Near-infrared and mid-IR spectroscopy of selected humite minerals
The combination of electronic and vibrational spectra has been applied to correlate the spectral properties, with composition, structure and cation substitutions such as Mn, Fe, Ca and Zn for Mg in humites: norbergite, alleghanyite, leucophoenicite and sonolite with increasing number of silicate layers, 1, 2, 3 and 4.
The observation of two broad bands in the visible range, near 550 nm and 450 nm (18180 and 22220 cm-1) and one sharp band around 410 nm (24390 cm-1) is characteristic of Mn2+ in alleghanyite and leucophoenicite. The study of UV-Vis (electronic) spectral features confirms Mn as a major substituent in these two samples.
Cation impurities like Zn and Ca as revealed from EDX analysis might be the cause for the absence of Mn-type spectrum in sonolite. The first observation is the near-infrared spectra of all four minerals in the first fundamental overtone OH-stretching mode are different and each mineral is characterized by its NIR spectrum. The feature in the range 7180-6600 cm-1 [1393 to 1515 nm or 1.39 to 1.52 µm] corresponds to the overtones of OH stretching vibrational modes of the humite groups observed in their IR spectra over the range, 3680-3320 cm-1. The infrared spectra of the hydrous components of OH and SiO4 groups in the mineral structure act as an aid to distinguish the minerals of the humite mineral group. A band at 541 cm-1 is assigned to MnO stretching mode.
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