Zincohogbomite-8H from Samos (Greece): crystal structure, polysomatism, and polytypism in hogbomite related structures

The crystal structure of type locality zincohogbomite-8H, Zn3.54Mg0.10Ni0.2 0Fe2.333+.2+Al14.73Cr0.03Ti0.98Sn0.09O30+x(OH)(2-x), (a = 5.7097(8), b = 18 .333(3) Angstrom, Z = 1, space group P6(3)mc) was refined from single-cryst al X-ray data to R = 1.79% with 43 variables and 505 observations > 4 sigma (F). Zincohogbomite-8H with close-packed oxygen layers stacked along the c- axis in the sequence ABCBACBC has the same symmetry and polyhedral articula tion as hogbomite-8H refined by GATEHOUSE and GREY (1982). The structure of Samos zincohogbomite may be interpreted as composed through alternation al ong c of two gahnite (spinel) layers of approximately Zn2Al4O8 composition and two layers of a nolanite-like unit with (Fe0.693+Al0.31)(IV)(Fe0.123+.2 +Ti0.12Sn0.01Al0.75)(4)O-VI(7)(OH) composition. Out of the seven crystallog raphically independent cation sites three are tetrahedrally (T) and four ar e octahedrally (M) coordinated; taking into account their multiplicities, t he formula T6M16O30(OH)(2) can be derived. It is suggested that in hogbomite related structures with either substantia l Ti4+ or Sn4+ (hogbomites, zincohogbomites, pengzhizongites, and nigerites ) the observed polysome depends not only on the Ti4+, Sn4+ concentration bu t more probably on a complex interaction of cations of various valence and size responsible for the formation of nolanite-like subunits ('ch') of TM4O 7(OH) composition within a spinel-like host structure (the number of remain ing 'c' layers describes the number of spinel-like layers with TM2O4 compos ition). In addition, the number of nolanite-like layers may also determine the amount of OH in the formula. With increasing number of nolanite units t he following stacking types will be favored: (1) 16H: 2 x (cccccc + ch), (2 ) 12H: 2 x (cccc + ch), (3) 10H: 2 x (ccc + ch), (4) either 8H: 2 x (cc + c h) or 24R: 3 x (cccc + hcch), (5) either 6H: 2 x (c + ch) or 18R: 3 x (cc chhc).