Angastonite
Chemical formula: CaMgAl<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>(OH)<sub>4</sub>·7H<sub>2</sub>O
Angastonite is a very rare hydrated calcium magnesium aluminum phosphate, forming characteristic rosette-like aggregates of white, bladed crystals.
Properties
- Mohs hardness
- 2
- Luster
- Vitreous, Pearly
- Streak
- White
- Density
- 2.33
- Cleavage
- Perfect on {010}
- Transparency
- Transparent to Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification The most characteristic feature of angastonite is its mode of occurrence – rosette-like or hemispherical aggregates of thin, bladed crystals. Low hardness (can be scratched with a fingernail), pearly luster on cleavage surfaces, and occurrence in paragenesis with other secondary phosphates in carbonate rocks are key diagnostic clues. ## Distinguishing from Similar Minerals Angastonite is sometimes confused with other white phosphate minerals with radial aggregates. - **Minyulite**: Forms similar aggregates, but its crystals are usually more acicular than bladed. It crystallizes in the orthorhombic system. - **Wavellite**: Often forms spherical aggregates with a radial structure, but is much harder (3.5-4) and usually has a greenish tint. ## Crystal Forms Angastonite crystallizes in the triclinic system. It forms thin, flattened crystals with a bladed or platy habit, which combine into characteristic, radial aggregates resembling rosettes or hemispheres.
Geological environment
## Genesis Angastonite is a secondary phosphate mineral. It forms as a result of low-temperature hydrothermal processes or weathering, within phosphate lenses found in carbonate rocks, such as marbles. Its crystallization occurs under the influence of magnesium-rich solutions acting on earlier phosphate minerals and aluminosilicates. ## Mineral Associations At the type locality in Australia, angastonite coexists with minerals such as minyulite, crandallite, apatite, variscite, montgomeryite, and goethite. ## Localities This is an extremely rare mineral, known from only a few localities worldwide. The most important and type locality is the Penrice Marble Quarry in Angaston, South Australia. Its occurrence has also been confirmed in the Pizzo Marcio phosphate deposit in Piedmont, Italy.
Rarity
Very rare
For collectors
## Quality Criteria The most highly prized angastonite specimens are those with well-formed, complete rosettes with a distinct luster. Aesthetics are important – the arrangement of aggregates on the rock matrix and the absence of damage to delicate crystals. A contrasting matrix color (e.g., darker rock) enhances visual appeal. Due to its rarity, any well-identified specimen, even a small one, has great value for collectors of rare minerals. ## Popular Localities By far the most sought-after and classic specimens come from the type locality – the Penrice Quarry in Angaston, Australia. They serve as the standard for this mineral.
Care and storage
## Cleaning Angastonite is an extremely delicate, soft mineral with perfect cleavage. It should be cleaned with the utmost care, preferably using compressed air or a very soft brush. Contact with water, especially tap water, is not recommended. If necessary, a small amount of distilled water can be used, applied precisely and the specimen immediately dried. ## What to Avoid Ultrasonic cleaners, all chemicals (especially acids), and strong streams of water should be absolutely avoided. The mineral is sensitive to high temperatures, which can cause dehydration and destruction of its structure. It should be protected from falls and scratches by harder minerals. ## Storage Angastonite specimens are best stored in separate, padded boxes (of the "micromount" type) to prevent mechanical damage. They should be kept in stable conditions, away from direct sunlight and heat sources.