The healthful springs at Epsom are no longer used to make this salt—it is more commonly refined from the mineral epsomite (mined or produced artificially), and it is abundant and inexpensive.
Ep"som salts` ∨ salt` (?). Med.
Sulphate of magnesia having cathartic qualities; -- originally prepared by boiling down the mineral waters at Epsom, England, -- whence the name; afterwards prepared from sea water; but now from certain minerals, as from siliceous hydrate of magnesia.
THE MINERAL EPSOMITE
Chemistry: MgSO4 - 7H2O, Hydrated Magnesium Sulfate.
Uses: A source of magnesium salts, various chemical and medicinal uses and as mineral specimens.
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 mineral hexahydrite, MgSO4 - 6H2O. It is perhaps hard to believe that the loss of one molecule of water can make a new mineral, but in this case the water molecules are intricately involved with the structure of epsomite. This is seen in the change of symmetry from orthorhombic in epsomite to monoclinic in hexahydrite.
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.
Color is colorless, white with pale shades of pink and green.
Luster is vitreous in large crystals; silky to earthy in most specimens.
Transparency: Crystals are translucent to transparent.
Crystal System is orthorhombic; 2 2 2.
Crystal Habits include fibrous, acicular, encrusting, stalactitic, botryoidal and granular masses. Typically formed on the walls of caves and mines as efflorescences and in playa lakes.
Cleavage is perfect in one direction and distinct in two other directions.
Fracture is conchoidal.
Hardness is 2 - 2.5
Specific Gravity is approximately 1.7 (well below average).
Streak is white to colorless.
Other Characteristics: Is very soluble in water. A fact that is a detriment to most collection specimens as they may absorb water from the air and deteriorate over time. The taste is bitter like epsom salt.
Associated Minerals include aragonite, gypsum, calcite , mirabilite, pyrite and pyrrhotite.
Notable Occurrences include Epsom (hence the name), Surrey, England; Mt. Vesuvius, Italy; Stassfurt, Germany; Herault, France; Kruger Mountain, Oroville, Washington; Albany County, Wyoming; El Tiro Mine, Arizona and Carlsbad, New Mexico, USA; Ashcroft, British Columbia, Canada and South Africa.
Best Field Indicators are crystal habit, low density, associations, solubility in water and taste.