Rare alkali metals in micas from Shakhdarinskaya miarolitic pegmatite (South-Western Pamir)

Rare alkali metals in micas from Shakhdarinskaya miarolitic pegmatite (South-Western Pamir)

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The study is focused on the mineralogy of rare-element rich tourmaline-bearing miarolitic pegmatities of the Shakhdarinskaya vein (SW Pamirs, Tajikistan).The results of the study are crucially important for understanding the regularities of distribution and behavior of Li, Rb and Cs in the course of crystallization of the most low-temperature pegmatitic and rare-element rich granitic magmas.The major objective of the study was the understanding of mineralogy and geochemistry of the pegmatite with special emphasis on chemistry, classification and nature of Li, Cs and Rb-rich micas.This paper represents the first information on mineralogy of the Shakhdarinskaya vein along with the attempt of comparison with the features of the mineral-forming media.

Optical microscopy, Raman spectroscopy, WDS and EDS X-ray spectral microanalysis and SIMS were applied in order to obtain the data on mineral compositions and structure of micas and inclusions of mineral forming media.Micas from three major assemblages: quartz-two-feldspar cubs foam finger one, which builds the most part of the Shakhdarinskaya pegmatite; near-miarolitic quartz-albite-mica assemblage; and miarolitic assemblage, which is composed of quartz, albite and mica from miarolitic pockets, were studied.Dark mica from the quartz-feldspar assemblage are represented by intermediate composition of the annite-siderophyllite series, enriched in Li, Cs, Rb, Nb.Micas of nearmiarolitic and miarolitic read more assemblages belong to trioctahedral lithium-fluorine mica - polylithionite.

The early Li-micas were enriched in Fe and Mn, which are almost absent in the rims of the niarmiarolitic mica and they are totally absent in miarolitic mica.Polylithionites of the Shakhdarinskaya pegmatite are strongly enriched in Rb2O up to 1,2 wt.% and Cs2O up to 0,6 wt.%.

It is demonstrated that Cs enrichment of the polylithionite increases more than Rb one from nearmiarolitic to miarolitic assemblage.Based on fluid and melt inclusion analysis it was established that Li, Rb and Cs-enriched mica were formed from mineral-forming media that were anomalously enriched in rare alkali metals, boron and fluorine.