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

Abstract

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 Mossbauer 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 : FeIII4+xFeII1–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 : FeIII4+xFeII1–x(PO4)3(OH)5–xOx with 0 < x < 1. In both cases, Mossbauer spectroscopy indicates that Fe2+ or Mn2+ substituents may be localized on the different crystallographic sites of the structures.