K-dominant tourmaline was synthesized in the system MgO-Al2O3-B2O3-SiO2-KCl-H2O at 700 °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 (K0.60(3)0.36(3))(Mg2.60(7)Al0.40(7))(Al5.98(3)Si0.02(3))Si6O18(BO3)3(OH)3.92(7)O0.08(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 XK(XNa)−1, magnesio-foitite via the coupled substitution XKYMg(XYAl)−1, and “K-olenite” via the coupled substitution YMgOH(YAlO)−1. Structural refinement of the powder X-ray diffraction data provides a unit-cell volume for the synthesized “K-dravite” of 1580.1(5) Å3, which is greater than that determined for K-bearing dravitic tourmaline synthesized at the same conditions [1574.9(4) Å3]. We interpret this to reflect expansion of the crystal structure due to incorporation of the relatively large K+ ion.

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