Local ordering of chromium(III) in trioctahedral hydroxide sheets of stichtite studied by ion exchange chromatography

Stichtite is the layered Mg-Cr(III) hydroxide carbonate of the pyroaurite group. The possibility of describing Cr(III) short range order (SRO) in stichtite by use of ion chromatography of the Cr(III)-hydroxo species following acid dissolution of the compound is described. Two synthetic stichtites of similar composition (average Mg5.73Cr(III)2.27(OH)15.93(O)0.07(CO3)1.10·xH2O) prepared in the absence or presence of hydroxy-bridged Cr(III) dimers and a sample from Dundas, Tasmania, Mg6.29Ni(II)0.02Cr(III)0.90Al0.65Fe(III)0.15(OH)15.26(OH2)0.74(CO3)1.23·xH2O have been examined. Ion chromatography showed the highest degree of SRO for the Dundas stichtite. For the synthetic stichtites the sample synthesized from Cr(III) dimers had the highest SRO. No information on cation ordering could be extracted from powder X-ray diffraction data. Visible spectroscopy indicates that for constant Mg:Cr(III) ratio the crystal field splitting increases with increasing SRO. Two different OH-stretching IR absorption bands (3585 and 3472 cm−1) are assigned to OH coordinated to 3Mg and OH coordinated to 2MgCr or Mg2Cr, respectively.