Mansfieldite
Chemical formula: AlAs<sup>5+</sup>O<sub>4</sub>·2H<sub>2</sub>O
Mansfieldite is a rare aluminum arsenate, forming small, inconspicuous crystals, and is the arsenic analogue of scorodite.
Properties
- Mohs hardness
- 3.5-4
- Luster
- Vitreous
- Streak
- White
- Density
- 3.03
- Cleavage
- Good on {010}, poor on {100}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Amateur identification of mansfieldite is practically impossible due to its inconspicuous appearance and small crystal size. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from similar minerals Mansfieldite is visually indistinguishable from scorodite (FeAsO₄·2H₂O), with which it forms an isomorphic series. Distinguishing these two minerals requires chemical analysis to determine the dominant element (aluminum in mansfieldite, iron in scorodite). It can also be confused with other secondary arsenates or phosphates of similar appearance. ## Crystal forms Crystals are most often pseudo-octahedral or thick tabular, often with corroded or rounded faces. It occurs as fine druses, crusts, radial aggregates, as well as earthy and compact masses.
Geological environment
## Genesis Mansfieldite is a secondary mineral, forming in the oxidation (weathering) zones of arsenic-rich ore deposits, especially in aluminum-rich rocks. It forms under low temperature and pressure conditions as a result of the interaction of arsenic acid-containing solutions with clay minerals or other aluminosilicate minerals. ## Mineral associations It co-occurs with other secondary arsenic and sulfate minerals. It is most commonly associated with scorodite, pharmacosiderite, beudantite, realgar, orpiment, kaolinite, as well as iron and manganese oxides. ## Localities Important localities worldwide include Hobart Butte, Oregon (type locality) and Gold Hill, Utah in the USA; the Tsumeb mine in Namibia; the Clara mine in Germany (Black Forest); Lavrion in Greece; as well as some deposits in Japan (Kiura mine, Oita) and Chile.
Rarity
Rare
For collectors
## Quality criteria For collectors, the most desirable specimens are those with well-formed, even if microscopic, crystals with distinct luster and pure color (colorless, white, pink). Rich crystalline druses on a contrasting rock matrix are highly valued. Due to its rarity, even microscopic specimens (so-called micromounts) have great value for systematic collectors. ## Popular localities Specimens from classic localities such as Tsumeb (Namibia) or Gold Hill (Utah, USA) are particularly prized for their historical significance and material quality. Well-formed microcrystals from the Clara mine in Germany are also highly regarded.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its low hardness and potential reactivity, washing in water, and especially using ultrasonics, should be avoided. ## What to avoid Avoid contact with chemicals, acids, and detergents. The mineral is sensitive to high temperatures, which can lead to its dehydration and destruction of its structure. It should be protected from scratching by harder minerals. ## Storage It is recommended to store specimens in separate, padded boxes or "micromount" containers to protect them from mechanical damage and contact with other minerals. Display should be in stable conditions, away from sources of heat and moisture.