Experimental determination of Na-Ca distribution between tourmaline and fluid in the system CaO-Na2O-MgO-Al2O3-SiO2-B2O3-H2O

We have carried out tourmaline-synthesis experiments in the system Na 2 O–CaO–MgO–Al 2 O 3 –SiO 2 –H 2 O–B 2 O 3 at 200 MPa between 500° and 700°C under various total concentrations of Na and Ca in order to investigate the distribution of Na and Ca between tourmaline and fluid. Starting materials were oxide–hydroxide mixtures with NaCl + CaCl 2 solutions of different concentrations. The main end-members of the synthetic tourmalines are dravite – uvite – “oxy-uvite” (deprotonated Ca-tourmaline). The dominant substitutions are □AlNa −1 Mg −1 , □Al 2 Ca −1 Mg −2 and NaHCa −1 , and their importance depends on temperature, molarity of the solution and on the ratio Ca/(Ca + Na). The Na–Ca substitution seems to be continuous at least in the steps of 25 mol.%. Unit-cell parameters, obtained from Rietveld refinement, show a systematic increase of c with increasing X-site occupancy in tourmaline and a decrease with increasing Al/(Al + Mg). Tourmalines coexist with chlorite + quartz ± plagioclase at 500° and 600°C, and with cordierite + quartz ± plagioclase at 700°C, rarely with other phases such as tremolite, enstatite or talc. The systematic response of tourmaline composition to changing Na and Ca concentrations and X Ca indicates that tourmaline can be used as a tracer of fluid composition in natural systems. Furthermore, tourmaline can be used as an equilibrium check on mineral parageneses, because the Ca/(Ca + Na) value in plagioclase is the same as that in tourmaline, and Na + Ca content in cordierite is strongly correlated with X -site occupancy in tourmaline.