Physico-chemical properties of MnCO3–CaCO3 and MnO–CaO solid solutions

Abstract

MnCO₃–CaCO₃ has been shown to form a continuous phase isomorphous with calcite as the Mn concentration is varied across the whole range of composition from CaCO₃ to MnCO₃. Magnetic susceptibility and e.p.r. studies show that the manganese-containing carbonate is paramagnetic, and diffuse reflectance spectroscopy confirms octahedral coordination for the Mn²⁺ ions.

The decomposition of the Mn_xCa_1–xCO₃ phase to form the oxide has been investigated at five compositions between x= 0 and x= 1 with a view to preparing Mn_xCa_1–xO solid solutions in a finely divided state. It is shown that this can be achieved by controlled decomposition in vacuo at 925 K. X-ray powder diffraction confirms the expected variation of unit-cell parameter with Mn content in the cubic Mn_xCa_1–xO phase. A special Gouy cell was devised to determine magnetic susceptibility without exposure to the atmosphere. The temperature range studied (77–300 K) included the Néel temperature for MnO, and attenuation of superexchange interactions as Ca²⁺ ions replace Mn²⁺ ions is manifested by a rapid decrease of the Weiss constant and by a decrease of linewidth in the e.p.r. spectrum. Mn²⁺ is present in the high-spin ⁶S_5/2 state (µ= 5.9µ_B) in all the solid solutions.

The transformation of carbonate to oxide has been followed by diffuse reflectance spectroscopy in vacuo. Mn_xCa_1–xCO₃(x= 0.65) is shown to decompose rapidly at 675–750 K, but there is evidence for an intermediate state to be passed before the Mn_xCa_1–xO solid solution is produced.