Alarsite
Chemical formula: AlAs<sup>5+</sup>O<sub>4</sub>
Aluminum arsenate, a very rare mineral forming tiny, colorless crystals in products of volcanic fumarole activity.
Description
## Characteristics Alarsite is an aluminum arsenate, belonging to the group of rare minerals. It is the arsenate analog of the much more common berlinite (aluminum phosphate). It typically forms very small, colorless or white crystals, which rarely exceed fractions of a millimeter. It occurs as coatings, crusts, and druses on volcanic rocks. Individual crystals most often have a short prismatic or pyramidal habit with a hexagonal outline. ## Physical Properties Alarsite crystals are characterized by a vitreous luster and are transparent to translucent. The mineral's hardness on the Mohs scale is 3, making it relatively soft. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 3.33 g/cm³. ## Colors and Varieties Alarsite is a colorless mineral; sometimes it is colored white by inclusions or structural imperfections. No colored varieties or trade names are known. ## History and Name The mineral's name, approved in 1993, directly refers to its chemical composition – aluminum (Latin: *aluminium*) and arsenic (English: *arsenic*). It was discovered in volcanic exhalation products of the Great Fissure eruption of Tolbachik volcano in Kamchatka, Russia, and described by a team of mineralogists led by Lidia P. Vergasova. ## Applications Due to its extreme rarity and microscopic crystal size, alarsite has no practical applications. It is solely an object of scientific and collecting interest.
Diagnostic features
## Identification Field identification of alarsite is practically impossible without specialized equipment. In a collection, it is recognized based on its specific occurrence environment (volcanic fumaroles), characteristic form of small, hexagonal crystals, and co-occurrence with other rare arsenates. Its low hardness (3 on the Mohs scale) distinguishes it from visually similar quartz (hardness 7). ## Distinguishing from Similar Minerals Alarsite is visually indistinguishable from its phosphate analog, berlinite (AlPO₄) – this requires chemical analysis (EDS, WDS). It can be confused with many other colorless fumarolic minerals, such as quartz or other arsenates. Certain identification is only possible using analytical methods, e.g., X-ray powder diffractometry (XRD). ## Crystal Forms Alarsite crystals are trigonal but exhibit a pseudohexagonal habit. They take the form of short, hexagonal prisms, often terminated by pyramidal faces. Most commonly, they form fine-grained aggregates, coatings, and druses covering the surface of lava or volcanic tuff.
Geological environment
## Genesis Alarsite is a mineral of volcanic origin. It forms by sublimation from hot gases (exhalations) in high-temperature fumaroles (above 400°C). It crystallizes directly from the gas phase on the walls of volcanic conduits and on rock surfaces. ## Mineral Associations This mineral co-occurs with other products of volcanic sublimation, such as hematite, orthoclase, tenorite, as well as other rare arsenates, e.g., johillerite, urusovite, arsemirandite, and eryclaxite. ## Localities The only confirmed and well-documented locality of alarsite in the world is the Tolbachik volcano on the Kamchatka Peninsula in Russia. This is its type locality, from which practically all available specimens originate.
Rarity
Extremely rare
Collector aspects
## Quality Criteria The collector's appeal of alarsite primarily depends on its rarity. Specimens with rich, crystalline coatings, on which well-formed, though microscopic, crystals with distinct luster are visible, are most highly valued. Contrast with the substrate and the presence of other rare associated minerals, which enhance the scientific and aesthetic value of the specimen, are also of great importance. ## Popular Localities The only source of collector specimens is the type locality – fumaroles formed during the Great Fissure eruption of Tolbachik volcano (1975-1976) in Kamchatka, Russia.
Care and storage
## Cleaning Alarsite specimens are typically microscopic and very fragile. The safest cleaning method is to gently remove dust with a soft brush or a photographic air blower (compressed air). Wet cleaning should be avoided, especially if the host rock is porous or may react with water. ## What to Avoid The mineral is sensitive to mechanical damage. Ultrasonic cleaners, strong chemicals, and acids should be avoided. As an arsenate, it contains arsenic, so it is recommended to wash hands after each contact with the specimen. Although stable, it should not be exposed to sudden temperature changes. ## Storage It is recommended to store specimens in specialized "micromount" boxes, which protect delicate crystals from damage, dust, and loss. A clear label with the mineral name and location should be ensured.