Chrystallochemical and luminescent characteristics of tourmaline group minerals from Malkhan pegmatite field (Transbaikalia)

Characteristics of luminescence of tourmaline group minerals aren't studied profoundly enough still. They are given in seldom publications. The purpose of the work is to show luminescence analysis possibilities in complex approach to tourmaline group minerals study. Studied samples were taken from Sosedka pegmatite body and piles of Mokhovaya pegmatite body of Malkhan pegmatite field in Transbaikalia. According to energy dispersive X-ray microanalysis results studied tourmalines belong to elbaite-liddicoatite isomor-phous series and have 10-35% of rossmanite component. X-ray luminescence (XRL) spectra of studied samples were obtained in optical range of wave lengths (200-800 nm). For Sosedka vein tourmalines (especially for calcium ones) appearing of luminescence bands related to rare earth elements presence in their XRL spectra is rather typical. Appearance of such luminescence bands as ones caused by Ce³+ (340-360 nm), Eu²+ (380-480 nm), Dy³+ (500-600 nm), Sm²+ (680-750 nm) presence may be an evidence of reductive conditions which had place in time of miaroles generation. On the contrary, Eu³⁺ (620-625 nm) and Sm³⁺ (600-670 nm) luminescence bands indicate a possible change from reductive conditions to oxidative ones and alkalinity increase, which could take place as a result of depres-surization of miarolitic cavities. Besides REE there is a possibility of Cr³⁺-related bands appearing at 690-820 nm in tourmaline XRL spectra. Fe³+-related band may appear in 700-750 nm range as a consequence of depressurization and an increase of alkalinity and oxidation potential. Most of intrinsic defects in borosilicates are holes trapped by oxygen atoms, which are, in turn, bonded with boron. For instance, such defects presence causes 650-660 luminescence band appearing. It is possible that excitons, vacancies, internodal oxygen and molecular oxygen also take part in luminescence (the latter two probably would indicate oxidative conditions). 280 nm band is related to oxygen vacancies, which may point that there was some oxygen deficiency in the mineral forming environment. There is a wide band with intensity maximum at 345-355 nm appearing in many spectra. It's probably caused by hole centers presence. Further investigations should be carried out for more precise intrinsic defects definition, but still. At the present moment it may be considered that all of intrinsic luminescence centers in Malkhan field tourmalines are oxygen excited states. XRL spectra of Malkhan pegmatite field tourmalines are rather diverse. XRL analysis results allow concluding that there were reductive and acidic conditions of minerals formation. Samples taken from adjacent miaroles may have different luminescence spectra due to initial differences between compositions of separate melt parts, which turned to miaroles.

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