Double salts are salts containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize in a regular pattern. A well-known double salt is alum containing two cations, sodium and aluminum and a sulphate anion. Other examples are potassium sodium tartrate and bromlite. A magnified crystal of a salt (halite/sodium chloride) In chemistry, a salt is any ionic compound composed of positively charged cations and negatively charged anions, so that the product is neutral and without a net charge. ... A cation is an ion with positive charge. ... An anion is an ion with negative charge. ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... A crystal of Alum Alum, in chemistry, is a term given to the crystallized double sulfates of the typical formula M+2SO4·M3+2(SO4)3·24H2O, where M+ is the sign of an alkali metal (lithium, sodium, potassium, rubidium, or caesium), and M3+ denotes one of the trivalent metal... Potassium sodium tartrate is a double salt first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. ... Bromlite, also known as Alstonite is a member of the aragonite group of minerals. ...