The elbaite-subtype pegmatite at Blizna, near Cerna v Posumavi in southern Bohemia, Czech Republic, intrudes dolomite-calcite marble. It can be divided into two distinct units, unit A and unit B, and has a narrow exocontact zone along the edge of the pegmatite. The crystallization sequence of tourmaline from unit A is Ca-rich schorl-elbaite --> Ca-rich elbaite-schorl-dravite --> Mn-rich elbaite --> Mn-rich liddicoatite --> liddicoatite-elbaite, typical of tourmaline from elbaite-subtype pegmatites. The crystallization sequence of tourmaline from unit B is Na-rich uvite --> Ca-rich schorl-elbaite-dravite --> Ca-rich Mg-bearing elbaite --> (Ca, Mn)-rich elbaite --> Mn-rich liddicoatite-elbaite --> Na-rich liddicoatite. Tourmaline from the exocontact ranges from Na-rich uvite --> Ca-rich dravite --> dravite-elbaite --> Ca-rich Mg-bearing elbaite --> liddicoatite-elbaite. The intermediate elbaite-schorl-dravite from unit A and schorl-elbaite-dravite from unit B are characterized by low F, moderate Mn and high Ti and Ca contents. The Mg-bearing elbaite from unit B and the exocontact is characterized by low Fe, moderate Ti, and high Ca and F contents. The dravite-elbaite from the exocontact is characterized by low Fe, Mn, Ti, Ca and F contents. The intermediate elbaite-schorl-dravite and elbaite-dravite solid-solutions crystallized as a result of mixing of (Ca, Mg)-rich fluids from the dolomite-calcite marble host-rock and exsolved (Na, Al, Li, F, B)-rich pegmatitic fluids. The factors controlling formation of elbaite-dravite in rocks may be summarized as follows: high activities of Li, Mg, Al and B, high Li/Mg ratio in the parent medium, acidic conditions with pHacidic conditions with pH <similar to 6, and lack of holmquistite. Consequently, elbaite-dravite solid-solutions should occur in highly contaminated elbaite-subtype pegmatites penetrating marble and in holmquistite-free contact aureoles around complex LCT pegmatites.
Tourmaline of the elbaite-dravite series from an elbaite-subtype pegmatite at Blizna, southern Bohemia, Czech Republic
Ottolini L
1999
Abstract
The elbaite-subtype pegmatite at Blizna, near Cerna v Posumavi in southern Bohemia, Czech Republic, intrudes dolomite-calcite marble. It can be divided into two distinct units, unit A and unit B, and has a narrow exocontact zone along the edge of the pegmatite. The crystallization sequence of tourmaline from unit A is Ca-rich schorl-elbaite --> Ca-rich elbaite-schorl-dravite --> Mn-rich elbaite --> Mn-rich liddicoatite --> liddicoatite-elbaite, typical of tourmaline from elbaite-subtype pegmatites. The crystallization sequence of tourmaline from unit B is Na-rich uvite --> Ca-rich schorl-elbaite-dravite --> Ca-rich Mg-bearing elbaite --> (Ca, Mn)-rich elbaite --> Mn-rich liddicoatite-elbaite --> Na-rich liddicoatite. Tourmaline from the exocontact ranges from Na-rich uvite --> Ca-rich dravite --> dravite-elbaite --> Ca-rich Mg-bearing elbaite --> liddicoatite-elbaite. The intermediate elbaite-schorl-dravite from unit A and schorl-elbaite-dravite from unit B are characterized by low F, moderate Mn and high Ti and Ca contents. The Mg-bearing elbaite from unit B and the exocontact is characterized by low Fe, moderate Ti, and high Ca and F contents. The dravite-elbaite from the exocontact is characterized by low Fe, Mn, Ti, Ca and F contents. The intermediate elbaite-schorl-dravite and elbaite-dravite solid-solutions crystallized as a result of mixing of (Ca, Mg)-rich fluids from the dolomite-calcite marble host-rock and exsolved (Na, Al, Li, F, B)-rich pegmatitic fluids. The factors controlling formation of elbaite-dravite in rocks may be summarized as follows: high activities of Li, Mg, Al and B, high Li/Mg ratio in the parent medium, acidic conditions with pHacidic conditions with pHI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
