LOCAL ORDERING OF CHROMIUM(III) IN HYDROXIDE SHEETS OF STICHTITE STUDIED BY ION-EXCHANGE CHROMATOGRAPHY

Stichtite is the layered Mg-Cr(III) hydroxide carbonate of the pyroaur ite group. The possibility of describing Cr(III) short range order (SR O) in stichtite by use of ion chromatography of the Cr(III)-hydroxo sp ecies following acid dissolution of the compound is described. Two syn thetic stichtites of similar composition (average Mg5.73Cr(III)(2.27)( OH)(1593)(O)(0.07)(CO3)(1.10). xH(2)O) prepared in the absence or pres ence of hydroxy-bridged Cr(III) dimers and a sample from Dundas, Tasma nia, .65Fe(III)(0.15)(OH)(15.26)(OH2)(0.74)(CO3)(1.23). xH(2)O have be en examined. Ion chromatography showed the highest degree of SRO for t he Dundas stichtite. For the synthetic stichtites the sample synthesiz ed from Cr(III) dimers had the highest SRO. No information on cation o rdering 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-stret ching IR absorption bands (3585 and 3472 cm(-1)) are assigned to OH co ordinated to 3Mg and OH coordinated to 2MgCr or Mg2Cr, respectively.