Synthesis of tourmaline solid solutions in the system Na2O-MgO-Al2O3-SiO2-B2O3-H2O-HCl and the distribution of Na between tourmaline and fluid at 300to 700 degrees C and 200 MPa

Tourmaline has been synthesized hydrothermally at 200 MPa between 300 and 7 00 degreesC from oxide mixtures with Mg-Al ratios for the end members dravi te NaMg3Al6(Si6O18)(BO3)(3)(OH)(3)(OH) and Mg-foitite square (Mg2Al)Al-6 (S i6O18)(B03)3(OH)3(OH) Six different Na concentrations were investigated to determine the distribution of Na between tourmaline and fluid in the SiO2-s aturated system Na2O-MgO-Al2O3-SiO2-B2O3H2O-HCl. Synthetic tourmaline range s from X-site vacant (square) tourmaline (Mg-foitite) to nearly ideal dravi te with Na = 0.95 apfu. There are small, but significant, amounts of proton deficiency and negligible tetrahedral Al. Chemical variation is primarily caused by the substitutions Al square Mg-1Na-1 and minor AlMg-1H-1. Varying amounts of Na and square determine the Mg/Al ratios. Besides tourmaline an d quartz, additional Mg-Al phases are chlorite and, at 700 degreesC, cordie rite. Albite is also present at high Na concentrations in the bulk composit ion. The c dimension of the tourmaline crystals increases with Na in tourma line. The amount of Na in the X-site depends strongly on the bulk concentra tion of Na in the system as well as on the temperature. These factors in tu rn control the phase assemblage and the composition of the fluid phase. For the assemblage tourmaline + quartz + chlorite/cordierite + fluid, a linear relationship exists between Na concentration in the fluid (quenched after the run) and tourmaline with temperature: T degreesC[+/- 25 degreesC] = (Na -fluid/Na-tur) x 558.878 -14.692 (r(2) = 0.96). For the assemblage tourmali ne + albite + quartz + fluid, it is: T degreesC[+/- 15 degreesC] = (Na-flui d/Na-tur) x 845.813-6.231 (r(2) = 0.95), where Na-fluid is the concentratio n of Na+ in the final fluid (mol/l) and Na-tur is the number of Na cations in the X-site of tourmaline. The equations are valid in the temperature ran ge of 500-715 degreesC. Our experiments demonstrate that: the occupancy of the X-site in combination with the changing concentrations of Al and Mg can be used to monitor changes in the fluid composition in equilibrium with a growing tourmaline crystal. Currently, this relation call be applied qualit atively to natural tourmaline to explain zoning in Na- and Al/(Al + Mg).