As regards the heterogeneous decomposition of double salts, it has long been known that such salts as carnallite and K2Mg(SO4)2 give up the more soluble salt if an insufficient quantity of water for their complete solution be taken. The complete saturation of 100 parts of water requires at 0° 14·1, at 20° 25, and at 60° 50·2 parts of the latter double salt (anhydrous), while 100 parts of water dissolve 27 parts of magnesium sulphate at 0°, 36 parts at 20°, and 55 parts at 60°, of the anhydrous salt taken. Of all the states of equilibrium exhibited by double salts the most fully investigated as yet is the system containing water, sodium sulphate, magnesium sulphate, and their double salt, Na2Mg(SO4)2, which crystallises with 4 and 6 mol. OH2. The first crystallo-hydrate, MgNa2(SO4)2,4H2O, occurs at Stassfurt, and as a sedimentary deposit in many of the salt lakes near Astrakhan, and is therefore called astrakhanite. The specific gravity of the monoclinic prisms of this salt is 2·22.
If this salt, in a finely divided state, be mixed with the necessary quantity of water (according to the equation MgNa2(SO4)2,4H2O + 13H2O = Na2SO4,10H2O + MgSO4,7H2O), the mixture solidifies like plaster of Paris into a homogeneous mass if the temperature be below 22° (Van't Hoff und Van Deventer, 1886; Bakhuis Roozeboom, 1887); but if the temperature be above this transition-point the water and double salt do not react on each other: that is, they do not solidify or give a mixture of sodium and magnesium sulphates. If a mixture (in equivalent quantities) of solutions of these salts be evaporated, and crystals of astrakhanite and of the individual salts capable of proceeding from it be added to the concentrated solution to avoid the possibility of a supersaturated solution, then at temperatures above 22° astrakhanite is exclusively formed (this is the method of its production), but at lower temperatures the individual salts are alone produced. If equivalent amounts of Glauber's salt and magnesium sulphate be mixed together in a solid state, there is no change at temperatures below 22°, but at higher temperatures astrakhanite and water are formed.