A Raman spectroscopic study of the basic carbonate mineral callaghanite Cu2Mg2(CO3)(OH)6⋅2H2O

Raman spectrum of callaghanite, Cu2Mg2(CO3)(OH)6⋅2H2O, was studied and compared with published Raman spectra of azurite, malachite and hydromagnesite. Stretching and bending vibrations of carbonate and hydroxyl units and water molecules were tentatively assigned. Approximate O–H…O hydrogen bond lengths were inferred from the spectra. Because of the high content of hydroxyl ions in the crystal structure in comparison with low content of carbonate units, callaghanite should be better classified as a carbonatohydroxide than a hydroxycarbonate.

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► We have studied the Raman spectrum of callaghanite, Cu2Mg2(CO3)(OH)6⋅2H2O.
► A comparison is made with published Raman spectra of azurite, malachite and hydromagnesite.
► Stretching and bending vibrations of carbonate and hydroxyl units and water molecules were tentatively assigned.
► Approximate O–H…O hydrogen bond lengths were inferred from the spectra.