Schorl-oxy-schorl to dravite-oxy-dravite tourmaline from granitic pegmatites; examples from the Moldanubicum, Czech Republic

Wet-chemical analyses of 41 tourmalines from granitic pegmatites 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 a combination of heterovalent substitutions (super Y) Al (super W) O (super Y) R (super 2+) (sub -1) (super W) (OH) (sub -1) , (super X) [] (super Y) Al (sub 2) (super W ) O (super X) Na (sub -1) (super Y) R (super 2+) (sub -2) (super W) (OH) (sub -1) , (super X) [] (super Y) Al (super X) Na (sub -1) (super Y) R (super 2+) (sub -1) and (super X) [] (super W) (OH) (super X) Na (sub -1) (super W) O (sub -1) , and homovalent substitutions Fe (super 2+) Mg (sub -1) and (OH)F (sub -1) . Tourmalines with the general formula (super X) (Na (sub 0.5) [] (sub 0.5) ) (super Y) (R (super 2+) (sub 2) Al) (super Z) Al (sub 6) (BO (sub 3) ) (sub 3) Si (sub 6) O (sub 18) (super V) (OH) (sub 3) (super W) (O (sub 0.5) OH (sub 0.5) ) crystallized in very similar P-T conditions in granitic systems saturated in Na, Al, Si and H (sub 2) O, indicating the importance of short-range order requirements on tourmaline chemical composition. The abundance of heterovalent substitutions involving the W site requires determination of light elements (H, B, Li, F) and Fe (super 2+) /Fe (super 3+) in tourmalines to specify substitution mechanisms with certainty. Normalization of EPMA data for (Fe,Mg)-rich, (Ca,Li,F)-poor tourmalines from granitic pegmatites on (OH,F) (sub 3.5) O (sub 0.5) , which is more probable than (OH,F) (sub 4) , appears to be advisable.