Images
Formula: Al12(SO4)5(OH)26.20H2O
Hydrated sulphate
Crystal System: Triclinic
Specific gravity: 2.007 measured, 2.01 calculated
Hardness: 3½
Colour: White to blue-green
Solubility: Dehydrates reversibly at room temperature
Environments
Metamorphic environments
Hydrothermal environments
Localities
At the type locality, the Salt Range, Punjab Province, Pakistan, zaherite is associated with
kaolinite, böhmite
and
and aluminite. Here, zaherite is found both in
relatively pure form as veinlets up to 1 cm across that cut massive
kaolinite-böhmite rock,
and intimately mixed with kaolinite and
böhmite in a massive white
claystone.
Aluminite is also found as veinlets in the vicinity.
Zaherite in fairly pure form was found as densely packed aggregates of extremely small grains
(AM 62.1125-1128).
Zaherite from the Salt Range -
Image
At the Hotson 6 Mine, Koenabib farm, Khâi-Ma Local Municipality, Namakwa District Municipality, Northern
Cape, South Africa, zaherite occurs as veins up to 2 cm wide in close association with
natroalunite and
hotsonite. It is derived from
primary
sillimanite
but alters to encrustations of hotsonite. The
sillimanite occurs as large irregular lenses with size
ranging from several metres to hundreds of metres. The zaherite occurs in an arid region with an
average rainfall of less than 3 inches per annum. The mode of occurrence suggests that the
zaherite is an intermediate mineral in the alteration of
natroalunite to
hotsonite, the former originally deriving from
sillimanite.
Zaheite occurs as sheaves of chalk-white to light bluish green material orientated perpendicular
to the walls of the veins
(MM 48.131-135).
The aluminous minerals occur in a
biotite-sillimanite
schist. The alteration products include silicates, oxides,
sulphates and sulphides. The prevalent silicates are kaolinite,
pyrophyllite, topaz,
muscovite and biotite.
The oxides are diaspore,
rutile, ilmenite and,
rarely, corundum. Hydrated oxides are common, particularly
goethite. The sulphides and sulphide alteration-products are
chalcopyrite,
pyrite, covellite,
chalcanthite,
atacamite and, rarely,
brochantite. Sulphates are represented by
natroalunite, zaherite and
gypsum. The zaherite is associated with
hotsonite; rare grains of
turquoise were also identified.
(CM 23.1.29-34).
Mineral Reactions at the Hotson 6 Mine
Evidence supports a petrogenetic model that involves four chemical reactions that sequentially entailed
the transformation of sillimanite to
natroalunite,
natroalunite to zaherite and zaherite to
hotsonite, which is also the order of relative abundance at
the Hotson 6 quarry. In a few instances zaherite has formed directly from
sillimanite.
The prevalent appearance of natroalunite as the first
member of the reaction chain was apparently controlled by the local availability of abundant sodium. All
the veins at Koenabib have natroalunite, never
alunite, which contrasts with many other comparable occurrences.
(1) sillimanite to
natroalunite
3(Al2O3.SiO2) + 2Na+ + 4(SO4)2- +
3H2O + 6H+ = Na2O.3Al2O3.4SO3.6H2O +
3SiO2
A source of sodium is close at hand in the feldspar of the
country rock and its alteration products.
(2) Natroalunite to zaherite
2(Na2O.3Al2O3.4SO3.6H2O) + 22H2O =
6Al2O3.5SO3.33H2O + 4Na+ +
3(SO4)2- + 2H+
The transformation merely involves the removal of sodium and some of the sulphate in a hydrous medium.
This could easily have been accomplished during a local hydrothermal or weathering event.
(3) Sillimanite to zaherite with no consumption of
sodium
6(Al2O3.SiO2) + 5(SO4)2- +
28H2O + 10H+ = 6Al2O3.5SO3.33H2O +
6SiO2
It would seem that this transformation merely requires a very hydrous environment and a source of
sulphate, but the fact that the transformation of sillimanite
to natroalunite was much more common indicates that a local
deficiency of alkalis must be essential as well. This is probably the reason for the relative rarity of
zaherite.
(4) Zaherite to hotsonite
11(6Al2O3.5SO3.33H2O) + 12P2O5 =
6(11Al2O3.2P2O5.6SO3.53H2O) + 19(SO4)2-
+ 26H2O + 38H+
The source of the phosphate essential for the formation of
hotsonite is at present obscure. The sulphate required for the
formation of natroalunite, zaherite and
hotsonite presents no problem because sulphides are ubiquitous
in the country rocks as well as in the sillimanite ore. The
fairly abundant chalcopyrite is extensively altered to
covellite and
goethite, and encrustations of
chalcanthite and
atacamite are common
(CM 23.1.29-34).
Zaherite from the Hotson 6 Mine -
Image
Back to Minerals