Single-crystal, polarized-light, FTIR spectroscopy of masutomilite: short range order of cations and orientation of the O-H dipoles

Masutomilite is a rare Mn-analogue of the Li-mica zinnwaldite; it was described by Harada et al. (1976) from the Shiga Prefecture, Japan, and later reported from few other localities (Němec, 1983, Čerńy and Burt, 1984). We relate here an infrared study of two masutomilite specimens from Central Urals, Russia (Mokrusha Pegmatite, Murzinka Region) and Idaho, USA (Sawtooth Mountains, Boise County), respectively. Both samples belong to the 1M polytype. The chemical composition, derived by combining electron microprobe analysis, inductively coupled plasma atomic emission spectrometry (ICP-AES) for Li, and results from the structure refinement are, on the basis of O10(F,OH)2 (Brigatti et al., 2007): (Si3.30Al0.70) (Al1.00Fe0.36Mg0.01Mn0.31Li1.32) (Ca0.01Na0.04K0.94) O10F1.91(OH)0.09 (Mokrusha mine, Russia) and (Si3.11Al0.89) (Al0.91Ti0.02Fe0.46Mg0.03Mn0.52Li1.06) (Na0.05K0.92Rb0.02) O10F1.89(OH)0.11 (Sawtooth Mountains, Boise County, Idaho, USA). Both samples have all Li > 1.0 apfu and variable Fe/Mn. The anionic site is dominated by F with low amounts of OH (0.09-0.12 apfu). Single-crystal, polarized-light FTIR spectra were collected using a NicPlan microscope equipped with a nitrogen-cooled MCT detector, a KBr beamsplitter and a gold wire grid polarizer, at 4 cm-1 resolution. The spot size was ~ 100 μm and 128 scans were averaged for each spectrum. The collected patterns are different as expected from the different chemical composition of the examined samples, however they show broad similarities and a similar behaviour under polarized light. They consist of a higher-frequency component which is centred at 3685 and 3659 cm-1 for masutomilite from Russia and Idaho, respectively; both bands are strongly polarised for E // α. A broader, multicomponent absorption is observed at around 3600 cm-1. In masutomilite from Idaho this band is split in two components of equal intensity, at 3685 and 3659 cm-1 respectively, plus a lower-frequency shoulder at ~ 3552 cm-1. This band shows maximum intensity for E // γ and a medium intensity for E // α, while showing reduced intensity for E // β. A lower frequency, broad band is finally present at ~3500 cm-1 in both samples; this feature is virtually unaffected by rotation of the polarizer. Following the model of Robert et al. (1989) for Li-bearing micas, the higher frequency bands at 3685 - 3659 cm-1 are assigned to OH-dipoles bonded to FeFeFe local environments (TRI-6 environments), while the multicomponent band around 3600 cm-1 is assigned to AlM2+M2+ (with M2+ = Fe or Mn) local environments (TRI-7 environments). The spectra show that all Li is associated to F at the anionic site. From measurement of the dichroic ratio (absorbance along a / absorbance along b, e.g. Serratosa and Bradley, 1962, Vedder and McDonald, 1963) the orientation of the O-H vectors with respect the (001) plane can be evaluated. References Brigatti, M.F., Mottana, A., Malferrari, D. and Cibin, G. (2007) Am. Mineral., 92, 1395-1400. Harada, K., Honda, M., Nagashima, K., Kanisawa, S. (1976) Min. J., 8, 95-109. Němec, J. (1983) Neues J. Miner., Mh., 537-540. Robert, J.-J., Beny, J.-M., Beny, C., Volfinger, M. (1989) Can. Mineral., 27, 225-235. Serratosa, J.M. and Bradley, W.F. (1962) Determination of the orientation of OH bond axis in layer silicates by infrared absorption. J. Phys. Chem., 62, 1164-1167. Vedder, W. and McDonald, R.S. (1963) Vibrations of the OH ions in muscovite. J. Chem. Phys., 38, 1583-1590.