Pure carnallit would have the following composition:
| Chlorin | 38.3 | per | cent. |
| Potassium | 14.0 | “ | “ |
| Magnesium | 8.7 | “ | “ |
| Water | 39.0 | “ | “ |
Equivalent to
| Potassium | chlorid | 26.8 | “ | “ |
| Magnesium | “ | 34.2 | “ | “ |
| Water | 39.0 | “ | “ | |
The commercial article as taken from the mines, as is seen above, has less potash (K₂O) than kainit, the mean content being about nine and nine-tenths per cent. Those proposing to use this body for fertilizing purposes should bear in mind that it contains less potash and more chlorin than kainit.
Carnallit occurs in characteristic brown-red masses. On account of its highly hygroscopic nature it should be kept as much as possible out of contact with moist air and should not be ground until immediately before using.
By reason of the greater bulk in proportion to its content of potash and its hygroscopic nature and consequent increased difficulty in handling, the price per unit of potash in carnallit is less than in kainit.
In some localities small quantities of ammonium chlorid have been found with carnallit but not to exceed one-tenth per cent. It has therefore no practical significance to the farmer but may be of interest to the analyst.
239. Polyhalit.—Polyhalit is a mineral occurring in Stassfurt deposits and consisting of a mixture of potassium, magnesium, and calcium sulfates, with a small proportion of crystal water. This mineral, on account of its being practically free of chlorin, would be one especially desirable for use in those cases, as in the culture of tobacco, where chlorids are injurious. Unfortunately, it does not occur in sufficient quantities to warrant the expectation of its ever being found in masses large enough to become a general article of commerce. It is found only in pockets or seams among the other Stassfurt deposits, and there is no assurance given on finding one of these deposits of polyhalit that it will extend to any great distance. The composition of the mineral is shown by the following formula: K₂SO₄·MgSO₄·(CaSO₄)₂·H₂O. Its percentage composition is shown by the following numbers:
| Potassium sulfate | 28.90 | per | cent. |
| Magnesium sulfate | 19.93 | “ | “ |
| Calcium sulfate | 45.18 | “ | “ |
| Water | 5.99 | “ | “ |