Formula: (H5O2)Fe3+(SO4)2.2H2O
Hydrated normal sulphate
Specific gravity: 2.23
Hardness: 2
Streak: White
Colour: Blue, colourless, white, yellow, light green, grey, colourless in transmitted light
Solubility: Slowly soluble in water, easily soluble in acids
Common impurities: As

Hydrothermal environments

Rhomboclase is an uncommon secondary mineral formed by alteration of pyrite, marcasite or pyrrhotite, especially in an arid climate; it is typically of post-mining origin. Associated minerals include szomolnokite, copiapite, römerite, voltaite, epsomite, halotrichite, melanterite, chalcanthite and pyrite (HOM, Mindat).


At the Socavon mine, Oruro City, Cercado Province, Oruro, Bolivia, rhomboclase occurs as thin platy crystals up to 2 cm in size with melanterite and copiapite (Dana).

At the Alcaparrosa Mine, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, colourless and faintly brownish-purple rhomboclase and intimately associated szomolnokite and römerite with bright angular pieces of pyrite scattered through the minerals (AM 23.669-760).

At the type locality, Smolník, Gelnica District, Košice Region, Slovakia, rhomboclase occurs in an altering pyritic ore body, associated with szomolnokite and copiapite (Mindat).

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