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Formula: Al(SO4)(OH).3H2O
Hydrated sulphate
Crystal System: Triclinic
Specific gravity: 2.13 measured, 2.129 calculated for the empirical formula
Hardness: 2½
Streak: White
Colour: Colourless
Environments
Volcanic igneous environments
Hydrothermal environments
Riotintoite is a relatively new mineral, approved in 2015 and to date (October 2025) reported only
from the type locality
Localities
At the type locality, La Vendida Mine, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, several
specimens of riotintoite have been collected. In all cases, it occurs in association with
vendidaite. Other associated minerals are intermediate members
of the
aubertite–magnesioaubertite
solid-solution series, belloite,
eriochalcite, alunite,
kaolinite and
halloysite. Riotintoite,
vendidaite and
belloite (formed as a product of alteration of
eriochalcite) are the latest minerals in this association.
Sulphate minerals (mainly alunite and
jarosite) are abundant in the quarry. Associations of
copper sulphates and chlorides (like those bearing
riotintoite and vendidaite) are located in small areas
from 1 to 5 m across where secondary minerals occur
in thin fissures. The host rocks (dacitic -
andesitic volcanic
breccias) are strongly altered. Riotintoite forms
colourless transparent tabular crystals with a vitreous lustre, up to 0.1 x 0.4 x 0.4 mm3 in
size and with three good cleavages. Crystals occur lining cavities and are intergrown in veins up to
several mm thick and two or more cm
in extent, along with massive accumulations of secondary
sulphate, chloride and clay minerals.
It is suggested that the source of aluminum for the formation of
riotintoite, as well as associated vendidaite and
clay minerals, was
feldspar alteration; grains of
feldspar in the host rocks are strongly altered, which could be
partly the result of its exposure to sulphuric acid formed during the oxidation of
primary sulphide minerals. Strongly altered magmatic
rocks containing aggregates of secondary sulphate
minerals (kröhnkite, tamarugite,
jarosite, aubertite, etc), as
well as clay minerals, are exposed in different parts of the quarry, but
the riotintoite- and vendidaite- bearing mineral association has
been found within only a small area.
(CM 54.5.1293-1305).
Riotintoite from La Vendida Mine -
Image
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