The term kerolite-pimelite series is used for a series of hydrous magnesium-nickel silicates with talc-like structure and composition but with additional water and with a highly disordered and non-swelling stacking of the layers. The layer spacing, obtained after correction for Lorentz-polarization and structure factor variations with diffraction angle, is near 9.6A and is larger than that of talc because of an absence of close-packing of adjacent layers. The average composition of nineteen samples is (Mg,Ni)3.04(Al,Fe)0.01(Si3.93,Al0.02Fe0.02)O10(OH)2·0.89H2O. The additional “water” is held in a variety of ways and is released gradually up to 700°C. Infrared spectroscopic data suggest the presence of surface hydroxyls yielding an absorption near 3700 cm−1, Si–OH groups with absorption near 4550 cm−1, and interlayer and surface adsorbed H2O absorbing at 1600, 3400, and 3600 cm−1. The splitting of OH stretching and librational modes in the infrared spectra clearly shows the effects of increasing Ni-for-Mg substitution. As expected, optical absorption spectra provide evidence for Ni2+ in octahedral coordination only. The thermal transformations of these minerals are examined, and evidence is obtained for a transitional face-centered cubic phase prior to the development of a high-temperature phase.