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Brine,abundant with sodium,potassium,lithium,rubidium,and borate,is an important source to produce variety inorganic chemicals.The Pingluoba underground brine is famous for its high ion content like sodium (96.79 g·L-1),potassium (53.27 g·L-1),lithium (8.99 × 10-2 g·L-1),rubidium (3.75 × 10-2 g·L-1).The brine belongs to a marine sedimentary deep brine,with distinguishing features of deep buried depth (over 4500 m),high pressure (about 97 MPa),high temperature (about 393 K),and high salinity (over 420 g/L).[1] The potassium (KCl) reserves of Pingluoba underground brine is about 4.55 million ton.The exploitation of this brine can reduce the burden of potassium requirement in China.To exploit brine and mineral resources,mineral equilibrium studies on different types of brine are necessary.The Pingluoba underground brine belong mostly to the complex system Li + K + Rb + Mg + Cl + borate + H2O.In this six-component system,magnesium chloride can form various hydrate salts; potassium and rubidium can easily formed a solid solution in the chloride solution because of their similar ion radius; the double salt can be formed between KCl and MgCl2.It is well known that the crystallization zone and crystallization form of solid solution,double salt and hydrated salt depend on coexist ion and temperature.[2] Aiming at understanding the crystallization form and crystallization area of the solid solution,double salt and hydrated salt changes with the temperature,some phase equilibria focused on the subsystems of Pingluoba underground brine at different temperatures have been done by our research group[3-7],relevant phase diagrams were constructed.Based on the phase diagrams,we can get the information about the crystallization path,crystalloid forms and crystallization zones of different salts: in the ternary system KCl + MgCl2+ H2O,the crystalloid form of the magnesium chloride is MgCl2·6H2O and the crystallization zone of carnallite decreases with the increase of temperature; in the ternary system KCl + RbCl + H2O,the miscible solid solution [(K,Rb)Cl] was formed and the crystallization zone of [(K,Rb)Cl] decreases with the increase of temperature; in the quaternary system LiCl + KCl + RbCl + H2O,there are four crystallization fields correspond to single salts KCl,LiCl·H2O,RbCl and the solid solution (K,Rb)Cl.The salt’s crystallization region decrease in the order of (K,Rb)Cl,KCl,LiCl·H2O and RbCl.The crystallization zone of the solid solution (K,Rb)Cl almost occupies the entire phase region.