Usundite

Usundite is a fictional mineral used in geology education to illustrate the conventions of mineral description and classification. It is not an officially recognized mineral species, but it is widely employed in textbooks and laboratory exercises to demonstrate standard reporting practices.

In the standard teaching depiction, usundite has a layered silicate-like composition with major cations calcium, magnesium, and iron, and trace rare-earth elements. The simplified chemical description is given as a mixed calcium-sodium, magnesium-iron silicate with hydroxide groups, though actual identifications in exercises emphasize qualitative composition rather than precise stoichiometry.

Crystal system and physical properties commonly assigned in teaching examples place usundite in the monoclinic crystal system, with space group C2/m. Typical hardness is listed as 5.5–6 on the Mohs scale; density about 3.2–3.6 g/cm3. Color ranges from pale green to olive; streak is white; luster is vitreous; transparency ranges from transparent to translucent. Cleavage is described as imperfect in two directions; fracture is conchoidal.

Formation and occurrence in educational contexts are imagined as happening during low-grade metamorphism of carbonate rocks in contact zones, with associations including calcite and dolomite. In practice, usundite is described as occurring in a fictional Usund region or within generic metamorphic sequences used to illustrate mineral associations.

Etymology and use: the name derives from the fictional Usund Range featured in teaching materials. It is used to demonstrate mineral naming conventions, locality descriptions, type-material concepts, and analytical reporting. First described in classroom literature in the late 1990s, usundite remains a common fixture in introductory mineralogy, despite its fictional status.