Synthesis of K-dominant tourmaline

K-dominant tourmaline was synthesized in the system MgO-Al2O2-B2O3-SiO2-KCl-H2O at 700 degrees C and 4.0 GPa. The crystals were zoned and characterized by less-potassic cores (1.46 wt% K2O) and more-potassic rims (up to 3.44 wt% K2O). The K-dominant tourmaline rims are represented by the average structural formula (K-0.60 square(0.36(3))) (Mg-2.60(7))(Al-5.98(3))Si6O18(BO3)(3)(OH)(3.92(7))O-0.08(8), which is analogous to the structural formula of dravite and is referred to here as K-dravite; the maximum analyzed K content (3.44 wt% K2O) represents occupancy of the X site by 0.71 K pfu. The addition of Na to the system in approximately equal molar proportions to K results in the crystallization of K-bearing, Na-rich dravitic tourmaline, dramatically reducing the K content to an average value of 0.47 wt% K2O, corresponding to 0.10 K pfu. This suggests that a K-dominated bulk composition is necessary for K-dominant tourmaline crystallization. Compositional zoning shows that solid solution exists between end-member compositions of K-dravite [KMg3Al6Si6O18(BO3)(3)(OH)(3)(OH)] and dravite via the isovalent exchange K-X(Na-X)(-1), magnesio-foitite via the coupled substitution (KMg)-K-X-Mg-Y((X)square Al-Y)(-1), and K-olenite via the coupled substitution (MgOH)-Mg-Y((AlO)-Al-Y)(-1). Structural refinement of the powder X-ray diffraction data provides a unit-cell volume for the synthesized K-dravite of 1580.1(5) angstrom(3), which is greater than that determined for K-bearing dravitic tourmaline synthesized at the same conditions [1574.9(4) angstrom(3)]. We interpret this to reflect expansion of the crystal structure due to incorporation of the relatively large IC ion.