MORPHOLOGICAL PECULIARITIES AND GENESIS OF TOURMALINE CRYSTALS IN THE PEGMATITE VEIN KARKADILOVAYA (MALKHANY, CENTRAL TRANSBAIKALIA)
The paper is dedicated to study of tourmaline crystals in the «alteration zone», connected with the micro-fracture in zonal potassium feldspar (PFS) from the rare-metal pegmatite Karkadilovaya in Malkhanskoye pegmatite field (Central Transbaikalia). This micro-fracture occurs on the boundary between two feldspar varieties: nontransparent pink (PFS-1) and water-transparent (PFS-2) ones, and it is filled with albite-tourmaline aggregate intensively replacing PFS-1. Microscopic investigation allows distinguishing three varieties of tourmaline: dark-green (Typ-1) represented by casing-like pipe-shaped crystals filled with PFS-2, kaolinized mass or sheaf-like crystals of the pink tourmaline (Typ-2); the third variety of tourmaline (Typ-3) is represented by its black «heads» on pink needle-shaped small Typ-2 crystals. Temperature of the plagioclase-tourmaline aggregate formation is estimated as 380 ± 20 °C, under pressure 1.5 kbar. Temperature of the pink tourmaline (Typ-2) crystallization is 390 ± 20 °C, with correction for pressure (according to data on homogenization of gas-liquid inclusions). Analysis of mineral equilibriums between tourmalines and potassium feldspar has shown that the «tourmalinization» process was accompanied by decrease of the mineralization media alkalinity, and the transition from Typ-1 to Typ-2 evidences the increasing activity of lithium, with decreased sodium and iron activities. Replacement of the Typ-2 variety by the later black Typ-3 was possible with increasing activities of sodium, magnesium, and decreasing of lithium activity. The most favorable conditions for co-existence of those three tourmalines include pH = 6, a K + =10 -2, a Na + =10 -3.5, a Li + =10 -2.4, a FeOH + =10 -3.8 and the elevated boric acid activity.