The thermochemistry of some iron phosphate minerals: vivianite, metavivianite, baraćite, ludlamite and vivianite/metavivianite admixtures

Abstract

Vivianite shows a major differential thermal response spanning 65 to 315°C attributable to an endothermal loss of structural water combined with the oxidation of Fe 2+ (which results in exothermic peak( s) being described within the endothermal record) and consequential breakdown of the original structure. Subsequently, a gradual sintering of the amorphous product culminates in an exothermic structural ordering at 660°C resulting in the formation of α-FePO 4, Fe(PO 3) 3 and, occasionally, Fe 2O 3. The cause of a small exotherm near 780°C is possibly similar; decomposition/melting occurring between 1000 and 1200°C. Meta-vivianite behaves similarly but with the initial dehydration record being less complicated by oxidation of iron and with small endotherms forming just above 320°C from dehydroxylation of the structure. Baraćite also has the same basic pattern but an additional exotherm near 700°C, possibly due to formation of high magnesium phosphate. Ludlamite shows a narrower dehydration endotherm at a higher temperature reflecting the difference in bond type between the parallel bands of octahedra in the two structures. The differences in thermal behaviour between vivianite and metavivianite are too slight to allow them to be used to detect the presence of one mineral in the other with any reliability. While oxidation of vivianite might exert some influence on the response of the mineral to heat, other factors such as grain size and instrumentation differences are believed to have a greater influence on dissimilarities between published results.