Are the iron carbonate minerals, ankerite and ferroan dolomite, like siderite, important in paleomagnetism?

Abstract

Magnetic, Mössbauer effect, and thermal properties have been evaluated for specimens of the mineral ankerite and are contrasted with similar measurements reported for the mineral siderite to determine if ankerite (and ferroan dolomite), like siderite, may be important in paleomagnetism as a producer of secondary, spurious remanent moments as the result of oxidation in air. Our data indicate that ankerite and siderite both have comparable thermal stabilities and that ankerite does break down to form iron oxides at temperatures as low as 250°C during thermal demagnetization. We conclude from our data that any iron carbonate, siderite, ankerite, or ferroan dolomite, may readily oxidize in air to maghemite or hematite thus producing a spurious remanent moment (RM) that can dominate the magnetic properties of the specimen. Thermal demagnetization of ankerite or siderite at lower temperatures (below 250°C) does not appear to produce such a spurious RM. As might be expected, the quantity of secondary iron oxides produced by oxidation is limited by the iron content in the carbonate crystal lattice. Oxidized siderite produces significantly greater amounts of iron oxides than equivalent percentages of ankerite, which in turn produces more iron oxides than equal amounts of ferroan dolomite in a given sample.