The hydrous nickel-magnesium silicates are one of the most poorly understood mineral groups. A principal member of this group was originally described by Professor Martin H. Klaproth in 1788 as “grüner Chrysopraserde”. He found this clay mineral, now called pimelite, to be the coloring matter of the gem material chrysoprase from the famous locality at Kosemütz, near Frankenstein in Silesia, Germany. Nickel analogues of the polymorphs of the serpentine-group, chrysotile, lizardite and antigorite are the most abundant members of the group. In addition, schuchardite, nickel chlorite; nickel-exchanged vermicu-lites; nickeloan talc; and nickeloan sepiolite have been observed in natural deposits. Other minerals described as hydrous nickel-magnesium silicates are shown by chemical, DTA, optical and x-ray powder diffraction studies to be either pimelite or nickel serpentine-group minerals or mixtures. Thus, genthite is a mixture of 20 per cent pimelite and 80 per cent nickeoloan serpentine-group mineral; röttisite is pimelite; de saulesite is pimelite; revdanskite is chiefly pimelite; garnierite, nepouite, noumeite are mixtures of pimelite, nickel serpentine-group minerals and other minerals. The tie-lines nickel serpentine-group minnerals-pimelite, and pimelite-quartz are established in this study.