double salt

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A salt or mineral, such as alum or dolomite, that has more than one type of cation or anion and forms when a solution of two simpler salts undergoes crystallization.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry A solid solution of two simple salts, often in a definite proportion

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a solution of two simple salts that forms a single substance on crystallization