We had a question in our last test about Pseudoalums. I never heard such a thing at that time. Such a thing can be asked in mains, so:
Since the general formula for alum is X_2 SO_4.Y_2SO_4.24H_2O but if any compound is of form;
or, X_2 SO_4.YSO_4.24H_2O
or, X SO_4.YSO_4.24H_2O
They are known as Pseudo-alums.