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Refinement of the Nacrite Structure

Published online by Cambridge University Press:  28 February 2024

Hong Zheng  and
Sturges W. Bailey
Hong Zheng
Affiliation:
Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706
Sturges W. Bailey
Affiliation:
Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706
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Abstract

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Nacrite crystals from a vug within a matrix of dickite at Red Mountain near Silverton, Colorado, have a = 8.906(2), b = 5.146(1), c = 15.664(3) Å, β = 113.58(3)°, V = 657.9(3) Å3, and space group Cc. The structure was solved by direct methods to determine phase angles, followed by electron density maps to locate all atoms. Refinement by least-squares ceased at R = 4.5%. Each 7 Å layer has structural detail very similar to those of dickite and kaolinite, although nacrite stacking is based on — a/3 interlayer shifts along the 8.9 Å axis (with octahedral cations alternating between the I and II sites in successive layers), whereas dickite and kaolinite are based on shifts of — a/3 along the 5.1 Å axis (with octahedral cations in the same set of sites in each layer). The angle of tetrahedral rotation is 7.8°, and the octahedral counterrotations are 7.6° and 8.1°. The H+ protons were located on DED maps. The inner 0..H1 vector points exactly toward the vacant octahedron and is depressed — 18.6° away from the level of the octahedral cations. All three surface OH groups have 0...H vectors at 50° to 66° to (001), although OH2 may not participate in interlayer hydrogen bonding. All three interlayer OH-H-O contacts are bent to angles between 132° and 141° and form contacts between 2.94 and 3.12 Å. The interlayer separation of 2.915 Å is slightly larger than in dickite, interpreted as due to a less favorable meshing of the oxygen and hydroxyl surfaces in nacrite—a direct consequence of layer shifts along the 8.9 Å axis.

Keywords

Bond lengthsCrystal structureHydrogen bondsNacriteRefinementTetrahedral rotation
Type
Research Article
Information
Clays and Clay Minerals , Volume 42 , Issue 1 , February 1994 , pp. 46 - 52
Copyright
Copyright © 1994, Clay Minerals Society

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