Sylvite

Sylvite is a mineral made of potassium and chlorine (potassium chloride, KCl). It looks very much like halite, the mineral form of table salt: it forms cube-shaped crystals, is soft, dissolves in water and tastes salty, but with a slightly bitter edge. It is one of the last salt minerals to settle out as sea or lake water evaporates in closed basins. Its real importance is as the main ore of potash, the potassium fertilizer.

Fertilizer: The potash obtained from sylvite is the main source of potassium, one of the three basic plant nutrients. In the USA the fertilizer industry uses about 85% of potash sales; the rest goes to chemical and industrial uses (USGS). There is no substitute for potassium as a plant nutrient. Production: According to the 2025 estimates in the USGS 2026 report, more than half of world potash production (about 49 million tonnes of K₂O) comes from two countries: Canada (about 15 million tonnes) and Russia (about 10 million tonnes).

It has a hardness of 2 on the Mohs scale. It is soft enough to scratch with a fingernail. Its specific gravity is 1.99 g/cm³.

Its best-known localities: Mount Vesuvius, Campania, Italy; Esterhazy area, Saskatchewan, Canada; Carlsbad potash district, Eddy County, New Mexico, USA; Verkhnekamsk deposit (Solikamsk–Berezniki), Perm region, Russia; Salihorsk (Soligorsk), Minsk region, Belarus.

Like halite, it forms cubes and octahedra and has perfect cleavage in three directions at right angles. It is usually colourless, white or pale grey-blue; inclusions of iron oxide (hematite) can colour it yellowish red to red. It is very soft (hardness 2), dissolves in water and tastes salty, but with a bitter note as well. Never try to identify unknown minerals by tasting them, because some minerals are toxic. Because it looks so much like halite, telling them apart by eye is hard. Sylvite is a little lighter (specific gravity about 2.0 against about 2.2 for halite) and bends light a little less (refractive index 1.49 against 1.54 in halite). The mineral it is most often confused with is halite (table salt). A chemical test or laboratory measurement is needed to tell the two apart for certain.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Sylvite’s value comes not from beauty, as with a gem, but from its potassium content as an ore: the grade of the deposit (K₂O), the thickness and depth of the bed, whether the ore is dug from underground or dissolved with hot water and pumped up, the cost of separating it from halite and other salts, and the distance to farmland and ports. Potash used in agriculture is expected to be at least 95% potassium chloride (USGS). For collectors, clear, sharp-edged or red, undamaged crystals and specimens from famous places such as Carlsbad and Esterhazy are sought; because it dissolves in water and reacts to moisture, it is hard to keep.

In the 17th century the potassium chloride studied by the Dutch physician Franciscus Sylvius was known as “Sylvius’s digestive salt” and “Sylvius’s fever-reducing salt”; these names show that the salt was believed to help digestion and fever. There is no evidence that sylvite or potassium chloride improves digestion or lowers fever; the mineral’s name merely preserves a trace of this old terminology. Potassium chloride is today a genuine medicine, used under medical supervision for a lack of potassium in the blood, but too much of it is dangerous enough to stop the heart. Sylvite specimens should therefore never be eaten or soaked in drinking water. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.

Sylvite is harmless to handle but must not be eaten: if too much potassium chloride is taken in, the potassium in the blood rises to a dangerous level (hyperkalaemia) and can disturb the heartbeat. Never taste mineral specimens or put them in drinking water. Because it dissolves in water, keep specimens somewhere dry.