Overview
Anthoinite is a rare hydrated aluminium tungstate mineral, a soft white species first described from the tungsten deposits of the Democratic Republic of the Congo. It forms in the weathered, near-surface parts of tungsten-bearing ore zones and is of interest to mineralogists and to specialists who study the alteration of tungsten ores. Its dull, chalky appearance and softness set it apart from the harder, more familiar tungsten minerals.
Composition & structure
Anthoinite is a hydrated aluminium tungstate, commonly written as Al(WO4)(OH)·H2O and equivalently as AlWO3(OH)3. It is white when free of iron and has a notably high specific gravity, reflecting its tungsten content. The mineral is very soft and earthy, with a dull lustre, and tends to occur in fine-grained, compact masses rather than as well-formed crystals.
| Formula | Al(WO4)(OH)·H2O |
| Crystal system | Triclinic |
| Mohs hardness | About 1 (very soft) |
| Lustre | Dull, earthy |
| Colour | White (when free of iron) |
| Type locality | Maniema, Democratic Republic of the Congo |
Formation & occurrence
Anthoinite is a secondary mineral that forms by the weathering and alteration of primary tungsten minerals in the oxidised parts of tungsten deposits. It is associated with the breakdown of tungsten ores in tropical, deeply weathered terrains, which is consistent with its discovery in the Congo. It occurs in tungsten-bearing pegmatite and quartz-vein systems and their alluvial and eluvial derivatives, where the original tungsten minerals have been chemically reworked.
Identification & similar species
Anthoinite's softness, white colour, dull lustre and high density are its most useful field characteristics, but as a fine-grained alteration product it is readily confused with other secondary tungsten and clay-like minerals. It is related to mpororoite, another hydrated aluminium tungstate, and reliable identification depends on chemical and X-ray analysis. Its high specific gravity, unusual for such a soft white material, is a helpful clue.
Notable localities & collecting
The type locality is in Maniema, in the Democratic Republic of the Congo, and the mineral has also been reported from tungsten deposits in the wider Central African region, including Rwanda. Anthoinite is rare and unspectacular in appearance, so it is collected primarily as a species of mineralogical and scientific interest rather than for display, and it is seldom offered in the general mineral trade.