Overview
Alarsite is a rare aluminium arsenate mineral that crystallises directly from hot volcanic gases. It was discovered in the fumaroles of the Tolbachik volcano on the Kamchatka Peninsula in Russia, one of the world's richest sources of unusual arsenate and sulphate minerals. As a fumarolic species, alarsite is fragile, scarce and of essentially scientific and specialist-collector interest; it has no commercial or ornamental use.
Composition & structure
Alarsite has the simple formula AlAsO4, the arsenate analogue of the aluminium phosphate berlinite and structurally related to quartz. It crystallises in the trigonal (hexagonal) system and occurs as small subhedral grains up to a fraction of a millimetre across, gathered in aggregates. The mineral is brittle, has a vitreous lustre, and is colourless, sometimes with pale yellow, green or blue tints caused by inclusions; in transmitted light it is colourless.
| Formula | AlAsO4 |
| Crystal system | Trigonal (hexagonal) |
| Mohs hardness | Not formally determined (microhardness roughly 5) |
| Lustre | Vitreous |
| Colour | Colourless, with pale yellow, green or blue tints |
| Type locality | Tolbachik volcano, Kamchatka, Russia |
Formation & occurrence
Alarsite is a fumarolic mineral, deposited from arsenic- and aluminium-bearing volcanic exhalations as gases cool and react near the surface. At Tolbachik it is found in association with a suite of equally exotic high-temperature minerals including fedotovite, klyuchevskite, lammerite, nabokoite, atlasovite, langbeinite, hematite and tenorite. This kind of "sublimate" mineralisation, where species condense directly from gas rather than from water, is uncommon and is the defining environment for alarsite.
Identification & similar species
Alarsite is the arsenate counterpart of berlinite (AlPO4) and shares the quartz-type framework structure, so the two are crystallographically analogous. Because its grains are tiny and colourless, alarsite cannot be identified by eye and is determined by microprobe analysis and X-ray diffraction. Its occurrence as a fumarolic crust at an active volcano, alongside the characteristic Tolbachik mineral assemblage, is itself a strong contextual indicator.
Notable localities & collecting
The Tolbachik volcanic field on Kamchatka is the type and principal locality, and the species remains closely tied to that setting. Fumarolic minerals like alarsite are difficult to preserve because they are fragile and can alter once removed from the hot, dry gas environment. As a result, alarsite is a research curiosity and a target for dedicated micromount collectors rather than a mineral available in the general trade.