Vibrational Spectroscopy of Stichtite

Raman spectroscopy complimented with infrared spectroscopy has been used to study the mineral stitchtite, a hydrotalcite of formula Mg6Cr2(CO3)(OH)16.4H2O. Two bands are observed at 1087 and 1067 cm-1 with an intensity ratio of ~2.5/1 and are attributed to the symmetric stretching vibrations of the carbonate anion. The observation of two bands is attributed to two species of carbonate in the interlayer, namely weakly hydrogen bonded and strongly hydrogen bonded. Two infrared bands are found at 1457 and 1381 cm-1 and are assigned to the antisymmetric stretching modes. These bands were not observed in the Raman spectrum. Two infrared bands are observed at 744 and 685 cm-1 and are assigned to the ν4 bending modes. Two Raman bands were observed at 539 and 531 cm-1 attributed to the ν2 bending modes. Importantly the band positions of the paragenically related hydrotalcites stitchtite, iowaite, pyroaurite and reevesite all of which contain the carbonate anion occur at different wavenumbers. Consequently Raman spectroscopy can be used to distinguish these minerals, particularly in the field where many of these hydrotalcites occur simultaneously in ore zones.