Raman spectra of Ni-Mg kerolite: effect of Ni-Mg substitution on O-H stretching vibrations

The Ni‐rich mineral phases constituting the Ni‐ores in New Caledonia are dominated in most cases by kerolite–pimelite series, also called talc‐like minerals. Despite their economic and geologic interests, the crystallographic structures of these minerals are not fully understood. In order to improve the knowledge of their crystallographic structure, a set of natural talc‐like minerals of varying Ni/Mg ratio was selected. A combination of SEM, EMPA, TEM, and Raman analysis shows that the studied mineral contains only one mineral phase identified as the kerolite–pimelite series. Although kerolite and pimelite are not currently recognized as distinct minerals by the IMA, this study shows that these minerals exist as distinct minerals without any mixed layering with other mineral phases such as serpentine or other sheet silicates. The solid solution between the Ni and Mg endmembers was complete without any gap, corresponding to a Ni–Mg substitution in the octahedral sheet. The Raman spectra in the OH stretching vibration region of representative samples covering the whole range of Ni–Mg substitution show a continuous and significant evolution from the Mg to the Ni endmember. The signal was decomposed into nine Gaussian–Lorentzian functions, but the large overlapping of the peaks and the complexity of the band structure prevent any interpretation of the spectra from a structural point of view. Finally, the problem is formalized as a multivariate curve resolution (MCR) which is solved using the Bayesian Positive Source Separation (BPSS) algorithm. This reveals that four possible arrangements of Ni and Mg in the octahedral sheet are encountered, and these arrangements are dependent on the Ni–Mg substitution rate. It also confirms that Raman microspectroscopy can be very efficient in quick evaluation of the Ni content of the mineral. Copyright © 2015 John Wiley & Sons, Ltd. Key findings Raman OH stretching vibrations in the talc‐like mineral Ni–Mg kerolite are used to go further in the understanding of its crystal chemistry. The study is based on samples covering the whole substitution range of Mg by Ni with constant crystal chemistry, features and interlayer spacing, and no mixed‐layering and on a chemometrics analysis of the Raman spectra. The heterogeneous distribution of Mg and Ni in the octahedral sheet is demonstrated, along with the complexity of hydroxyl site geometry in comparison with talc.