REFINEMENT OF THE NACRITE STRUCTURE

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) angstrom, beta = 113.58(3)-degrees, V = 657.9(3) angstrom3, and sp ace group Cc. The structure was solved by direct methods to determine phase angles, followed by electron density maps to locate all atoms. R efinement by least-squares ceased at R = 4.5%. Each 7 angstrom layer h as structural detail very similar to those of dickite and kaolinite, a lthough nacrite stacking is based on -al3 interlayer shifts along the 8.9 angstrom axis (with octahedral cations alternating between the I a nd II sites in successive layers), whereas dickite and kaolinite are b ased on shifts of -a/3 along the 5.1 angstrom axis (with octahedral ca tions in the same set of sites in each layer). The angle of tetrahedra l rotation is 7.8-degrees, and the octahedral counter-rotations are 7. 6-degrees and 8.1-degrees. The H+ protons were located on DED maps. Th e inner 0..H1 vector points exactly toward the vacant octahedron and i s depressed -18.6-degrees away from the level of the octahedral cation s. All three surface OH groups have 0...H vectors at 50-degrees to 66- degrees to (001), although OH2 may not participate in interlayer hydro gen bonding. All three interlayer OH-H-O contacts are bent to angles b etween 132-degrees and 141-degrees and form contacts between 2.94 and 3.12 angstrom. The interlayer separation of 2.915 angstrom is slightly larger than in dickite, interpreted as due to a less favorable meshin g of the oxygen and hydroxyl surfaces in nacrite-a direct consequence of layer shifts along the 8.9 angstrom axis.