senarmontite

Senarmontite is a mineral species in the oxide category with the chemical formula Sb2O3. It is one of the two polymorphs of antimony trioxide, representing the tetragonal form; the other polymorph is valentinite, which is monoclinic. Senarmontite commonly forms as a secondary mineral in the oxidation zones of antimony-bearing ore deposits.

Crystals are typically small to tabular or needle-like, and may occur as aggregates. The color is usually colorless to white, occasionally with a pale yellow tint. It has a vitreous luster, a Mohs hardness of about 2.5, and a relatively high specific gravity around 5.6 g/cm3. It is insoluble in water and may be more stable under oxidizing conditions.

Senarmontite forms in oxidized environments associated with antimony minerals such as stibnite, valentinite, quartz, and other oxide phases. It has been reported from a number of localities worldwide, with notable occurrences in historic European mining districts and in other antimony-rich regions.

The mineral is named after the French general Sénarmont. Identification in practice can be challenging because senarmontite and valentinite are dimorphs of Sb2O3; definitive separation typically requires crystallographic analysis such as X-ray diffraction, as their optical and morphological properties can be similar.