About Nb and Ta in lithium micas

Vasil’ev N.V.*, Borodulin G.P.**

* Institute of mineralogy and geochemistry of rare elements, Moscow, Russia; ** Institute of experimental mineralogy RAS, Chernogolovka, Russia, Gleb-Borodulin@yandex.ru

 

Interest to trace element contents in micas (especially amounts of Nb and Ta) enhanced in the 1960-th. It was linked with searching of rare metal deposits, mainly located in rare metal pegmatites (Ivanov et al., 1973). Since then accumulation of insolated data in the area of the concentration of impurity metals in micas in different origin has been taking place. For instance we may notice the article by M.A.Wise (1995), who has investigated trace element content behavior of micas from granitic pegmatites and also has generalized obtained data. The study was carried out with X-Ray fluorescent method, the analytical charge weighted about 1.7 gram. The main problems of Li-mica analyzing are its possible heterogeneity, zonality and existence of micro amounts of «alien» minerals (mainly Nb,Ta-minerals). That’s why, when one analyze trace element contents, competent pure fraction selection of mica is required.

These days some micro beam methods such as Secondary ionic mass spectrometry (SIMS) and Laser ablation inductively coupled plasma mass spectrometry (LA ICP MS) are developed. Location area of this methods is about 20-50 µm. It provides to study concrete sample zone for a wide group of elements. Detection limit fluctuates depending on an installation and analyzing condition within 0.05- 5 ppm.

We have conducted study of local trace element content distribution in Li-micas using SIMS (Cameca ims-4f, IMI RAS, Yaroslavl’-city, analyzer Simakin S.G.) and LA-ICP-MS (ELAN 6100 DRC with laser ablation system LSX-200, IMGRE, Moscow, analyzer Vasil’ev N.V.) methods. When it was analyzed with LA ICP MS method we used calibration glass NIST-612 with the concentration of each element at ≈ 40 ppm level as «external» standard. We used Si as internal standard. Using this technique, we obtain relative responsivity of investigated element (from Li to U) by measuring of calibration glass. And we get absolute responsivity from comparing MS signal of internal standard with just obtained independent concentration data which provided by micro beam method. Comparing of concentration data for the same series which obtained using SIMS and LA ICP MS methods shows good convergence of this methods, even regarding considerable mica heterogeneity. It is important, that SIMS is more responsible, but LA ICP MS is more productive.

We have investigated some Li-macas of Li-muscovite – lepidolite series (rare metal pegmatites), some polylithionites from Nb-Ta deposits (Aryskan, Ulug-Tanzek) and some muscovites, biotites, zinnwaldites, and lepidolites from rare metal granites and greisens (Orlovka and Etyka deposits). In spite the mica were analyzed for a wide spread of micro elements (considerable variations were found for Li, Be, Rb, Cs, Tl, Pb, Zn, W, Sn, Ta, Nb, Cr, Ti), our main purpose was to study behavior of Nb and Ta. According to M.A. Wise, the most Li-mica from rare metal pegmatites have average concentration of Nb and Ta at the level of 40-200 ppm. Peak concentration are: 400 ppm for Nb and 1200 ppm for Ta. Our results are presented on the picture 1. There are two reference trends for Nb and Ta behavior in Li-micas. The first is indicated by grey dotted arrow and it is in accordance with (Ivanov, 1973): Ta and Nb concentration change in regularly order, and Nb concentration changes more considerably then Ta. This trend describes polylithionites from Ta-Nb deposits. The second trend is typical for pegmatites: Nb and Ta concentration change closely with similar concentrations. The concentration range is in accordance with M.A. Wise data. Li-micas from Orlovka and Etyka deposits have both this trends. Li-mica compositions drift to the lepidolite series and it leads to Nb and Ta concentration decreasing and F concentration increasing.

Another interesting problem is associated with extremely possible Nb and Ta concentration in Li-micas. The Ta concentration at about 600 and 1200 ppm, which appeared in Wise’s analyzes, seemed to be caused by «mechanical» pollution of some other minerals (micro crystals of tantalite). In contrast to deformed octahedrons TiO₆ and NbO₆ with nonequivalent metal-oxygen links, TaO₆ octahedrons have symmetric structure, and it causes some restrictions to isomorphic (Ta) – (Ti,Nb) substitutions in silicates. Nb concentration in Li-micas reaches 11000 ppm (1.1 wt.%). Such Nb concentrations in polylithionite are mentioned in «Minerals» reference book (Vol. 3 – «Laminated Silicates») with an appropriate reference (Stevens, 1938). This concentration might be produced by Ta,Nb-mineral mechanical impurity in micas. That’s why we investigated a sample of Greenland polylithionite (№ 65-231), from the Fersman Mineralogy Museum (Moscow) collection. Electron microscope observation and micro beam analyzes confirmed the existence of pyrochlore presence (20-50 µm) in mica scales. Nevertheless the Nb concentration at the level of 0.7-1.5 wt.% were obtained in clear area of mica. It was confirmed by three methods: microbeam analyzes, SIMS and LA ICP-MS. Ta concentration was shown at the level of 1 ppm. IR spectra of polylithionite 65-231 demonstrate significant crystal structure distortion, so we expect more detail investigation being done.

Fig. 1. Nb and Ta concentration in micas from rare metal granites, pegmatites and greisens. Letters: M – lepidolites of central pegmatites zones of Malkhan ridge (Western Transbaikalia, Russia), UТ – polylithionites of Ulug-Tanzek deposit (Western Sayan, Russia), A – polylithionite of Aryskan deposit (Western Sayan, Russia).

 

This work is dedicated to memories of our teachers T.N.Shuriga and G.P.Zaraisky. The work is executed under financial support of RFBR grants  08-05-00835, 08-05-00865 and ONZ RAS program 2.

 

 References:

Ivanov V.V., Belevtin V.V., Borisenko L.F. Average concentrations of elements-impurities in minerals. Moscow: Nedra, 1973. 208 p. (in Russian)

Wise M.A. Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites // Mineralogy and Petrology. 1995. Vol. 55. P. 203-215.

Stevens R.E. New analyses of lepidolites and their interpretation // American Mineralogist. 1938. Vol. 23. P. 607-628.