The hydrated‐calcium silicates are of particular geological interest as typical minerals formed by hydrothermal reactions at contact‐zones of different mineral species. They are also of technical significance as compounds which may be formed in the steam‐treatment of Portland cement and pozzolanic materials and in the manufacture of sand‐lime brick. To ascertain which of the hydrated‐calcium silicates reported in the literature are distinct minerals. X‐ray diffraction‐patterns have been made on samples obtained mostly from the United States National Museum. Patterns were also made on the following anhydrous calcium silicates: Wollastonite, pseudo‐wollastonite, beta‐ and gamma‐ di‐calcium silicates, tricalcium disilicate, and tricalcium silicate. The other phase of the investigation now in progress, and on which this paper is a preliminary report, is an attempt to synthesize some of the hydrated‐calcium silicates by hydrothermal methods.