Crystal Structure Refinement and Mössbauer Spectroscopy of an Ordered, Triclinic Clinochlore

Joseph R. Smyth1, M. Darby Dyar2, Howard M. May3, Owen P. Bricker4 and James G. Acker4
1 Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0250
2 Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383
3 US Geological Survey, Water Resources Division, 3215 Marine Street, Boulder, Colorado 80303
4 US Geological Survey, Reston, Virginia 22092

Abstract: 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 (3σ). Unit cell parameters are a = 5.3262(6) Å; b = 9.226(1) Å; c = 14.334(3) Å; α = 90.56(2)°; β = 97.47(2)°; and γ = 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 > 3σ and 0.064 for all reflections. The chemical formula is (Mg0.966Fe0.034)M1(Mg0.962Fe0.038)M22(Si2.96Al1.04)O10 (OH)2(Mg0.996Fe0.004)M32(Al0.841FeIII0.102Cr0.004Ti0.004)M4(OH)6, which is consistent with electron microprobe (EMP), wet chemical analyses, Mössbauer 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 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 tetrahedral sites have nearly identical mean oxygen distances and volumes, and thus show no evidence of long-range cation ordering.

Key Words: Cation Ordering • Chlorite • Clinochlore • Crystal Structure • Mössbauer

Clays and Clay Minerals; August 1997 v. 45; no. 4; p. 544-550; DOI: 10.1346/CCMN.1997.0450406
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)