Images
Formula: Al5(SO4)(PO4)(OH)10.8H2O
Hydrated phosphate
Crystal System: Triclinic
Specific gravity: 2.060 to 2.068 measured, 2.06 calculated
Hardness: 2½
Streak: White
Colour: White
Environments
Localities
At the type locality, the Hotson 6 Mine, Koenabib farm, Khâi-Ma Local Municipality, Namakwa District Municipality,
Northern Cape, South Africa, hotsonite occurs in close association with
zaherite and natroalunite, as
secondary encrustations on
zaherite derived from
primary sillimanite.
The sillimanite occurs as large irregular lenses with size ranging from
several metres to hundreds of metres. Preliminary results indicate that
sillimanite is locally altered to
natroalunite and zaherite,
which is in turn transformed to hotsonite.
Hotsonite occurs as white chalk-like material with a dull to silky lustre and earthy fracture
(AM 69.979-983).
The evidence concerning the mineral association sillimanite -
natroalunite - zaherite -
hotsonite supports a petrogenetic model that involves four chemical reactions:
(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.
(3)
In a few instances zaherite has formed directly from
sillimanite; the reaction consumes no 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
The reaction responsible for hotsonite, the final event in the postulated chain of reactions, differs markedly
from the preceding transformations.
11(6Al2O3.5SO3.33H2O) + 12P2O5 =
6(11Al2O3.2P2O5.6SO3.53H2O) + 19(SOsub>4)2- + 26H2O + 38H+
This is the only reaction of the series that exhibits prograde characteristics, involving the removal of some of the
sulphate and water.
The prevalent appearance of natroalunite as the first member of the
reaction chain was apparently controlled by the local availability of abundant sodium. It should be noted that all the
veins at Koenabib have natroalunite, never
alunite, which contrasts with many other comparable occurrences
(CM 23.1.29-34).
Hotsonite from the Hotson 6 Mine -
Image
Back to Minerals