The crystal-chemical basis of the relationship between hydrated calcium silicate and clay minerals is discussed, and a review is given of recent work on the tobermorite minerals, the group of hydrated calcium silicates which most nearly resembles the clay minerals. New data are presented for the specific gravities, refractive indices, basal spacings, and dehydration isobars of synthetic tobermorite minerals, and a tentative explanation is proposed for the variability of Ca : Si ratio in these substances.