Mineralogy of pockets of the Malkhan tourmaline deposit (Transbaikalia): feldspars of the Sosedka vein

A detailed study was given to the composition and structure of alkali feldspars from the pockets of the Sosedka pegmatite vein, a large source of gems within the Malkhan tourmaline deposit. The vein is of concentric-zonal structure. Three types of pockets were recognized by mineral composition: A—quartz–lepidolite–Mn–Li–Al–tourmaline (± pollucite, hambergite, borocookeite, boromuscovite, danburite, light pink beryl); B—quartz–adularia–axinite (± laumontite); and C—quartz and laumontite (± B-containing cookeite). Each type of pockets contains feldspars of specific composition and structure. This evidences that pockets formed in strongly different conditions, though some pockets of different types are localized as close as 0.5–2 m from each other within a zone. The reported data disagree with the common model implying the formation of zonal pegmatite bodies as a result of crystallization differentiation within the vein.