LOW-Al TOURMALINES OF THE SCHORL-DRAVITE - POVONDRAITE SERIES IN REDEPOSITED TOURMALINITES FROM THE WESTERN CARPATHIANS, SLOVAKIA

Abstract

Tourmalines of the schorl–dravite – povondraite series were studied in three samples of tourmalinite pebbles from Lower Triassic Lužna Formation in the Tatric Unit (Bratislava, Devinska Kobyla hill) and the Upper Cretaceous Upohlav conglomerates of the Pieniny Klippen Belt (Orlove), in the western Carpathians, Slovakia. A combination of analytical techniques was used to analyze for major elements (electron microprobe), Fe 3+ and Fe 2+ (Mossbauer spectroscopy). The tourmaline-group minerals studied belong to the schorl–dravite subgroup; they attain the composition of povondraite by virtue of an obvious dravite–schorl – povondraite trend, partially overlapped by a schorl – feruvite trend. A significant amount of Fe 3+ in the tourmaline was determined directly by Mossbauer spectroscopy (at least 59.1% of iron is Fe 3+ ) and indirectly by the cation charge-deficiency (Fe 3+ up to 3.7 apfu ). The Al 3+ content is less than 5.0 apfu , locally reaching below 3.0 (2.86) apfu in sample TQ6. A decreasing Al 3+ content can be charge-balanced by two different mechanisms of substitution: (1) feruvite–uvite substitution [CaMg(NaAl) −1 ], and (2) povondraite substitution (Fe 3+ Al −1 ). The feruvite–uvite substitution saturates the X site with Ca 2+ and the octahedral sites with Mg 2+ to balance Al 3+ below 6.0 apfu . Subsequent decreases in Al content below 5.0 apfu cannot be compensated by the input of divalent cations, but by Fe 3+ using a povondraite substitution. Enlargement of the tourmaline structure in Al-poor povondraitic tourmaline due to Fe 3+ Al −1 substitution allows introduction of K into the X site (up to 0.03 apfu ). The genetic environment of the tourmalines studied remains obscure, but their high Fe 3+ contents and K-enrichment suggest highly oxidized conditions in association with evaporitic processes.