Publication details
Hydrothermal replacement of garnet by tourmaline in elbaite subtype LCT pegmatites
| Authors | |
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| Year of publication | 2015 |
| Type | Appeared in Conference without Proceedings |
| MU Faculty or unit | |
| Citation | |
| Description | Replacement mineral assemblages (tourmalines, micas) after primary garnet are indicator of composition of residual fluids. Mineral assemblages of tourmaline replacing garnet from elbaite-subtype pegmatites cutting marble and calc-silicate rock (Tsarafara Nord and Tamponilapa, Madagascar) and enclosed in gneiss and amphibolite (Ctidružice, Czech Republic) were examined. Primary mineral assemblages in all three pegmatites include spessartine-dominant garnet (Sps42-96) and black schorl to Mg-rich schorl, albite and quartz. Garnet replacement reactions were observed only in the vicinity of the Li-enriched albite unit. The replacement takes place mainly on the direct contact of garnet and albite. The total volume of this replacement process related to the whole pegmatite volume is negligible. Secondary assemblages replacing garnet and albite consist of mostly dark blue Fe-rich fluor-elbaite to very rare fluor-liddicoatite and quartz. Secondary tourmaline differs from primary ones in low to negligible contents of Mg, Ti, and mostly also Fe, and apparently elevated contents of Al, Mn, Li and F. However, behavior of Ca and Na is distinct in tourmalines from the individual localities. The schorl – fluor-elbaite – liddicoatite is present mainly at Tsarafara, whereas at Tamponilapa, most compositions represent schorl – elbaite – rossmanite solid solution, with rare schorl-uvite trend. Textural relations showing replacement of garnet and albite by tourmaline and quartz suggest the simplified reaction: 23 NaAlSi3O8 + 3 (Mn2Fe1)Al2Si3O12 + 12 H3BO3 + 3 LiF + HF = 4 (Na0.75)(Al1.25Fe0.75Li0.75Mn0.25)Al6 (Si6O18)(BO3)3(OH)3F + 44 SiO2 + 10 Na2SiO3 + 5 MnO + 12 H2O + OH- |