Leguernite

Chemical formula: Bi<sup>3+</sup><sub>38</sub>O<sub>42</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>15</sub>

Leguernite is an extremely rare bismuth sulfate, forming microscopic, colorless to white crystals as volcanic fumarole products.

## Characteristics Leguernite is a very rare mineral from the sulfate group, chemically a basic bismuth sulfate. It occurs as extremely small, tabular or lamellar crystals, rarely exceeding 0.2 mm in length. The crystals are colorless to white and usually form disordered aggregates or thin coatings on the rock surface. Due to their size, visual features are difficult to observe without a microscope. ## Physical Properties Leguernite crystals exhibit a luster described as vitreous to pearly. They are transparent to translucent. Hardness and density have not been precisely measured due to the small size and rarity of the material. ## History and Name The mineral was named in honor of François Le Guern (1942-2014), a French volcanologist who for many years studied volcanic gases and their condensation products (fumaroles). Le Guern was a pioneer in direct gas sampling from active volcanoes. Leguernite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2013 (IMA2013-031). The type locality is the Arsenatnaya fumarole on Tolbachik volcano in Kamchatka, Russia.

Properties

Luster
Vitreous to pearly
Streak
White
Cleavage
Perfect on {0001}
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of leguernite is possible only through advanced analytical methods, such as Raman spectroscopy and powder X-ray diffraction (PXRD), combined with chemical analysis (EDS). Visual identification in the field or even in a collection is impossible due to its microscopic size and similarity to many other fumarole products. ## Distinguishing from similar minerals Leguernite can be confused with other bismuth sulfates, such as pusscharite, bismutite, or natrotantite, which form in similar environments. Definitive differentiation requires specialized laboratory equipment. ## Crystal forms It forms very small, thin, tabular crystals with a hexagonal outline, often elongated and flattened. They occur as single, dispersed crystals or form disordered aggregates and thin crusts.

Geological environment

## Genesis Leguernite is a mineral of volcanic origin, formed by sublimation from volcanic gases in high-temperature fumaroles (at temperatures of 450-600°C). It crystallizes directly from the gas phase on the surface of volcanic rocks near exhalation vents. ## Mineral associations This mineral co-occurs with other volcanic sublimation products, such as anglesite, galena, pusscharite, bismutite, as well as rare bismuth arsenates, tellurides, and selenates. ## Localities The only confirmed occurrence of leguernite in the world is its type locality - the Arsenatnaya fumarole, located on the second cinder cone of the Northern Breakthrough of the Great Fissure Eruption of Tolbachik volcano on the Kamchatka Peninsula in Russia.

Rarity

Extremely rare

For collectors

## Quality criteria For a mineral as rare as leguernite, the only criterion is its confirmed presence on a fragment of the host rock. Every specimen, even microscopic, is extremely valuable from a scientific and collecting point of view. The size and richness of crystal aggregates, though still on a micro scale, can increase the value of a specimen. ## Popular localities The only source of specimens is Tolbachik volcano in Kamchatka. Material from this locality is extremely difficult to obtain and appears on the collector's market sporadically, usually offered by specialized dealers focusing on minerals from this specific locality.

Care and storage

## Cleaning Due to its extreme rarity, small crystal size, and fragility, leguernite specimens should not be cleaned mechanically or chemically. Any attempts at cleaning risk irreversible damage to the mineral. They should be left in their pristine state. ## What to avoid Avoid contact with water, chemicals, ultrasound, and any vibrations or impacts. Specimens are very sensitive to changes in temperature and humidity. Do not expose them to direct sunlight. ## Storage Leguernite specimens must be stored under stable conditions, preferably in sealed "micromount" containers that protect them from dust, humidity, and mechanical damage. Store in a dry place at a constant temperature.

Sources

Read more