Johntomaite
Chemical formula: BaFe²⁺₂Fe³⁺₂(PO₄)₃(OH)₃
Johntomait is a rare, greenish-black phosphate mineral of the bjarebyite group, discovered in South Australia.
Properties
- Mohs hardness
- 4.5
- Luster
- Vitreous
- Streak
- Dark grey greenish
- Density
- 4.05
- Cleavage
- {100}
- Fracture
- Irregular/Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of johntomait include its greenish-black color, vitreous luster, and occurrence as small, tabular crystals forming radial or rosetted aggregates. A dark gray-green streak is also an important diagnostic clue. ## Distinguishing from Similar Minerals Johntomait can be confused with other dark minerals of the bjarebyite group or other black phosphates. Final distinction from similar minerals, such as bjarebyite or kulanite, requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Johntomait crystals are tabular, flattened parallel to the {100} plane. They most often form radial or rosetted clusters and aggregates.
Geological environment
## Genesis Johntomait is a secondary mineral. It forms in the oxidation zones of phosphate-bearing deposits, in this case, as a result of the alteration of triplite in pegmatites or phosphate-rich sedimentary rocks. ## Mineral Associations This mineral co-occurs with other phosphates, such as rockbridgeite, strengite, cyrilovite, wardite, as well as with quartz and goethite. ## Localities The only confirmed and described occurrence of johntomait is its type locality – the Spring Creek mine, near Wilmington in South Australia.
Rarity
Very rare
For collectors
## Quality Criteria The most desirable johntomait specimens are those with well-formed, sharp crystals forming aesthetic, rosetted aggregates on a contrasting substrate (matrix). The size of the aggregates and the absence of mechanical damage significantly increase their collector value. Due to its rarity, any well-defined specimen is valuable. ## Popular Localities The only source of johntomait specimens is the Spring Creek mine in South Australia, which makes them extremely sought after by collectors specializing in rare minerals and those from type localities.
Care and storage
## Cleaning Due to its rarity and potential fragility, only gentle cleaning with compressed air or a very soft brush is recommended. Avoid water and any chemical agents. ## What to Avoid Avoid contact with water, acids, detergents, and ultrasonic cleaners. The mineral may be sensitive to sudden temperature changes. As an opaque mineral, it is not particularly sensitive to light, but prolonged exposure is not recommended. ## Storage Johntomait specimens are best stored in their original "perky box" or in a stable, sealed container to protect them from dust, moisture, and mechanical damage. Due to the small size of the crystals, display under magnification is the best way to present them.