Synthesis of Ca-tourmaline in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl

Ca-tourmaline has been synthesized hydrothermally in the presence of Ca(OH) (2) and CaCl2-bearing solutions of different concentration at T = 300-700 d egrees C at a constant fluid pressure of 200 MPa in the system CaO-MgO-Al2O 3-SiO2-B2O3-H2O-HCl. Synthesis of tourmaline was possible at 400 degrees C, but only above 500 degrees C considerable amounts of tourmaline formed. El ectron microprobe analysis and X-ray powder data indicate that the syntheti c tourmalines are essentially solid solutions between oxy-uvite, CaMg3Al6(S i6O18) (BO3)(3)(OH)(3)O, and oxy-Mg-foitite, square(MgAl2)Al-6(Si6O18)(BO3) (3)(OH)(3)O. The amount of Ca ranges from 0.36 to 0.88 Ca pfu and increases with synthesis temperature as well as with bulk Ca-concentration in the st arting mixture. No hydroxy-uvite, CaMg3(MgAl5)(Si6O18)(BO3)(3)(OH)(3)(OH), could be synthesized. All tourmalines have < 3 Mg and > 6 Al pfu. The Al/(A l+ Mg)-ratio decreases from 0.80 to 0.70 with increasing Ca content. Al is coupled with Mg and Ca via the substitutions Al(2)square Mg-2Ca-1 and AlMg- 1H-1. No single phase tourmaline could be synthesized. Anorthite (+quartz i n most runs) has been found coexisting with tourmaline. Other phases are ch lorite, tremolite, enstatite or cordierite. Between solid and fluid, Ca is strongly fractionated into tourmaline (+ ano rthite). The concentration ratio D = Ca(fluid)/Ca(tur) increases from 0.20 at 500 degrees C up to 0.31 at 700 degrees C. For the assemblage turmaline + anorthite + quartz + chlorite or tremolite or cordierite, the relationshi p between Ca content in tourmaline and in fluid with temperature can be des cribed by the equation T[+/-32 degrees C] = Ca(fluid)/Ca(tur)*2058.67 + 87.42 r(2) = 0.95 (whereby T = temperature in degrees C, Ca(tur) = amount of Ca on the X-site in tourmaline, Ca(fluid) = concentration of Ca2+ in the fluid in mol/l). T he investigations may serve as a first guideline to evaluate the possibilit y to use tourmaline as an indicator for the fluid composition.