THE MINERAL POLYHALITE

Chemistry: K₂Ca₂Mg(SO₄)₄ - 2H₂O, Hydrated Potassium Calcium Magnesium Sulfate.
Class: Sulfates
Uses: As a minor source of potassium and as mineral specimens.
Specimens

Polyhalite is named in allusion to its many metal ions in its formula or literally translated "many salts". Polyhalite is a potassium, calcium and magnesium sulfate salt. Although it is an evaporite mineral, polyhalite is otherwise not related to the mineral halite. It forms in marine evaporite deposits where sea water has been concentrated and exposed to prolonged evaporation. Polyhalite precipitates only after calcite, dolomite, gypsum, anhydrite and halite have precipitated first. This does not occur often as it requires significant evaporation, but some extensive beds of potassium evaporates have formed and are excavated for their potassium content. Other potassium evaporates include kainite, picromerite, carnallite and sylvite.

Polyhalite is relatively easy to distinguish from other evaporates. Its taste is bitter, unlike halite. It does not completely dissolve in water leaving a calcium sulfate residue, unlike sylvite. It gives a purple flame result when it is put is a gas flame due to its potassium content, unlike kieserite and other non-potassium salts. 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 in order to postulate their specific characteristics of formation.

PHYSICAL CHARACTERISTICS:

Color is white, colorless or gray. Iron oxides may color specimens red, brick-red or pink.
Luster is vitreous to resinous.
Transparency: Crystals are transparent to translucent.
Crystal System is triclinic: bar 1.
Crystal Habits include fibrous, lamellar, granular and foliated masses. Individual crystals are rare.
Cleavage is perfect in one direction.
Fracture is fibrous or splintery.
Hardness is 3.5
Specific Gravity is approximately 2.8 (average for translucent minerals).
Streak is white.
Other Characteristics: Does not completely dissolved in water leaving a residue, has a bitter taste and can color a flame purple (for potassium).
Associated Minerals include halite, anhydrite, kainite, carnallite, sylvite and other more rare potassium evaporite minerals.
Notable Occurrences include Carlsbad, New Mexico and Western Texas, USA; Hallstatt, Austria; Galicia, Poland and Strassfurt, Germany.
Best Field Indicators are incomplete dissolution, associations, cleavage, habit and taste.