Compressibility and structure behaviour of maruyamaite (K-tourmaline) from the Kokchetav massif at high pressure up to 20 GPa

The structural behaviour of maruyamaite (K-dominant tourmaline) ^X(K_0.54Na_0.28Ca_0.19)^Y(Mg_1.3Al_1.17Fe_0.39Ti_0.14)^Z(Al₅Mg)[Si_5.95Al_0.05O₁₈](BO₃)₃^V,W[O_1.69(OH)_2.31] from the ultrahigh-pressure metamorphic rocks of Kokchetav massif was studied using synchrotron based single-crystal diffraction up to 20 GPa. Within the whole pressure range the compression is regular and anisotropic, with the c direction being more compressible than the a direction. Fitting the V/P data with the 2nd and 3rd order Birch-Murnaghan equations of state gives: V₀ = 1587.2(7) ų, K₀ = 115.6(9) GPa at fixed K′ = 4, and V₀ = 1588(1) ų, K₀ = 112(3) GPa, K′ = 4.5(4). The bulk modulus values are slightly higher as compared to those found for dravite and cation-deficient synthetic K-dravite. The pressure evolution of the main structural parameters of K-tourmaline is similar to those of dravite. However, a minor change in the rigidity of local contacts of the X site with 6-membered ring, due to the presence of K, is apparently critical for stabilization of tourmaline structure within 15–20 GPa, which is evinced by the absence of the phase transition observed in dravite near 15.4 GPa. The stabilizing function of K becomes apparent at P > 15 GPa. The comparison of the HP structural behaviour of maruyamaite and dravite supports the recent suggestion that the large X site plays a secondary role in the elastic behaviour of tourmaline, compared to the octahedral framework. In addition, the present study reveals several new features of polyhedra distortions, which demonstrate their complex interaction on compression.