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 sp ace group P31m using neutron diffraction data, measured at 8, 150, and 294 K, and full-profile Rietveld refinement techniques. The lattice p arameters at 8 K [a = 5.3267(2), c = 7.2539(6) Angstrom], 150 K [a = 5 .3260(2), c = 7.2574(6) Angstrom], and 294 K [a = 5.3332(2), c = 7.271 8(6) Angstrom] 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 ove r three symmetry-related positions 0.24 Angstrom away from the ternary axis. The outer H3 atom location is consistent with the previous sing le-crystal X-ray structure refinement. On the basis of the present the rmal expansion data and previous compressibility measurements, the fol lowing equation of state for lizardite-1T is proposed: V-P,V-T = V-0[1 + 32.8 x 10(-6)(T - 294) - 15.5 x 10(-4)(P - 0.001)]. Accordingly, th e constant volume condition requires geothermal gradients on the order of 15 degrees C/km.