抄録
The crystal structures of clinoclase from Ikuchi-shima, Hiroshima, Japan, Cu3AsO4(OH)3 [monoclinic, space group P21/c, with a = 7.2798(3), b = 6.4694(2), c = 12.4172(5) Å; β = 99.5060(10)° V = 576.77(4) Å3, R1 = 0.0222 for 1734 independent reflections] and callaghanite from Sierra Magnesite Mine, Nevada, USA, Cu2Mg2(CO3)(OH)6·2H2O [monoclinic, space group C2/c, with a = 10.0168(9), b = 11.7671(10), c = 8.2383(6) Å; β = 107.416(6)° V = 926.53(14) Å3, R1 = 0.0368 for 1057 independent reflections] were refined using single-crystal X-ray diffraction. In clinoclase, Cu1Cu2(OH)4O4 and Cu32(OH)4O4 dimers share corners to form sheet structures with As tetrahedra. These sheets are held together by long Cu-O bonds [2.524(3) and 2.877(2) Å] and O6-H61…O3 bonds [O6-O3 = 2.852(3) Å]. In callaghanite, tunnel structures are formed within a three-dimensional framework constructed by sharing edges among polyhedra in the zigzag chains of Mg octahedra and the Cu2(OH)6·2H2O units. The hydrogen atoms H31, H61, and H62 are located in the tunnels and the O2 corner of the carbonate ion protrudes into the tunnel. The Cu square bases of tetragonal pyramid in clinoclase are highly distorted with extremely large variations in bond distances. The differences between the shortest and longest bond distances, Δdistance, in square base are 0.179 Å for Cu1 site, 0.106 Å for Cu2 site, and 0.126 Å for Cu3 site. In callaghanite, the variety of edge sharing manners and chemical species of ligand led to large variations in the Mg-O distances in the octahedron. Δdistance between the shortest Mg-O distance [2.059(2) Å] and the longest distance [2.123(2) Å] is 0.064 Å, which is larger than that in Cu-O distances in the square base of the Cu tetragonal pyramid (0.037 Å).
The experimentally determined unit cell parameters and atomic coordinates of clinoclase and callaghanite were optimized using first-principles calculations. Three-dimensional representations of covalent and anti-bonding interactions around Cu atoms in both minerals were constructed. The values of bond overlap population in the square bases of clinoclase vary from 0.44 to 0.16, while those in callaghanite show lesser variation, ranging from 0.37 to 0.29. The absolute values of partial orbital-orbital anti-bonding correlations between As and Cu in clinoclase (−0.22 to −0.11) are larger than those between Cu and Cu or Mg in callaghanite (−0.14 to −0.11), indicating the significant influence of As on both antibonding and covalent bonding behaviors in clinoclase. In conclusion, the large dispersion in bond distances within square planar covalent bonds in clinoclase can be attributed to the effects of adjacent highly covalent cations, mediated via oxide ions.
