Calclacite is the name that was given by the author in 1945 to a silky, acicular efflorescence that sometimes is found on the surface of limestone fossils and artifacts long stored undisturbed in wooden museum cases. From well developed synthetic calclacite recently prepared by Elisabeth West the author has been able to make more extensive and precise crystallographic measurements with the optical goniometer and the Weissenberg camera; also density measure ments and x-ray powder data. Spacings and intensities of the powder lines are tabulated. -- AATA