New crystal-chemical data on natrodufrénite, NaFe3+Fe3+5(PO4)4O(OH)5(H2O)2

The study the holotype specimen of natrodufrénite from Rochefort-en-Terre (Morbihan, Brittany, France) and selected comparative samples from Poniklá (Liberec Region, Czech Republic) provided new structural, chemical, and spectroscopic data on this natural sodium containing hydrated basic iron phosphate. Its crystal structure, solved using single-crystal X-ray diffraction data obtained for the greenish variety from the Poniklá locality, was refined to R1 = 2.75% for 3879 unique reflections with Fo > 4σ(Fo) and confirmed for the holotype specimen by three-dimensional electron diffraction technique combined with Rietveld analysis of powder X-ray data. Natrodufrénite is monoclinic with space group C2/c and cell parameters a = 25.9973(4), b = 5.15580(13) Å, c = 13.8990(7) Å, β = 111.466(3)°, V = 1733.75(10) Å3 and Z = 4. The mineral is isostructural with dufrénite, having the structural framework built by two different types of PO4 tetrahedra, four different face or corner sharing FeØ6 octahedra and NaØ8 polyhedra in the channels along the b axis. The characteristic structural unit is the trimer of FeØ6 coordination octahedra. Natrodufrénite is so far the only member of the group with Fe3+-dominance at all octahedral sites. The revised ideal formula NaFe3+Fe3+5(PO4)4O(OH)5(H2O)2 (IMA 25-F) is in good agreement with that given in the original description of natrodufrénite.