Crystal Chemistry of High-Aluminum Schorls From the Shevchenkivske Rare-Metal Deposit (Western Azov Region, Ukraine)

Two generations of tourmaline found in the Shevchenkivske rare-metal deposit (Western Azov region, Ukraine) were investigated by EPMA, XRD and IR spectroscopy analysis. For comparison, the results of previous studies of tourmalines from the nearby Sorokynske pegmatite field and the Airtau ridge (Kazakhstan) were used. Based on the chemical composition, structural and IR spectroscopic characteristics, black tourmalines of the later generation II were identified as schorl and schorl with an alkaline-deficient structure and low aluminum content. The sample from Kazakhstan was identified as schorl with increased magnesium content. Green and greenish-black small tourmaline crystals of the earlier generation I are represented by high-alumina oxy-schorl and schorl with a cation-deficient structure. The formation of oxy-schorl occurs as a result of the isomorphic substitution of Fe2+ by Al3+ according to the scheme XNa+ + YFe2+ → X□ + YAl3+ with the formation of vacancies in the X-position and is accompanied by deprotonation WOH– + YFe2+ = WO2– + YAl3+. As a result of such coupled isomorphic substitutions in the tourmaline structure, the amount of Al3+ increases significantly, while the amount of alkalis in the X position and OH groups in the W- and partially in V-positions, decreases greatly. This leads to the formation of oxy-schorl with a disordered distribution of cations between the Y and Z octahedra, and of OH groups and oxygen atoms between the W and V positions. The formation of high-alumina oxy-schorl from fluide with high-aluminum content is characteristic for the rocks of the Shevchenkivske rare-metal deposit, which contain mullite and sillimanite that was detected as inclusions in the green grains of schorl of the generation I. The formation of schorls of the later generation II took place from an environment comparatively depleted in Al. The existence of tourmalines of the oxy-schorl — alkali-deficient schorl — schorl series in the rocks of the Shevchenkivske deposit is the result of the gradual formation of its rocks from fluides, the composition of which changed from high to low alumina content and increasing in ferrum and alkalis. The Shevchenkivske schorls and oxy-schorls differ from tourmaline from the Sorokynske field, which, due to the large number of vacancies in the X position, falls into the foitite field, and according to other features of the structure, it is close to alkali-deficient schorl and was diagnosed as alkali-deficient schorl. Oxy-schorl, the member of tourmaline oxy-subgroup, was found in Ukraine for the first time.