MINERAL ASSEMBLAGES, COMPOSITIONAL VARIATION, AND CRYSTAL STRUCTURE OF FERUVITIC TOURMALINE FROM A CONTAMINATED ANATECTIC PEGMATITE AT MIROSOV NEAR STRAZEK, MOLDANUBIAN ZONE, CZECH REPUBLIC

摘要

Amphibole-bearing, simply zoned, anatectic pegmatite, up to 20 cm thick, cuts calc-silicate rocks at Mirosov, near Strazek, in the Moldanubian Zone of the Czech Republic. Common amphibole (K-rich magnesio-hornblende to ferro-hastingsite), plagioclase An_(30-42) and titanite, along with accessory feruvitic tourmaline—Ca-rich schorl to Na-rich feruvite—are indicators of Ca and Fe-contamination from the host rock. Interstitial tourmaline from the outer unit (TU1) and tourmaline from graphic intergrowths with quartz in more evolved parts of the pegmatite (TU2) have similar chemical compositions (EMPA, LA-ICP-MS) with Ca = 0.37-0.58 apfu, Al = 4.77-5.77 apfu; TU1 has higher Mg/(Mg + Fe_(tot)) = 0.52-0.41 compared to TU2 (0.39-0.15). TU1 is enriched in Ti, V, Cr, Sr, and TU2 in Li and Zn, respectively. Late microscopic veinlets of Na,Al,Mg-enriched tourmaline (TU3)—Ca-rich dravite/schorl—replace both TU1 and TU2. A combination of EMPA, single crystal XRD data, and bond-valence optimization applied to the TU2 data yielded the formula ~X(Ca_(0.494)Na_(0.451)□_(0.055)) ~Y(Mg_(0.422)Al_(0.365)Fe~(2+)_(1.143)Fe~(3+)_(0.952)Mn_(0.029) Ti_(0.084)Zn_(0.005)) ~Z(Al_(5.106)Fe~(3+)_(0.377)Mg_(0.518)) ~T(Si_(5.879)Al_(0.121)O_(18)) (BO_3)_3 [(OH)_(2.656)O_(1.286)F_(0.0.058)], which represents disordered Na,Fe~(3+)-rich feruvite (to "oxy-feruvite"). Contaminated pegmatite from Mirosov and its Ca-rich tourmalines are similar to other simple pegmatites from the Moldanubian Zone which have been contaminated by Ca,Fe-rich host rocks. The chemical compositions of Ca-rich schorl to Na-rich feruvite from various geological environments are similar, showing only minor dominance of Ca over Na (< ~0.65 apfu Ca), low X-site vacancy, and low Al (mostly < ~5.5 apfu Al), but highly variable in Mg/(Mg + Fe_(tot)). The formation of feruvite requires specific geochemical conditions with high activities of Ca and Fe, but rather low Mg, conditions typical for some contaminated granitic pegmatites.