PSEUDOSINHALITE, MAGNESIOTAAFFEITE-6N' 3S AND MAGNESIOTAAFFEITE-2N' 2S AS REPLACEMENT PRODUCTS OF SPINEL IN DOLOMITE MARBLE FROM STUBENBERG, STYRIA, AUSTRIA

Abstract

Pseudosinhalite [ideally Mg 2 Al 3 O(BO 4 ) 2 (OH)] and magnesiotaaffeite-6 N ′3 S (ideally BeMg 2 Al 6 O 12 ), with rare lamellae of magnesiotaaffeite-2 N ′ 2 S (ideally BeMg 3 Al 8 O 16 ) occur as replacement products of spinel in geikielite–ilmenite – baddeleyite – zirconolite veins within polymetamorphic dolomite marble at Stubenberg, Styria, Austria. The crystallization sequence of Mg–Al phases is spinel → pseudosinhalite → magnesiotaaffeite minerals, and spinel formed after or during the replacement of baddeleyite by zirconolite. Pseudosinhalite contains up to 2.2 wt.% TiO 2 , 1.4% Cr 2 O 3 , and 1.3% FeO, whereas magnesiotaaffeite minerals contain up to 6.6 wt.% ZnO, 2.4% FeO, and 1.8% Cr 2 O 3 . The replacement of spinel by a Mg–Al borate and by Mg–Al–Be oxides requires an influx of B and Be, respectively, since no B- or Be-bearing minerals are present in the original assemblage. These elements are likely derived from fluids released from a Permian granite and pegmatites, or from Eo-Alpine metamorphic fluids that have interacted with B- and Be-bearing lithologies.