On the crystal chemistry and elastic behavior of a phlogopite 3T

The crystal chemistry and the elastic behavior under isothermal conditions up to 9 GPa of a natural, and extremely rare, 3T-phlogopite from Traversella (Valchiusella, Turin, Western Alps) [(K(0.99)Na(0.05)Ba(0.01))(Mg(2.60)Al(0.20)Fe (0.21) (2+) )[Si(2.71)Al(1.29)O(10)](OH)(2), space group P3(1)12, with a = 5.3167(4), c = 30.440(2) , and V = 745.16(9) a"<< A(3)] have been investigated by electron microprobe analysis in wavelength dispersion mode, single-crystal X-ray diffraction at 100 K, and in situ high-pressure synchrotron radiation powder diffraction (at room temperature) with a diamond anvil cell. The single-crystal refinement confirms the general structure features expected for trioctahedral micas, with the inter-layer site partially occupied by potassium and sodium, iron almost homogeneously distributed over the three independent octahedral sites, and the average bond distances of the two unique tetrahedra suggesting a disordered Si/Al-distribution (i.e., aOE (c) T1-O > similar to 1.658 and aOE (c) T2-O > similar to 1.656 a"<<). The location of the H-site confirms the orientation of the O-H vector nearly perpendicular to (0001). The refinement converged with R (1)(F) = 0.0382, 846 unique reflections with F (O) > 4 sigma(F (O)) and 61 refined parameters, and not significant residuals in the final difference-Fourier map of the electron density (+0.77/-0.37 e (-)/a"<<(3)). The high-pressure experiments showed no phase transition within the pressure range investigated. The P-V data were fitted with a Murnaghan (M-EoS) and a third-order Birch-Murnaghan equation of state (BM-EoS), yielding: (1) M-EoS, V (0) = 747.0(3) a"<<(3), K (T0) = 44.5(24) GPa, and K' = 8.0(9); (2) BM-EoS, V (0) = 747.0(3) a"<<(3), K (T0) = 42.8(29) GPa, and K' = 9.9(17). A comparison between the elastic behavior in response to pressure observed in 1M- and 3T-phlogopite is made.