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The crystal structure of a natural, ordered IIb-4 triclinic clinochlore has been refined in space group C1 from 4282 unique X-ray intensity measurements of which 3833 are greater than 3 times the statistical counting error (3??). Unit cell parameters are a = 5.3262(6) A??; b = 9.226(1) A??; c = 14.334(3) A??; ?? = 90.56(2)??; ??= 97;47(2)??; and ?? = 89.979(9)??, which represents the greatest deviation from monoclinic symmetry yet recorded for a triclinic chlorite. The final weighted R is 0.059 for reflections with I> 3??- and 0.064 for all reflections. The chemical formula is (Mg0.966)Fe0.034)MI(Mg0.962Fe 0.038)M2:(Si2.96Al1.04)O 10 (OH)2(Mg0.996Fe0.004m32(Al0.841 FeIII0.102Cr0.004 Ti0.004)M4 (OH)6, which is consistent with electron microprobe (EMP), wet chemical analyses, Mo??ssbauer spectroscopy and X-ray structure refinement. The high degree of ordering of the divalent versus trivalent octahedral cations in the interlayer is noteworthy, with FeIII and A1 in M4 and virtually no Fe in M3. In the 2:1 layer, M1 and M2 each contain similar amounts of Fe. The 2 tetrahedral sites have nearly identical mean oxygen distances and volumes, and thus show no evidence of long-range cation ordering.
Additional Publication Details
Crystal structure refinement and Mo??ssbauer spectroscopy of an ordered, triclinic clinochlore