beta-Ca(2)P(2)O(7).4H(2)O is monoclinic, P2(1)/c, a = 12.287 (6), b = 7.511 (3), c = 10.775 (5) A, and beta = 112.54 (1) degrees. Five of the terminal O atoms from a pyrophosphate group bind to Ca atoms, together with O atoms from three of the water molecules. The fourth H(2)O forms only hydrogen bonds. Both Ca atoms have coordination number 7 and show characteristics between those of a capped octahedron and a pentagonal bipyramid. The analysis of coordination distortions suggests that regularity and volume efficiency of a Ca coordination polyhedron increases with the number of bound water O atoms. The structure is layered after ¿100¿, reflected also in the morphology of crystals which are formed as extremely thin plates. The central parts of the layers are formed by chains of Ca coordination polyhedra which run along the b axis and are interconnected by pyrophosphate groups. Water molecules form the surfaces of the layers. A peculiar auto-inhibition of growth from supersaturated solutions is supposed to be caused by a direct attachment of CaP(2)O(7)(2-) and P(2)O(7)(4-) to the water molecules on the surfaces of layers. Ca(2)P(2)O(7).4H(2)O is known in two polymorphs. The unit-cell volume of the beta form compared with that of alpha suggests that the former is a low-temperature modification.