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

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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 MnxCa1–xCO3 phase to form the oxide has been investigated at five compositions between x= 0 and x= 1 with a view to preparing MnxCa1–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 MnxCa1–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 6S5/2 state (µ= 5.9µB) in all the solid solutions. The transformation of carbonate to oxide has been followed by diffuse reflectance spectroscopy in vacuo. MnxCa1–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 MnxCa1–xO solid solution is produced.

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