Experimental determination of Na-Ca distribution between tourmaline and fluid in the system Cao-Na (sub 2) O-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -B (sub 2) O (sub 3) -H (sub 2) O

We have carried out tourmaline-synthesis experiments in the system Na (sub 2) O-CaO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O-B (sub 2) O (sub 3) 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 + CaCl (sub 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 (sub -1) Mg (sub -1) , []Al (sub 2) Ca (sub -1) Mg (sub -2) and NaHCa (sub -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 + or - plagioclase at 500 degrees and 600 degrees C, and with cordierite + quartz + or - plagioclase at 700 degrees 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 (sub 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.