The use of glycerol intercalates in the exchange of CO32−with SO42−, NO3−or Cl−in pyroaurite-type compounds

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Abstract
Strongly held carbonates anions in the interlayer region of pyroaurite-type compounds, [Ml II 1−x M2 III x (OH)2] x+[(CO3) x/2 yH2O] x− (x = 0·15−0·44), can be relatively easily exchanged by other anions such as NO3 −, Cl− or SO4 2− dissolved in heated glycerol. Some Fe(III) of pyroaurite is reduced to Fe(II) by the glycerol treatment. If pyroaurite is treated with glycerol in the absence of dissolved anions or with glycerol vapour at 120°C, glycerol becomes intercalated in the interlayers. In all examples the interlayer CO3 2− is partially or completely lost as CO2. In aqueous salt solutions a glycerol intercalated form allows other anions to substitute in the interlayer. Expansion in the c-axis direction following glycerol treatment depends on (1) the method of glycerol treatment, (2) the trivalent metal in the octahedral sheet, and (3) the interlayer anions. Heated glycerol causes a different expansion to that produced by its vapour phase at 120°C. The variation in d(003) follows the series: hydrotalcitevap. > pyroauritevap. > hydrotalciteliquid > pyroauriteliquid > SO4 2−-pyroauritevap. > SO4 2−-hydrotalcitevap., where liquid refers to glycerol used at 160–180°C and vap. refers to glycerol vapour at 120°C Carbonate-free, glycerol-intercalated pyroaurite compounds may serve as re-usable anion absorbers. Glycerol intercalates of pyroaurite-type compounds may also be useful to study and identify stacking sequences and interlayer compositions in these minerals.