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Formula: Mg(SO4).6H2O
Hydrated normal sulphate, hexahydrite group
Crystal System: Monoclinic
Specific gravity: 1.757 measured, 1.745 calculated
Hardness: 2 to 2½
Streak: White
Colour: Colourless, white, pale greenish white, colourless in transmitted light
Luminescence: Fluoresces and phosphoresces dull cream white (Mindat)
Solubility: Soluble in water
Environments
Evaporite deposits
Hydrothermal environments
Fumeroles
Martian environments
Hexahedrite is formed by dehydration of epsomite, as efflorescences
on magnesian rock exposures and in mine workings and as speleothems in caves; it may be widespread as an evaporative
precipitate in saline lakes and soils and it occurs rarely a fumarolic precipitate (HOM). It may dehydrate to
starkeyite. Associated minerals include
calcite, epsomite,
niahite, siderotil and
starkeyite
(Mindat).
Localities
The type locality is the Bonaparte River, Lillooet Mining Division, British Columbia, Canada.
The Former Sherritt Gordon Mine, Sherridon District, Manitoba, Canada, was mined for
zinc, copper,
silver and gold between 1930 and 1951.
Since the mine closed, high-sulphide tailings impoundments underwent extensive oxidation, and surface seeps developed
along the flanks of the impoundments, discharging groundwater with a pH as low as 0.39 (extremely acid) and high
concentrations of dissolved sulphate and iron and other metals. Several
secondary efflorescent minerals were observed within
groundwater-seepage areas including melanterite,
rozenite, halotrichite,
chalcanthite, alpersite,
copiapite, hexahydrite,
jurbanite, pickeringite,
jarosite and gypsum
(CM 53.961-977).
At the Kladno District, Central Bohemian Region, Czech Republic, hexahydrite is
pseudomorphous after epsomite
(Dana).
Kelčany, Hodonín District, South Moravian Region, Czech Republic, hexahydrite is
pseudomorphous after epsomite
(Dana).
In the Gyöngyösoroszi–Mátraszentimre region, Mátra Mountains, Hungary, intensive
zinc-lead-copper
mining took place for about 40 years, then the last mine was closed in 1986. In 2006, a project was launched for the
remediation of the waste dumps and for the sealing of the abandoned shafts and adits still releasing acid mine-drainage
owing to the large amount of oxidising pyrite in the walls. In the Bányabérc
waste dump the bulk pH was around 3.7 (acid). Minerals found there included
pyrite, gypsum,
jarosite, hydroniumjarosite,
plumbojarosite, anglesite,
quartz, the feldspars,
goethite, hematite,
pickeringite, alunogen,
epsomite, hexahydrite, a
smectite-group phase, galena,
sphalerite and trace calcite
(CM 47.509-524).
At the Kujawy salt deposits, Poland, hexahydrite occurs with syngenite and other sulphates
(Dana).
On the banks of the Rio Tinto, Minas de Riotinto, Huelva, Andalusia, Spain, the soluble metal sulphate salts
melanterite, rozenite,
rhomboclase, szomolnokite,
copiapite, coquimbite,
hexahydrite and halotrichite, together with
gypsum, have been found
(MM 67.263–278).
At Oroville, Oroville Mining District, Okanogan county, Washington, USA, hexahydrite occurs as efflorescence
on epsomite
(Dana).
Extra-terrestrial.
In the sulphate-bearing strata on Mars the hydrated sulphates observed in the surface thin layer by remote sensing are
likely weathering products. Widespread kieserite was identified in situ on
the weathered surface of the Mg-sulfates-rich section, which formed from hexahydrite dehydration after exposure
to the ambient conditions
(AM 99.283-290).
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