Formation of synthetic analogues of double metal-hydroxy carbonate minerals under controlled pH conditions: II. The synthesis of desautelsite

Desautelsite, a Mg(II)Mn(III) hydroxy carbonate first described in 1979 and represented by the general formula Mg (8− x ) Mn(III) x (OH) 16 (CO 3 ) x /2 (in which 2 < x < 2·67), has been synthesized under controlled pH conditions (induced hydrolysis) and coprecipitation techniques. In the pH-stat syntheses, the desautelsite is formed by aerial oxidation of MnCO 3 in 0·05–0·15 m Mg(NO 3 ) 2 solutions at ∼pH 9 and temperatures of 35°C where, in the absence of Mg, manganite would normally form. The synthesis product from this technique is generally more crystalline than that formed by the coprecipitation method. Composition, particle shape and aggregation may be controlled through variations of the Mg concentration, pH, aeration rate or temperature. Indirect evidence suggests Mn to be present in the +3 oxidation state in the desautelsite.