Thermal behaviour and physico-chemical characterization of synthetic and natural iron hydroxyphosphates

The thermal behaviour of five hydroxyphosphates, lipscombite [Fe₃(PO₄)₂(OH)₂], barbosalite [Fe₃(PO₄)₂(OH)₂], giniite [Fe₅(PO₄)₄(OH)₂· 2H₂O], rockbridgeite Fe₅(PO₄)₃(OH)₅] and Fe₄(PO₄)₃(OH)₃, has been studied. Thermogravimetric analysis revealed the hydroxyphosphates to have distinct behaviours: some underwent dehydroxylation between 623 and 893 K like other phosphates [lipscombite, barbosalite, Fe₄(PO₄)₃(OH)₃], while rockbridgeite and giniite differed. Rockbridgeite, which appeared to contain only iron(III) ions, underwent a first partial dehydroxylation at only 438 K, while giniite totally dehydroxyated only at between 623 and 823 K. Comparison of the results obtained by thermogravimetric analysis with those obtained by other techniques, including chemical analysis, X-ray diffraction and Mössbauer spectroscopy, allowed us to propose the existence of a new solid solution for the giniite compound and to confirm that previously observed for rockbridgeite. The solid solution of the former corresponds to a variation in composition due to dehydroxylation–hydration : Fe^III_4+xFe^II_1–x(PO₄)₄(OH)_2–x(2 +x)H₂O with 0 < x < 1. The latter corresponds to a variation in the composition due to dehydroxylation–oxidation : Fe^III_4+xFe^II_1–x(PO₄)₃(OH)_5–xO_x with 0 < x < 1. In both cases, Mössbauer spectroscopy indicates that Fe²⁺ or Mn²⁺ substituents may be localized on the different crystallographic sites of the structures.