THE MINERAL PICROMERITE

Chemistry: K₂Mg(SO₄)₂ - 6H₂O, Hydrated Potassium Magnesium Sulfate.
Class: Sulfates
Group: Picromerite
Uses: As a minor source of magnesium and as mineral specimens.
Specimens

Picromerite, also known as "schoenite", forms in marine evaporite deposits where sea water has been concentrated and exposed to prolonged evaporation. Other marine evaporite minerals include calcite, dolomite, gypsum, anhydrite, halite, kieserite, polyhalite, kainite, carnallite and sylvite. Evaporite minerals are geologically important because they clearly are related to the environmental conditions that existed at the time of their deposition, namely arid. They also can be easily recrystallized in laboratories enabling sedimentologists to obtain their specific characteristics of formation. Picromerite is also found as a crusty deposit around fumaroles and in some dry caves. Specimens of picromerite in dry air will dehydrate forming the mineral langbeinite and will turn cloudy. For this reason specimens of picromerite should be stored in sealed containers.

Picrmerite lends its name to a group of sulfates called the Picrmerite Group. These sulfates are all monoclinic, have medium to large sized cations and have six water molecules. The general formula of this group is A₂B(SO₄)₂ - 6H₂O. The A cation can be either potassium or ammonium, NH₄. The B cation can be either iron, copper, magnesium and/or nickel.
These are the members of the Picromerite Group:

Boussingaultite (Hydrated Ammonium Magnesium Sulfate)
Cyanochroite (Hydrated Potassium Copper Sulfate)
Mohrite (Hydrated Ammonium Iron Sulfate)
Nickel-boussingaultite (Hydrated Ammonium Nickel Magnesium Sulfate)
Picromerite (Hydrated Potassium Magnesium Sulfate)

THE PHYSICAL CHARACTERISTICS OF PICROMERITE:

Color is white, colorless, reddish, gray or yellowish.
Luster is vitreous.
Transparency: Specimens are translucent to opaque.
Crystal System is monoclinic; 2/m.
Crystal Habits include granular, earthy and encrusting masses. Individual short prismatic crystals are rare.
Cleavage is perfect.
Fracture is conchoidal.
Hardness is 2.5
Specific Gravity is approximately 2.0 - 2.1 (light even for translucent minerals).
Streak is white.
Other Characteristics: Bitter taste.
Associated Minerals include trona, blodite, halite, alunite, thernardite, sylvite and other more rare evaporite minerals.
Notable Occurrences include Mt. Etna, Sicily and Mt. Vesuvius, Napoli, Campania, Italy as well as Galicia, Poland and Stassfurt, Saxony, Germany.
Best Field Indicators are associations, density, habit, fracture and environment of formation.