Chalcophanite, ZnMn 3 O 7 .3H 2 O; new crystal-structure determinations (1988)

Single-crystal X-ray structure refinements of the layer zinc-manganese oxide mineral chalcophanite, using crystals from Bisbee, Arizona, and Sterling Hill, New Jersey, yielded unit-cell parameters a = 7.533(3) A, c = 20.794(7) A (Bisbee) and a = 7.541(3) A, c = 20.824(8) A (New Jersey) and converged to residuals of R = 0.043 and R = 0.041, respectively, in space group RJ. The structures are similar to one previously reported as triclinic but with significant differences in atom positions and bond lengths. Modeling of the orientation of the water molecule using structure-energy calculations has delineated the hydrogen-bonding scheme in chalcophanite. Probable hydrogen bonds occur between water molecules and between the watcr oxygen atom and two oxygen atoms in the octahedral sheets, contributing to the binding force between layers. measuring approximately 0.60 mm x 0.30 mm x 0.16 mm (Bisbee) and 0.30 mm x 0.30 mm x 0.01 mm (Sterling Hill). Precession and Laue photographs show an R-centered lattice in Laue Class 3 for both crystals, indicating space group R3 or Rl The threefold axis is normal to the platelets. This does not agree with the triclinic (pI) symmetry found by Wadsley (1955), but is consistent with Moore's (1875) initial description of chalcophanite as rhombohedral, based on crystal morphology. It is puzzling that Wadsley reported not observing any symmetry in a Laue photograph taken normal to the platelets. Wadsley's triclinic cell (a = 7.54 A, b = 7.54 A, c = 8.22 A, a = 90°, (3 = 117.2°, 'Y= 120°) is related to our trigonal cell (Table I) by I I, 0,010, -1,0/2, I, 3/. A pronounced R-centered subcell (a = 2.88 A, c = 20.79 A) is apparent in the precession photographs. The weaker reflections defining the true cell result from the ordering of vacancies in the octahedral layer (see below). Electron-microprobe analyses of chalcophanite from Sterling Hill, New Jersey, reported by Ostwald (1985), yield a chemical formula that is very close to ZnMn30,' 3HzO, which is also consistent with microprobe analyses of the specimen from Bisbee, Arizona. In both cases, water was determined by difference. Intensity data were collected with a Krisel-automated Picker four-circle diffractometer by the step-scan mode, using Zr-filtered MoKa radiation. The experimental parameters are summarized in Table I. The reflections were examined graphically and background points adjusted as needed. Reflections with intensities greater than 4cr and 3cr, for the Bisbee and New Jersey data sets, respectively, were tagged as observed. Many of the reflections are broad or partially split, especially for the Bisbee crystal, indicating slight misalignment between platelets. The data were corrected for absorption using the Gaussian integration method in the XTALcrystallographic computing package (Stewart and Hall, 1985) for!J. = 88 cm-1 (International Tables for X-ray Crystallography, 1974). Refinement of setting angles by the method of Hamilton (International Tables for X-ray Crystallography. 1974) for 20 reflections (45° < 21J< 60°) yielded the unitcell parameters listed in Table I. Single crystals of chalcophanite were selected from samples The structure refinements were performed using the XTAL from Sterling Hill, New Jersey (NMNH CI814), the same samcomputing package (Stewart and Hall, 1985). Statistical distripie that supplied the crystal used by Wadsley (1955), and from butions of the normalized structure factors strongly indicate a Bisbee, Arizona (NMNH RI2334). The crystals are platelets centrosymmetric structure (R3). Scattering factors (Mn4+, Zn2+, 0003-004X/88/l112-1401$02.00 140