Sylvite
Sylvite is potassium chloride, KCl: cubic, hardness 2, density 1.993, soluble in water. Worked, it is 63.18% K2O-equivalent, the unit USGS reports potash in.
Sylvite is potassium chloride, KCl: a cubic halide of evaporite beds, hardness 2, density 1.993, refractive index 1.4903, soluble in water. Worked on standard atomic weights it is 52.45% potassium, or 63.18% K2O-equivalent, the unit USGS reports potash in; carnallite works out at 16.95% and halite, sodium chloride, at none.
In short
- One formula, one row. The September 2026 IMA-CNMNC Master List lists sylvite as KCl, status G, 1832, Italy. The Handbook of Mineralogy sheet gives the same formula and says the mineral is typically nearly pure KCl. On standard atomic weights that is 52.45% potassium and 47.55% chlorine.
- The potash ledger. USGS Mineral Commodity Summaries 2025, Potash says potash denotes a variety of mined and manufactured salts that contain potassium in water-soluble form, and reports its data in K2O equivalent. Worked as (K2O/2) per formula mass: sylvite 63.18%, carnallite 16.95%, halite 0. USGS Open-File Report 2016-1167 calls sylvite potassium chloride in natural mineral form, and sylvinite potassium chloride and halite.
- Where the USGS table slips, documented. The same 2016 report’s Table 1 prints 100.00 against sylvite and 63.17 against sylvinite. Worked, 63.17 to 63.18 is the figure for pure KCl; 100.00 is the K2O line’s own figure, and a mixture of KCl and halite has no single figure. Its carnallite, kainite, langbeinite, leonite and polyhalite lines recompute on its own formulas to within 0.01.
- Sylvite or halite. Both are cubic with perfect {001} cleavage. On their sheets sylvite reads density 1.993 and n 1.4903, halite 2.168 and 1.5443: halite is 8.8% denser and 0.054 higher in index. The halite page carries the humidity figures, which are not repeated here.
- Water, not weighing. The sylvite sheet says soluble in H2O, so the in-water weighing used on other species pages here does not apply to it. Carnallite, KMgCl3·6H2O, its associate, is printed as deliquescent in high humidity, then dissolving; the sylvite sheet prints no such line.
| Material | Formula (USGS / IMA) | What it is (source) | K by mass, worked | K2O-equivalent, worked | USGS Table 1 prints | Density; n (sheet) |
|---|---|---|---|---|---|---|
| Sylvite | KCl / KCl | Potassium chloride in natural mineral form (USGS); IMA G, 1832 | 52.45% | 63.18% | 100.00 (slip; see text) | 1.993; 1.4903 |
| Sylvinite | KCl+NaCl / no row | Potassium chloride and halite (USGS); a mixture, 0 IMA lines contain sylvin | Depends on the mix | Between 0 and 63.18% | 63.17 | - |
| Carnallite | KCl•MgCl2 6•H2O / KMgCl3·6H2O | Potash salt (USGS); IMA G, 1856 | 14.07% | 16.95% | 16.95 | 1.602; α 1.465 to γ 1.496 |
| Kainite | KCl•MgSO4•3H2O / KMg(SO4)Cl(H2O)2.75 | Potash salt (USGS); IMA G, 1865 | - | 18.92% (USGS formula); 19.27% (IMA formula) | 18.92 | Not fetched |
| Langbeinite | K2SO4•2MgSO4 / K2Mg2(SO4)3 | Potash salt (USGS); IMA G, 1891 | - | 22.70% | 22.70 | Not fetched |
| Polyhalite | K2SO4•MgSO4•2CaSO4•2H2O / K2Ca2Mg(SO4)4(H2O)2 | Potash salt (USGS); IMA G, 1817 | - | 15.62% | 15.62 | Not fetched |
| Halite | NaCl / NaCl | Sodium chloride or common salt (USGS); IMA G, 1847 | 0 | 0 | Not listed | 2.168; 1.5443 |
What is sylvite? Potassium chloride
Sylvite is potassium chloride, KCl, a grandfathered species of 1832 whose country of first description the IMA list gives as Italy. The Handbook of Mineralogy sheet prints the same formula and gives no analysis, only the line typically nearly pure KCl. On standard atomic weights the formula mass is 74.55 and the composition 52.45% potassium and 47.55% chlorine.
What the sheet records. Cubic; cubes, cubo-octahedra and octahedra to 15 cm, also columnar, in crusts, coarse granular to compact and massive. Cleavage perfect on {001}; fracture uneven; brittle, ductile under low strain. Hardness 2; density 1.993 measured, 1.987 calculated. Transparent; colourless to white, pale grey or pale blue, and yellowish red to red when included with hematite; vitreous lustre. Isotropic, n = 1.4903, anomalously weakly anisotropic when strained. The sheet has no streak field.
Where it forms. The Occurrence field gives sedimentary basins, forming thick bedded deposits with halite; a sublimate in volcanic fumaroles; nitrate beds and cave deposits; a product of burning coal; and inclusions in other minerals from intermediate-grade metamorphism. Its associates are halite, kieserite, kainite, carnallite, polyhalite, gypsum and anhydrite.
Is sylvite potash? The K2O-equivalent ledger
Potash is a commodity word, not a mineral name: USGS uses it for a variety of mined and manufactured salts that contain potassium in water-soluble form, and sylvite is one of them. USGS Mineral Commodity Summaries 2025, Potash gives that definition and reports its tables in thousand metric tons of potassium oxide (K2O) equivalent. The USGS potash statistics page repeats it. A whole-line scan of the September 2026 IMA list finds 0 lines containing potash.
The ledger, worked. K2O-equivalent is the K2O a salt would yield, (K2O/2) per formula unit for one potassium: 47.10 against KCl’s 74.55 gives 63.18% for sylvite. Carnallite, KMgCl3·6H2O, formula mass 277.84, gives 16.95%; halite, NaCl, gives none. USGS Open-File Report 2016-1167 says the world’s major potash resources are related to evaporite deposits that principally occur as potash salts such as sylvite, potassium chloride in natural mineral form, and sylvinite, potassium chloride and halite. It says muriate of potash (MOP) is potassium chloride.
What the ledger does not say. It gives composition, not grade; the commodity reports’ market and production lines are not used here. The sylvite sheet’s Distribution names the Carlsbad potash district and the Verkhnekamsk sylvite deposit; it does not rank them, and neither does this page.
Where the USGS potash table slips: sylvite at 100.00
A documented failure: Table 1 of the 2016 USGS report prints 100.00 as sylvite’s potassium oxide percentage and 63.17 against sylvinite, and the arithmetic says 63.17 belongs to sylvite. Pure KCl worked on standard atomic weights is 63.18% K2O-equivalent, 63.17 on the table’s rounding. A 100.00 would need a material that is all K2O, which is 83.02% potassium; sylvite is 52.45%. Sylvinite is potassium chloride and halite in the report’s own words, and a mixture has no single figure: from 0 for pure halite to 63.18% for pure sylvite.
The rest of the table holds. On the report’s own formulas, its carnallite line recomputes to 16.95, kainite to 18.92, langbeinite to 22.70, leonite to 25.69 and polyhalite to 15.62, each within 0.01. The IMA list writes kainite KMg(SO4)Cl(H2O)2.75, with a quarter less water than the report’s formula, and on that formula the figure is 19.27%. The table says it is modified from Garrett (1996); this page has not seen Garrett.
Why it matters on a label. A specimen labelled potash and analysed as 63% K2O-equivalent is consistent with near-pure sylvite; a figure in between points to a sylvite–halite mixture. Neither tells you the crystal on the label is sylvite rather than halite; the next section does.
Sylvite or halite? Density 1.993 against 2.168
Sylvite and halite share a habit, a cleavage and a colour range, and separate on density and refractive index. Both sheets give cubic crystals with perfect {001} cleavage, colourless to white. Sylvite reads hardness 2, density 1.993 and n 1.4903; halite reads 2 to 2.5, 2.168 and 1.5443. Halite is 8.8% denser and its index 0.054 higher. Carnallite, the third halide of the same beds, reads 1.602, is orthorhombic and biaxial (+), and has no cleavage printed.
A documented failure: the water weighing. The method on how to measure specific gravity at home weighs a piece in water. Both sheets say soluble in H2O, so a sylvite or halite weighed that way loses mass while it is measured, and the result is not a density. This page gives no substitute recipe. The classification question, halide or halite, is on is sylvite a halide.
What is not covered here. This page gives no handling guidance; the site’s general note is toxic minerals in a collection, which is not advice. The halite species page has its own humidity table, not repeated here.
Does sylvite dissolve? What the sheet prints
The sylvite sheet says soluble in H2O and nothing more about water. It prints no deliquescence humidity, no rate and no storage line, and this page adds none. The carnallite sheet goes further for its own species: soluble in H2O, deliquescent in high humidity, then dissolving. The live how to store soluble mineral specimens page covers the subject as a collecting note.
Magnetism. USGS Open-File Report 99-529, Table 1 ran 1 sylvite sample on a laboratory separator: total and best range above 1.70 amp, the separator’s maximum, which the report calls diamagnetic. Halite, 2 samples, behaved the same. See which minerals are magnetic.
Colour. The sheet prints pale blue among the colours and yellowish red to red when included with hematite. This page gives no cause for the blue.
Where sylvite is found
The Distribution field names Vesuvius, Campania, Italy; Hallein, Salzburg, Austria; Westeregeln and Stassfurt, 34 km south of Magdeburg, Saxony-Anhalt, and Bernburg, Germany; Aislaby, near Whitby, Yorkshire, England; Kalusz, Ukraine; the Verkhnekamsk sylvite deposit, Solikamsk-Berezniki region, Ural Mountains, Russia; the Permian salt basin of southeastern New Mexico, in the Carlsbad potash district, Eddy Co., with large crystals in the Potash Corporation of America mine, and adjacent parts of Texas; and the Smoky Hills, Peace River area, Alberta, Canada.
The British locality. Aislaby, near Whitby, is the one English entry, and the carnallite sheet lists Aislaby, Yorkshire too. The halite sheet’s Distribution does not name it. No other British sylvite locality is on the sheet, and none is added.
The name. The sheet derives it from the alchemical Sal digestivus Sylvii, or Salt of Sylvius, the digestive salt of François Sylvius de le Boë (1614–1672), Dutch physician and chemist of Leiden. A secondary database, webmineral, gives the namesake as Sylvia de la Boe; the sheet is followed here. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.