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 Na2O-CaO-MgO-Al2O3-SiO2-H2O-B2O3 at 200 MPa between 500 degrees and 700 degrees 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 + CaCl2 solutions of different concentrations. The main end-members of the synthetic tourmalines are dravite - uvite - "oxy-uvite" (deprotonated Ca-tourmaline). The dominant substitutions are. square AlNa-1Mg-1, square Al2Ca-1Mg-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 degrees and 600 degrees 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.