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) [(K0. 99Na0.05Ba0.01)(Mg2.60Al0.20Fe0.212+)[Si2.71Al1.29O10](OH)2, space group P3112, with a = 5.3167(4), c = 30.440(2) Å, and V = 745.16(9) Å3] 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., 〈T1-O〉̃ 1.658 and 〈T2-O〉̃ 1.656 Å). The location of the H-site confirms the orientation of the O-H vector nearly perpendicular to (0001). The refinement converged with R1(F) = 0.0382, 846 unique reflections with FO > 4σ(FO) and 61 refined parameters, and not significant residuals in the final difference-Fourier map of the electron density (+0.77/-0.37 e-/Å3). 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, V0 = 747.0(3) Å3, KT0 = 44.5(24) GPa, and K′ = 8.0(9); (2) BM-EoS, V0 = 747.0(3) Å3, KT0 = 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.