The crystal structure of khinite and polytypism in khinite and parakhinite

摘要

The crystal structure of khinite, Pb~(2+)Cu_3~(2+)Te~(6+)O_6(OH)_2, orthorhombic, a = 5.7491(10), b = 10.0176(14), c = 24.022(3) A, V = 1383.6(4) A~3, space group Fdd2, Z = 8, D_(calc) = 6.29 g/cm~3, from the Empire mine, Tombstone, Arizona, USA, has been solved by direct methods and refined to R_I = 3.2 percent on the basis of 636 unique observed reflections. There is one distinct Te site occupied by Te and coordinated by six O atoms in an octahedral arrangement with a distance of 1.962 A, typical of Te~(6+). There are three octahedrally-coordinated Cu sites, each of which is occupied by Cu~(2+) with distances of 2.132, 2.151 and 2.308 A, respectively. Each Cu octahedron shows four short meridional bonds (approx 1.95 A) and two long apical bonds (2.46-2.99 A) characteristic of Jahn-Teller-distorted Cu~(2+) octahedra. There is one distinct Pb site occupied by Pb and coordinated by six O atoms and two (OH) groups with a distance of 2.690 A. Te PHI_6 and Cu PHI_6 octahedra share edges and corners to form an [M PHI_2] (where PHI = O, OH) layer of composition [TeCu_3 PHI_8]. These layers stack along the c axis at 6 A intervals with Pb atoms between the layers. Identical layers occur in the structure of parakhinite, Pb~(2+)Cu_3~(2+)Te~(6+)O_6(OH)_2, hexagonal, a = 5.765(2), c = 18.001(9) A, V = 518.0(4) A~3, space group P3_2, Z=3, D_(calc) = 6.30 g/cm~3. It is only the relative stacking of the TeCu_3PHI_8 layers in the c direction that distinguishes the two structures, and hence khinite and parakhinite are poly types.