Formula: NaAl(SO4)2.11H2O
Hydrated normal sulphate
Specific gravity: 1.73 to 1.765
Hardness: 3
Streak: White
Colour: Colourless, turns white upon exposure, colourless in transmitted light.

Hydrothermal environments

Mendozite is formed rarely by oxidation of pyrite in reaction with clays; it also may occur in volcanic fumaroles. Tamarugite is an associated mineral, and mendozite alters to tamarugite on exposure to air (HOM).


At the Alcaparrosa mine, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, mendozite occurs in some abundance; it is a late mineral in the sequence and occurs after alunogen. The sequence at Alcaparrosa is, starting from the earliest, metavoltine, copiapite, pickeringite, mendozite, tamarugite and lastly fibroferrite (AM 23.722).

The type locality is San Juan, Capital Department, Mendoza Province, Argentina.

At Eureka, St. Louis county, Missouri, USA, mendozite occurred as a white powder in a recently excavated road cut in dolomite. Ground water migrating down the slope of the hill and seeping out on this surface would be dried by the sun. The evaporating water, fed by seepage and capillarity, would leave behind its soluble load as a residue on the surface. The powdered efflorescence was examined microscopically and other than impurities of fine quartz, dolomite, and clay, the material was found to consist of mendozite and tamarugite (AM 20.537-539).

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