Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KCl.MgCl2·6(H2O).

The mineral is deliquescent (absorbs moisture from the surrounding air) and specimens must be stored in an airtight container.

Carnallite occurs with a sequence of potassium and magnesium evaporite minerals: sylvite, kainite, picromerite, polyhalite, and kieserite.

Carnallite is an uncommon double chloride mineral that only forms under specific environmental conditions in an evaporating sea or sedimentary basin.

In contrast, both Israel and Jordan produce potash from the Dead Sea by using evaporation pans to further concentrate the brine until carnallite precipitates, dredging the carnallite from the pans, and processing to remove the magnesium chloride from the potassium chloride.

[6][7] The ions combine with similarly large but low valence and weakly polarized cations.

This prevents the magnesium and the chloride from interacting directly; instead, the water molecules act as charge transmitters.

These two aspects render the rare face-sharing described by the second and third of Pauling's rules acceptable in the carnallite structure.

[10] The fragmented shards of iron oxide produce red tints in the thin laminae of hematite.

[11] The potassium that carnallite contains fuses easily within a flame, creating a violet color.

[11] Mineral associations based on some physical properties include, but not limited to, halite, anhydrite, dolomite, gypsum, kainite, kieserite, polyhalite, and sylvite.

In controlled environment experiments, the halides form when 10%–20% of the original sample of water remains.

[14] Carnallite is mostly found in saline marine deposits,[11] although beds exist in the endorheic Qaidam Basin of China's Qinghai Province near Dabusun Nor.