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

The structure of lizardite-1T 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) Å], 150 K [a = 5.3260(2), c = 7.2574(6) Å], and 294 K [a = 5.3332(2), c = 7.2718(6) Å] 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 0,0,z positions and is disordered over three symmetry-related positions 0.24 Å 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-1T is proposed: V_P.T = V_o[1 + 32.8 × 10^-6(T - 294) - 15.5 × 10^-4(P - 0.001)]. Accordingly, the constant volume condition requires geothermal gradients on the order of 15 °C/km.