Abstract. This study presents and discusses infrared spectroscopic
data of well characterised, naturally occurring trioctahedral layer silicates of the serpentine (Srp), talc (Tlc), and sepiolite (Sep) mineral
groups, which are found in reactivated faults and sequences of white and green
clay veins (deweylite and garnierite) of the New Caledonian Ni-silicate ores.
Bands assigned to the OH stretching vibrations of these 1:1 and 2:1 layer
silicates in both the fundamental and first overtone regions of mid- and near-infrared (MIR and NIR) spectra, respectively, are compared to those reported in the
literature for synthetic Mg–Ni series of the Srp and Tlc mineral groups.
They are also presented according to the sequences of infillings recognised
in the white and green veins of the Ni-silicate ores. The study reveals that
serpentine-like (SL) minerals of the first sequences of clay infillings are
residues of larger crystals of serpentines (lizardite, chrysotile, and
antigorite) and that the newly formed talc-like (TL) minerals and Sep are the main Ni-bearing carriers of the Ni-silicate ores. Decreasing
crystal size and order in serpentine species have major effects on
vibrational bands. They favour the broadening of the OH stretching bands,
the degradation of the signals assigned to the interlayer OH, and the
enhancement of the signal related to weakly bound water molecules. The
replacement of Mg by Ni in octahedral sites of the 2:1 layer silicates (TL,
Sep) of the greenish clay infillings can be traced by specific OH stretching
bands related to the Mg3OH, Mg2NiOH, MgNi2OH, and Ni3OH
configurations in the fundamental (MIR) and first overtone (NIR) regions of
the spectra. The dominance of the Mg3OH and Ni3OH configurations
with respect to mixed configurations in the Mg–Ni mineral series of the
clay infillings (mostly in the dominant TL minerals) suggests that Mg and Ni
segregation is related to separate Mg-rich and Ni-rich mineral phases rather
than to a cationic clustering within the individual particles. This
segregation of Mg and Ni in discrete mineral phases is related to Mg–Ni oscillatory
zoning patterns (banded patterns) and is reproduced at the scale of the
Ni-silicate ores between the white (deweylite) and greenish (garnierite)
veins of the reactivated faults.