KCl, potassium Chloride
As a major source of potash and as mineral specimens
Sylvite, also called sylvine, is a major source of potassium or potash used in fertilizer products. So great is the need for potassium that sylvite deposits are considered very valuable economically. As a mineral specimen sylvite does not get much attention. The crystals can be well formed and are often reddish due to inclusions of hematite. However, sylvite is very soluble in water and specimens need to be stored in closed containers because even the moisture in the air can degrade its appearance. Never clean a sylvite specimen with water.
Sylvite is closely related to the more common halite, NaCl, and they share so many properties that identification is sometimes difficult. Sylvite commonly has octahedral faces truncating the corners of the cubic crystals. So does halite, but this characteristic is much more prevalent in sylvite than in halite. Better tests include a taste test in which halite, salt, will taste salty and sylvite tastes bitter. This test is good if you need to distinguish one or two specimens, but what if you are testing hundreds of feet of core samples for beds of sylvite verses halite. A good test in those cases is the knife test in which a knife blade when scratched across the surface of the sample will produce a powder in halite and not in sylvite.
The name sylvite is easily confused with the much more valuable sylvanite. Sylvanite is a silver gold telluride, AuAgTe4 and should never be mistaken for sylvite.
Colour: colourless or white, sometimes tinted red, blue or yellow
Transparency: Crystals are transparent to translucent
Crystal System: isometric; 4/m bar 3 2/m
Crystal Habits: cubes with frequent octahedral faces truncating the corners of the cube, crystals will often have a cavernous appearance from dissolution. More commonly massive and granular
Cleavage: good in three directions forming cubes
Hardness: 2 – 2.5
Specific Gravity: 3.9 – 4.1 (heavier than average for translucent)
Other: Dissolves easily in water, does not powder when the blade of a knife is scratched across its surface and has a bitter taste, not salty like halite
Associated Minerals: include halite, carnallite, kieserite, gypsum, anhydrite and other evaporite minerals
Major Occurrences: include Strassfurt, Germany; Kalush, Russia; New Mexico, Texas and Kern Co., California, USA; Saskatchewan, Canada; France, Mt. Vesuvius, Italy and Spain
Best Indicators: bitter taste, associations and crystal habit