This comparatively newly-discovered metal is widely distributed in minute quantities in the minerals containing other alkali metals, such as lepidolite (0.24 Per cent. of rubidium oxide), carnallite, and mica, as well as in sea-water, brine springs, and mineral waters, and in many plants and the potashes made from them. The best material to work upon for the preparation of the metal is the saline residue left in procuring lithium from lepidolite (see p. 363), containing sodium, potassium, lithium, and traces of rubidium and caesium, all in the form of chlorides. About 2} lb. of this residue is dissolved in 5 1/2 lb. water, and the cold solution is precipitated by adding a solution of 30 grm. platinum in aqua-regia; when the precipitate has settled, it is collected and boiled 25 times with successive small quantities of water, in each case poured upon the original solution, and amounting in all to about 3 1/3 lb.; thus is formed a fresh precipitate, which is again washed and treated as the first; this series of operations is repeated 7 or 8 times, till nearly all the rubidium is exhausted, when each platinum precipitate is dried, the platinum is reduced by hydrogen, and the alkaline mass is dissolved in water, giving about 125 grm. rubidium chloride, with 3 or 4 Per cent. potassium chloride and a little caesium chloride; the potassic impurity can be almost completely removed by repeating the precipitation and washing.
The metal itself is best prepared by the same process as that used for potassium (p. 405), charred rubidium tartrate being heated to whiteness in an iron tube, and yielding about 5 oz. of the metal from 75 oz. of the tartrate. The metal has a silver-white colour, and a sp. gr. of 1.52; it is soft and wax-like at 14° F. (-10° C), and melts at 101 J° F. (38 1/2° C.); on exposure to the air, it is immediately coated with a bluish suboxide, and ignites even more readily than potassium.