Abstract: Least-squares analyses of powder X-ray diffraction data have been undertaken for minerals and synthetics of composition (Mg,Mn,Fe,Co,Ni)_3−xSi₂O₅ (OH)₄. New polytypes of nepouite and greenalite have been established, and eleven new or altered unit cells have been calculated. Baumite is an unnecessary varietal name for a manganoan ferroan lizardite-1T; tosalite is an unnecessary varietal name for a manganoan greenalite; clinochrysotile is an unnecessary polytype name for chrysotile-2M_c1; orthochrysotile is an unnecessary polytype name for chrysotile-2Or_c1; ortho-antigorite and ortho-hexagonal serpentine are unnecessary names for lizardite-6T₁; and septechlorite should not be used. The powder data of the serpentine group are in general, poor.