The stability of the structural phases in the fluorosilicate hexahydrates of some 3d transition metals where M = Mn, Fe, Co and Ni is studied in a wide range of temperatures and pressures. The results are generalized in the form of a P - T phase diagram of crystal states. The triple points, new high-pressure phases and wide ranges of metastable states between them have been discovered. The correspondence between structural and magnetic P - T phase diagrams of the series is established.