CRYSTAL-STRUCTURE REFINEMENT AND MOSSBAUER-SPECTROSCOPY OF AN ORDERED, TRICLINIC CLINOCHLORE

The crystal structure of a natural, ordered IIb-4 triclinic clinochlor e has been refined in space group C (1) over bar from 4282 unique X-ra y intensity measurements of which 3833 are greater than 3 times the st atistical counting error (3 sigma). Unit cell parameters are a = 5.326 2(6) Angstrom; b = 9.226(1) Angstrom; c = 14.334(3) Angstrom; alpha = 90.56(2)degrees; beta = 97.47(2)degrees; and gamma = 89.979(9)degrees, which represents the greatest deviation from monoclinic symmetry yet recorded for a triclinic chlorite. The final weighted R is 0.059 for r eflections with I > 3 sigma and 0.064 for all reflections. The chemica l formula is .034)(M1)(Mg0.962Fe0.038)(M2)(2)(Si2.96Al1.04)O-10 (Al0.8 41Fe0.102Cr0.004Ti0.004)-Cr-III)(M4)(OH)(6), which is consistent with electron microprobe (EMP), wet chemical analyses, Mossbauer spectrosco py and X-ray structure refinement. The high degree of ordering of the divalent versus trivalent octahedral cations in the interlayer is note worthy, with Fe-III and Al in M4 and virtually no Fe in M3. In the 2:1 layer, M1 and M2 each contain similar amounts of Fe. The 2 tetrahedra l sites have nearly identical mean oxygen distances and volumes, and t hus show no evidence of long-range cation ordering.