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Formula: Mg(SO4).5H2O
Hydrated sulphate, chalcanthite group
Crystal System: Triclinic
Specific gravity: 1.896 measured, 1.90 calculated
Hardness: 2½
Streak: White
Colour: Colourless, pale blue
Solubility: Soluble in water
Environments
Localities
At the Basse-Casamance Valley, Senegal, pentahydrite occurs in acid sulphate soils associated with
alunogen and chalcanthite
(HOM).
Near Miami, Arizona, USA, copper-bearing pentahydrite was collected
from an efflorescent rim on a depression that had held water in a large waste-rock area. After dissolution of the
efflorescence in water, and evaporation of the resulting liquid,
alpersite precipitated and quickly dehydrated to
copper-bearing pentahydrite. It is suspected that
alpersite and copper-bearing
pentahydrite are widespread in mine wastes that contain
copper-bearing sulphides, but in which Fe2+ ions are not
available for melanterite crystallisation because of oxidation to
Fe3+ in surface waters of near-neutral pH
(AM 91.261-269).
At the type locality, the Cripple Creek Mining District, Teller County, Colorado, USA, pentahydrite occurs
as an efflorescence on shales associated with
alunogen and chalcanthite
(HOM).
At Virginia City, Comstock Mining District, Storey County, Nevada, USA, pentahydrite occurs
as an efflorescence on mine timbers associated with
alunogen and chalcanthite
(HOM).
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