by analysis is also crucial. ## Popular Localities Specimens from the Wessels Mine in South Africa and the Hoskins Mine in Australia are the most classic examples of this mineral for collectors specializing in micromounts.
Care and storage
## Cleaning Due to its extreme rarity and microscopic nature, coombsite specimens are generally not cleaned. Any attempts at mechanical or chemical cleaning of the host rock can irrevocably damage the delicate crystals. ## What to Avoid Contact with chemicals, ultrasound, and all forms of mechanical cleaning (abrasion, brushing) should be avoided. Specimens should be protected from dust and vibrations. ## Storage Specimens containing coombsite should be stored under stable conditions, preferably in sealed "micromount" boxes, which protect them from physical damage and dust. Exposure to moisture and extreme temperatures should be avoided.