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Acicular crystals and flakes of Mascagnite
Category Sulfate mineral
(repeating unit)
Strunz classification 07.AD.05
Color Colourless, grey, yellowish-grey, yellow
Crystal system Orthorhombic
Cleavage Distinct/ good
On {001}
Fracture Irregular/ uneven
Tenacity Sectile
Mohs scale hardness 2 - 2½
Luster Vitreous, dull
Diaphaneity Transparent, translucent, opaque
Density 1.768 g/cm3

Mascagnite is a rare ammonium sulfate mineral (NH4)2SO4. It crystallizes in the orthorhombic system typically forming as stalactitic masses exhibiting good cleavage. It is soft (not higher than 2.5 on the Mohs Scale) and water soluble. Optical properties are variable; the purest form is transparent and colorless, but opaque gray or yellow deposits are also known.

It occurs in fumaroles, as at Mount Vesuvius and associated with coal seam fires. It was named for Italian anatomist Paolo Mascagni (1752–1815) who first described the mineral.