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Formula: NaAl3(SO4)2(OH)6
Sulphate, alunite group
Crystal System: Trigonal
Specific gravity: 2.6 to 2.9 measured, 2.819 calculated
Hardness: 3½ to 4
Streak: White
Colour: White, greyish, yellowish, reddish, red-brown; colourless in transmitted light
Solubility: Insoluble in water
Electrical: Strongly pyroelectric
Common impurities: K
Environments
Sedimentary environments
Hydrothermal environments
Natroalunite is formed by solfataric or hydrothermal sulphate-bearing solutions reacting with
clays, rarely with
sillimanite; may be in
laterites and as an authigenic sedimentary mineral. Associated
minerals include alunite,
kaolinite, halloysite,
sillimanite and quartz
(HOM).
Localities
At Ichetuyskoye, Dzhida Basin, Buryatia, Russia, the mineral assemblage consists of
lazulite with variable amounts of
scorzalite, trolleite,
augelite, monazite, and members
of the alunite supergroup
(crandallite,
florencite-(Ce),
woodhouseite and
svanbergite).
Numerous electron microprobe analyses are listed for strontian
natroalunite
(AM 86.200).
At Pofadder, Khâi-Ma, Namakwa District Municipality, Northern Cape, South Africa, large lenses of
sillimanite occur in
biotite-sillimanite
schist. In one of the abandoned
sillimanite quarries on the farms Hotson and Koenabib, the paragenesis
sillimanite - natroalunite -
zaherite - hotsonite was found.
It is associated with silicates (kaolinite,
pyrophyllite, topaz,
muscovite and biotite), oxides
(diaspore, rutile,
ilmenite and corundum),
sulphides (chalcopyrite,
covellite and pyrite) and sulphide
alteration products (covellite and
goethite, atacamite and
chalcanthite). The prevalent sequence of alteration is from
sillimanite to natroalunite to
zaherite to hotsonite,
which is also the sequence of decreasing relative abundance, but
sillimanite was directly transformed to
zaherite wherever alkali elements were locally scanty. The necessary
sulphate was apparently supplied during the oxidation of sulphides such as
chalcopyrite and pyrite
(CM 23.29-34).
The type locality is the Red Mountain Mining District, Colorado, USA. The Red Mountain district lies in the
Tertiary volcanic rocks along the western border of the Silverton cauldron subsidence. It is characterised by
strong hydrothermal alteration of the volcanic rocks and by the principal occurrences of ore deposits in
chimney-like forms
(Mindat).
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