Schorl-oxy-schorl to dravite-oxy-dravite tourmaline from granitic pegmatites; examples from the Moldanubicum, Czech Republic
Novák, Milan; Povondra, Pavel; Selway, Julie B.
European Journal of Mineralogy Volume 16 Number 2 (2004), p. 323 - 333
published: Mar 29, 2004
ArtNo. ESP147051602014, Price: 29.00 €
Wet-chemical analyses (41) of tourmaline from granitic pegmatites (barren, barren-pocket, beryl, lepidolite types) in the Moldanubicum, Czech Republic revealed that members of the oxy-subgroup - common oxy-schorl, minor oxy-dravite and rare oxy-foitite are more abundant relative to the relevant members of the hydroxy-subgroup. The primary substitution mechanisms in tourmaline show combination of heterovalent substitutions: YAlWO YR2+-1 W(OH)-1, XYAl2 WO XNa-1 YR2+2- W(OH)-1, XYAl XNa-1 YR2+-1 and X W(OH) XNa-1 WO-1, and homovalent substitutions: Fe2+Mg-1 and (OH)F-1. Tourmalines with the chemistry expressed by the general formula X(Na0.50.5)Y(R2+2Al)ZAl6(BO3)3Si6O18V(OH)3W(O0.5OH0.5) crystallized in very similar PT conditions in granitic systems saturated on Na, Al, Si and H2O, it indicates the importance of short-range order requirements on tourmaline chemical composition. Abundance of heterovalent substitutions involving the W-site requires determination of light elements (H, B, F, Li) and Fe2+/Fe3+ in tourmalines to specify substitution mechanisms with certainty. Normalization of electronmicroprobe data of (Fe,Mg)-rich, (Ca,Li,F)-poor tourmalines from granitic pegmatites on (OH,F)3.5O0.5, which is more probable than (OH,F)4, seems to be suitable.