An isosymmetric displacive structural transformation in the hydrous layer silicate dickite [Al2Si2O5(OH)4, monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) Å, β = 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 β = 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.

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