Hydrogen positions and thermal expansion in lizardite-1T from Elba; a low-temperature study using Rietveld refinement of neutron diffraction data

Abstract

The structure of lizardite-l T from Monte Fico, Elba, was refined in space group P31m using neutron diffraction data, measured at 8, 150, and 294 K, and full-profile Rietveld refinement techniques. The lattice parameters at 8 K [a = 5.3267(2), c = 7.2539(6) A], 150 K [a = 5.3260(2), c = 7.2574(6) A], and 294 K [a = 5.3332(2), c = 7.2718(6) A] show nonlinear expansion, with nearly all volume change above 150 K. H positions were precisely refined at 8 K. The inner H4 atom deviates from the idealized O,O,zpositions and is disordered over three symmetry-related positions 0.24 A away from the ternary axis. The outer H3 atom location is consistent with the previous single-crystal X-ray structure refinement. On the basis of the present thermal expansion data and previous compressibility measurements, the following equation of state for lizardite-I T is proposed: Vp,T = Vo[1+ 32.8 x 10-6(T - 294) - 15.5 x 1O-4(P- 0.001)]. Accordingly, the constant volume condition requires geothermal gradients on the order of 15 °C/km.