Hexahydrite

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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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