double salt

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A salt, such as alum, that ionizes in solution as if it were two separate salts but forms a single substance upon crystallization.

Wiktionary

1. n. chemistry A solid solution of two simple salts, often in a definite proportion

GNU Webster's 1913

1. adj. (Chem.) A molecular combination of two distinct salts, as common alum, which consists of the sulphate of aluminium, and the sulphate of potassium or ammonium.
2. n. (Chem.) a salt regarded as formed by the union of two distinct salts, as common alum, potassium aluminium sulphate. See under Double.

WordNet 3.0

1. n. a solution of two simple salts that forms a single substance on crystallization