Bettertonite
Chemical formula: Al<sub>6</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>3</sub>(OH)<sub>9</sub>·16H<sub>2</sub>O
Bettertonite is a very rare aluminum arsenate, forming microscopic, white, spherical aggregates and rosettes.
Properties
- Luster
- Pearly
- Streak
- White
- Cleavage
- Perfect on {010}
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Bettertonite is recognized by its characteristic form – white, spherical or radial aggregates composed of tiny, acicular crystals. It occurs in paragenesis with other rare arsenates in the oxidation zones of ore deposits. Final identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Distinguishing from similar minerals It can be confused with other white secondary minerals forming similar aggregates, such as mansfieldite, scorodite, or wavellite. Distinguishing them "by eye" is practically impossible and requires chemical analysis to confirm the presence of aluminum and arsenic in appropriate proportions and the absence of other elements (e.g., iron in scorodite). ## Crystal forms Crystals are elongated and flattened, with a bladed or acicular habit. They usually form radial, rosette-like, or spherical (spherulitic) aggregates up to 0.2 mm in diameter.
Geological environment
## Genesis Bettertonite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic deposits rich in arsenic. It forms as a result of arsenic-rich waters acting on aluminum-bearing minerals in an acidic or neutral environment. ## Mineral associations At the type locality in Cornwall, it co-occurs with mansfieldite, scorodite, quartz, arsenopyrite, chalcopyrite, and fluorite. In other localities, it is also accompanied by olivenite, conichalcite, and pharmacosiderite. ## Localities The most important confirmed occurrences of this very rare mineral are: - Penberthy Croft Mine, St Hilary, Cornwall, United Kingdom (discovery site). - Clara Mine, Wolfach, Black Forest, Germany. - Cap Garonne Mine, Le Pradet, Var, France.
Rarity
Very rare
For collectors
## Quality criteria The quality of a bettertonite specimen is assessed based on the size and development of the spherical aggregates and their abundance on the rock matrix. Specimens with clearly formed, isolated rosettes or spheres, contrasting with the substrate, are most valued. Aesthetic association with other rare secondary minerals is also important. ## Popular localities All known localities are highly valued by micromount collectors. Specimens from the Clara Mine in Germany and the Cap Garonne Mine in France are as sought after as those from the type locality in Cornwall.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using compressed air to remove dust. Any contact with water or chemicals is inadvisable due to the delicacy and small size of the crystals. ## What to avoid Avoid contact with water, acids, and other chemical agents. The crystals are extremely brittle and sensitive to mechanical shocks and ultrasound. Specimens should not be heated, as the mineral contains water in its structure. ## Storage Bettertonite specimens, as micromounts, should be stored exclusively in specialized, sealed boxes (so-called micromount boxes) that protect them from dust, humidity, and mechanical damage.