Powder neutron diffraction study of 2M1 muscovite at room pressure and at 2 GPa

Abstract

A powder sample of 2M 1 muscovite, (K 0.90 Na 0.07 )(Al 1.63 Fe 0.23 Mg 0.16 Ti 0.03 )(Si 3.20 Al 0.80 )O 10 (OH) 2 , was studied by time-of-flight neutron diffraction at 1 bar and at 2 (± 0.1) GPa. Rietveld refinements of the crystal structure were performed, varying all atomic coordinates in the room-pressure case (profile R w = 0.013, Bragg intensities R w = 0.040) and only the z coordinates of basal O atoms plus xyz of O and H of the OH group at 2 GPa (profile R w = 0.035, Bragg intensities R w = 0.181). In both cases the lattice constants were refined: a = 5.2108(4), b = 9.0399(8), c = 20.021(2) A, β = 95.76(1)° (1 bar), a = 5.187(2), b = 8.995(4), c = 19.502(4) A, β = 95.78(2)° (2 GPa). The unit-cell compression shows deviations from linear elastic behaviour. Inter-layer K-O bonds are compressed along z more than the c lattice constant. The O-H group forms an angle of 85(1)° with c* and establishes three very loose contacts (H..O from 2.62 to 2.67 A) with neighbouring O atoms. This environment does not change appreciably at 2 GPa