Pressure-controlled polytypism in hydrous layered materials

Abstract

An isosymmetric displacive structural transformation in the hydrous layer silicate dickite [Al 2 Si 2 O 5 (OH) 4 , monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) Å, b = 96.77(1)∞], occurring under hydrostatic compression above 2.0 GPa, has been studied using single-crystal X-ray diffraction and diamond-anvil cell techniques. The structure of the high-pressure phase, determined in situ, is monoclinic with space group Cc with unit-cell parameters a = 5.082(3), b = 8.757 (6), c = 13.771(9) Å, and b = 89.60(2)∞ at 4.1 GPa. The positions of all hydrogen atoms at both ambient and high pressure have been determined by a combination of simulated annealing and energy minimization. The mechanism of the transformation, which may be general for other hydrous layered materials, involves a shift of the 1:1 layers with respect to each other by the vector [1/6, 1/6, 0] and is accompanied by the formation of new hydrogen bonds.

Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle

“…We conducted a reversed Monte Carlo (rMC) optimization (40) without bias by atomic potentials and Rietveld refinement to check possible constraints on the proton position. rMC has been found to be useful for assignment of proton positions from X-ray diffraction data even those obtained in high pressure diamond cells (41). The resulting proton position from rMC agrees with the result of the Rietveld refinement within 5% but we note that the profile-weighted Rwp for modelled pattern with and without protons differs by only 2.6% (Rwp = 5.72 versus 5.87%).…”

An Overview of the High‐Pressure Vibrational Spectra of Clays and Related Minerals

“…Recently, Johnston and his colleagues have reported the first documented case of a pressure-induced phase transformation in the 1 : 1 phyllosilicate dickite, which is a polytype of kaolinite. This phyllosilicate undergoes a reversible pressure-induced phase transition at 25 kbar (16,21), as evidenced by the changes observed in the n(OH) region (Figures 15 and 16). Interestingly, above 25 kbar, the individual 1 : 1 layers remain unaffected, whereas stacking of the individual layers and the interlayer topology are altered significantly.…”

“…Some representative highpressure infrared spectra of gibbsite are shown in Figure 11, while the pressure dependences of the OH stretching and deformation bands are plotted in Figure 12. These researchers attributed the band at 3619 cm 21 to the intralayer O-H stretching mode and the two bands at 3395 and 3375 cm 21 to the interlayer O-H stretching modes. The two latter bands, however, are more likely to be associated with H-bonded water molecules.…”

Kaolin polytypes revisited ab initio at 10 GPa

“…All current reliable observations about synthesis, diagenesis, and phase transformations of members of the kaolin system were rationalized with this graph. In particular, the layer-slipping mechanism of Dera et al (2003) for the reversible dickite HP-dickite transformation at ~2 GPa allowed rationalization of the observed existence of dickite at zero pressure and of the puzzling absence from the literature of reliable observations of any solid-to-solid transformation of kaolinite, dickite, or nacrite into each other under pressure.…”

“…In an extension of the work in MLP8, Mercier and Le Page (2009) (MLP9) assumed the generality of the layer-slipping mechanism for kaolin polytype transformations proposed by Dera et al (2003). Application to a systematic search for post-kaolinite phases through an examination of 19 possible structure models has led MLP9 to the prediction of two prime candidate model phases for post-kaolinite, one with space group symmetry P1 and the other with Cm at 12 GPa, both new structures (MLP9).…”