Hostname: page-component-586b7cd67f-rdxmf Total loading time: 0 Render date: 2024-11-26T03:18:49.387Z Has data issue: false hasContentIssue false

Structures of Intergrown Triclinic and Monoclinic IIb Chlorites from Kenya

Published online by Cambridge University Press:  02 April 2024

Hong Zheng
Affiliation:
Department of Geology & Geophysics, University of Wisconsin, Madison, Wisconsin 53706
Sturges W. Bailey
Affiliation:
Department of Geology & Geophysics, University of Wisconsin, Madison, Wisconsin 53706
Rights & Permissions [Opens in a new window]

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Triclinic IIb-4 and monoclinic IIb-2 clinochlore polytypes from Kenya are intergrown laterally in (001) platelets and are separated optically in some places by sharp lines of demarcation and in other places by embayed boundaries. Electron microprobe analyses show no compositional differences between the two polytypes. Structural refinements of each show that the 2:1 layer is identical in the two structures with no tetrahedral or 2:1 octahedral cation ordering in either ideal or subgroup symmetries. The two interlayer sheets dilfer only in the distribution of A1 and Mg over interlayer sites M3 and M4, but not in the distribution of Fe3+, Fe2+, or vacancies. The monoclinic IIb-2 polytype is less ordered than the triclinic IIb-4 polytype. The mean M-O,OH bond length of 1.990 Å indicates significantly less of the smaller A1 present in the M4 site of IIb-2 than for the mean M-O,OH value of 1.957 Å for the M4 site of IIb-4. The textural relations and the structural refinements suggest that the monoclinic IIb-2 structure is inherently less stable than the triclinic form. This is in accord with the observed lesser abundance in nature of the monoclinic form.

Type
Research Article
Copyright
Copyright © 1989, The Clay Minerals Society

References

Bailey, S. W. and Gieseking, J. E., 1975 Chlorites Soil Components, Vol. 2 New York Springer-Verlag 191263.CrossRefGoogle Scholar
Bailey, S. W., 1986 Re-evaluation of ordering and local charge balance in la chlorite Can. Mineral. 24 649654.Google Scholar
Bayliss, P., 1975 Nomenclature of the trioctahedral chlorites Can. Mineral. 13 178180.Google Scholar
Busing, W.R. Martin, K. O. and Levy, H. A., 1962 ORFLS, a FORTRAN crystallographic least-squares refinement program U.S. Natl. Tech. Inf. Serv. .CrossRefGoogle Scholar
Cromer, D. T. and Mann, J. B., 1968 X-ray scattering factors computed from numerical Hartree-Fock wave functions Acta Crystallogr. A24 321324.CrossRefGoogle Scholar
Dollase, W. A., 1980 Optimum distance model of relaxation around substitutional defects Phys. Chem. Minerals 6 255304.CrossRefGoogle Scholar
Guggenheim, S., Schulze, W. A., Harris, G. A. and Lin, J.-C., 1983 Noncentric layer silicates: An optical second harmonic generation, chemical, and X-ray study Clays & Clay Minerals 31 251260.CrossRefGoogle Scholar
Joswig, Werner, 1989 Neutron Diffraction Study of a One-Layer Monoclinic Chlorite Clays and Clay Minerals 37 6 511514.CrossRefGoogle Scholar
Joswig, W., Fuess, H., Rothbauer, R., Takéuchi, Y. and Mason, S. A., 1980 A neutron diffraction study of a onelayer triclinic chlorite (penninite) Amer. Mineral. 65 349352.Google Scholar
Neuhaus, A., 1960 Über die lonenfarben der Kristalle und Minerale am Beispiel der Chromfärbungen Z. Kristallogr. 113 195233.CrossRefGoogle Scholar
North, A. C. T. Phillips, D. C. and Mathews, F. S., 1968 A semi-empirical method of absorption correction Acta Crystallogr. A24 351359.CrossRefGoogle Scholar
Parkinson, J. (1947) Outlines of the geology of the Mtito Andei-Tsavoarea, Kenya Colony: Geol. Survey Kenya Rep. 13, 40 pp.Google Scholar
Phillips, T. L., Loveless, J. K. and Bailey, S. W., 1980 Cr3+ coordination in chlorites: A structural study of ten chromian chlorites Amer. Mineral. 65 112122.Google Scholar
Rule, A. C. and Bailey, S. W., 1987 Refinement of the crystal structure of a monoclinic ferroan clinochlore Clays & Clay Minerals 35 129138.CrossRefGoogle Scholar
Shannon, R. D., 1976 Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides Acta Crystallogr. A32 751767.CrossRefGoogle Scholar
Steinfink, H., 1958 The crystal structure of chlorite. I. A monoclinic polymorph Acta Crystallogr. 11 191195.CrossRefGoogle Scholar
Steinfink, H., 1958 The crystal structure of chlorite. II. A triclinic polymorph Acta Crystallogr. 11 195198.CrossRefGoogle Scholar
Steinfink, H., 1961 Accuracy in structure analysis of layer silicates: Some further comments on the structure of prochlorite Acta Crystallogr. 14 198199.CrossRefGoogle Scholar
Weiss, Z., Rieder, M., Chmielova, M. and Krajicek, J., 1985 Geometry of the octahedral coordination in micas: A review of refined structures Amer. Mineral. 70 747757.Google Scholar