Single-crystal FTIR spectroscopy of masutomilite, the Mn-analogue of zinnwaldite

Masutomilite is a rare Mn-analogue of zinnwaldite, which was firstly described from a granitic pegmatite at Tanakamiyama, Shiga Prefecture, Japan. Masutomilite forms extensive solid-solutions with zinnwaldite; few samples on this system have been actually reported from Tawara, Gifu Prefecture, Japan from western Moravia, Czech Republic and from few other localities. We relate here an infrared study of three mica samples labeled as masutomilite from Japan (Hirukawa Mine, Gifu Prefecture), Central Urals, Russia (Mokrusha Pegmatite, Murzinka Region) and Idaho, USA (Santooth Mountains, Boise County). Microchemical data, although incomplete due the lack of H2O determination, showed that the studied micas from Japan and USA have different Fe/Mn contents, the Idaho sample being significantly enriched in Fe relative to Mn. The Russian sample did not show any Mn and a only a very small amount of Fe (< 1 wt%). Single-crystal, polarized-light FTIR spectra were collected for all samples 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, with a spot size ~ 100 μm. Single cleavage flakes, 10 to 20 μm thick, were oriented under the optical microscope on the basis of the interference figure (masutomilite is biaxial negative, with Y = b and 2V < 35°), and polarized spectra were collected with the electric vector E//γ and E//β. The flakes were then mounted on a glass capillary and stepwise tilted under the beam, in order to collect the α-polarization spectra. The collected patterns are significantly different, due to the different chemical composition of the examined samples, however they show broad similarities. In particular they consist of a higher-frequency (~ 3700 cm-1) component, which is assigned to OH groups in local tri-octahedral environments, and broader absorptions at frequencies < 3600 cm-1, assigned to OH-groups in local di-octahedral environments. This feature indicates that all analyzed samples have vacant octahedral sites in the vicinity of the OH-group. In both samples from Utah and Japan, the most intense band at around 3600 cm-1 consists of several (at least four) well-resolved components, while the same absorption in the Russian specimen consists of a single band. This difference is explained considering the presence of Mn and Fe in the former samples, and of a unique octahedral cation (Al) in the latter sample. The higher-frequency, 3700 cm-1 band, is strongly polarized for all specimens, with maximum absorption for E//α, indicating that the trioctahedral OH groups are aligned along the c crystallographic axis. The di-octahedral bands are also significantly polarized and show maximum absorption for E//γ. From measurement of the dichroic ratio (absorbance along a / absorbance along b) the orientation of the di-octahedral O-H vector in the (001) place can also be evaluated.