UV and VIS Raman spectra of natural lonsdaleites: towards a recognised standard.

A UV laser has now been used to measure the Raman spectrum of lonsdaleite. This mineral species is a little-known hexagonal form of carbon having no known P-T field of stability. Lonsdaleite is known to coexist with diamond and/or graphite in certain impact structures and meteorites. Its presence in microinclusions in some ultrahigh-pressure eclogites is under discussion as there is a considerable wavenumber overlap of the sp(3) Raman band of lonsdaleite in the 1200-1400 cm(-1) region with certain bands of haematite, graphite and diamond, and also with "disordered-diamond" having a downshifted wavenumber. Various incoherent previously published values of the Raman bands are briefly reviewed and an attempt is made to establish a reference spectrum. Four samples of lonsdaleite from the Zapadnaya and Popigai impact structures (Ukraine) were measured with three different laser sources (488, 514.5 and 325 nm) with two Raman spectrometers. UV-Raman was less fluorescent. All the new data were coherent in establishing an sp(3) band centred at 1324+/-4 cm(-1) with a FWHM about five times wider than that of diamond and an intensity about 500 times weaker. The presence of a second band giving a weak shoulder around 1225 cm(-1) is discussed with respect to the alternative of one continuous asymmetrical band.