Abstract
The crystal structure of anapaite from Messel (Hesse), Ca2Fe (PO4)2 • 4H2O, has been refined (R = 0.027) in space group P-1 by 1257 reflections measured on an automatic single-crystal diffractometer (MoKα radiation) ; all the hydrogen atoms have been localized and included in the refinement. The unit-cell parameters are : a = 6.447(1), b = 6.816(1), c = 5.898(1) Å, α = 101.64(3), β = 104.24(3), γ = 70.76(4)° ; Z = 1. Within the framework structure, (100) layers of coordination polyhedra are emphasized in order to describe a plausible crystallochemical path for a phase transition to the bihydrated term (messelite). Bond valence calculations show that Ca is 7 + 1 coordinated and justify the presence of hydrogen bonds which are unusually short for water molecules (2.597 and 2.600 Å).
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