Images
Formula: Al4(SO4)(OH)10.15H2O
Hydrated sulphate
Crystal system: Monoclinic
Specific gravity:
Streak: White
Colour: White
Environments
Localities
At the Chickerell brick pit, Chickerell, West Dorset, Dorset, England, UK, hydrobasaluminite occurs as a
reaction rim surrounding carbonate concretions, and is believed to have resulted from the neutralisation of
aluminium-bearing acid sulphate solutions formed by oxidation of pyrite and
subsequent leaching of clay.
Felsőbányaite (basaluminite) is found only on concretions that have
fallen to the floor of the pit, suggesting that it is formed as a dehydration product of hydrobasaluminite.
Hydrobasaluminite dehydrates irreversibly to Felsőbányaite
(basaluminite) under normal laboratory conditions, but can be preserved indefinitely at high relative humidity
(MM 43.931-937).
At Newhaven Cliffs, Newhaven, Lewes, East Sussex, England, UK, hydrobasaluminite has been found
(Mindat photo).
Hydrobasaluminite from the Newhaven Cliffs -
Image
At the type locality, the Old Lodge Pit, Irchester, Wellingborough, Northamptonshire, England, UK, the mineral
assemblage, including hydrobasaluminite and felsőbányaite
(basaluminite), occurs along joints as a coating on the wall-rock and as filling narrow fissures, or as matrix to
the infilling. The uppermost bed of the ironstone is a massive
siderite mudstone. In the
underlying shelly
siderite-chamosite
oolite, weathering has oxidised much of the massive joint blocks to
ochreous brown limonitic stone with surviving cores of fresh green
carbonate stone. An outermost skin to the joint blocks consists of dense dark brown
limonite, clearly a
secondary enrichment. Deposited on the
limonite where it forms the wall of a fissure there is usually a thin
layer of fairly compacted brown goethite. Following this
goethite outwards is a second layer. This is usually fairly hard white
yellowish allophane, with well-defined banding.
Secondary fibrous
gypsum in places is found between this deposit and the rock-wall.
Calcite in masses the size of a small pin head,
aragonite needles, and long hair-like
gypsum crystals all occur locally as a later deposit on the
allophane in open fissures.
The central part of the infilling below the siderite
mudstone is a breccia
of fragments of the wall-rock in a white matrix consisting of
felsőbányaite (basaluminite) and hydrobasaluminite associated
with allophane. In the upper part fragments are mostly
limonitised
siderite mudstone, and in
the lower part brown oolitic
ironstone. Massive
halloysite occurs immediately below the
siderite mudstone
(MM 29.1-17).
Near Shoals, Martin County, Indiana, USA, hydrobasaluminite is found as irregular patches and lenses in
allophane and halloysite. The
clay mineral deposits are underlain by
limestone and are overlain by
quartz pebble
conglomerate. The hydrobasaluminite appears to have formed
by alteration of allophane which replaces
limestone, or by direct replacement of the
limestone.
The Shoals hydrobasaluminite dehydrates to felsőbányaite
(basaluminite) at room temperature (26oC), and decomposes to amorphous material at about 200oC.
The hydrobasaluminite consists of plates up to 5 microns across and about 0.05 microns thick, some with
rhombic shapes
(AM 54.1363-1373).
At Cottonwood Cave, Guadalupe Mountains, Eddy County, New Mexico, USA, hydrobasaluminite occurs in cave
passages in argillaceous dolostone by the reaction of sulphuric
acid with kaolinite. Associated minerals include
gibbsite and amorphous silica
(HOM).
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