Epsomite, or heptahydrite as it is known in chemistry circles, is one of only a few water soluble sulfate minerals. It is actually well known in most households as the artificially created epsom salt. Magnesium sulfate's medicinal uses were discovered from mineral waters at Epsom, England from where epsom salt and epsomite get their names.
Epsomite forms from as an efflorescent (a precipitation from vapors), on limestone cave walls and on the walls and timbers of various mines. It has been found in deposits from hot springs and fumaroles such as on Mt. Vesuvius, Italy. Epsomite forms large sedimentary beds having been included in some very unusual marine salt deposits in South Africa.
As might be expected, large crystals are few. Crusts and massive forms are more common. Some of the best specimens have attractive fibrous almost cotton-like aggregates with a silky luster.
Identification is usually pretty easy. Besides the easy solubility, the taste is very diagnostic. If you have ever tasted epsom salt, you have tasted epsomite and know of the bitter taste.
The easy solubility is a detriment to most specimens in collections.
To keep epsomite it is recommended to clean the specimens gently with alcohol
and then store them in sealed containers. In dry air conditions, epsomite
may lose one molecule of water and convert to the very closely related
The symmetry of epsomite happens to be rather notable. It is one of the very few minerals that crystallize in the Orthorhombic Disphenoidal Class. This symmetry class has just three 2 fold rotational axes at right angles to each other. No mirror planes or other symmetry operations. It is unfortunate that epsomite does not typically form well shaped crystals that show the unusual symmetry.
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