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Formula: NaAl(SO4)2.11H2O
Hydrated normal sulphate
Crystal System: Monoclinic
Specific gravity: 1.730 measured, 1.781 calculated
Hardness: 3
Streak: White
Colour: Colourless, turns white upon exposure, colourless in transmitted light.
Environments
Hydrothermal environments
Fumeroles
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).
Localities
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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