Named in honor of the Austrian mineralogist Josef Zemann (born 1923 and died just three weaks ago on the 16th of October 2022 at the age of 99), who had worked extensively on tellurium minerals. An obituary on Zemann in German can be found here.
Type Locality: The Mina La Bomballa near Moctezuma, Sonora, Mexico
Formula: Mg0.5ZnFe3+[TeO3]3 · 4.5 H2O
Space group: P63/m
Crystal system: hexagonal
Crystal class: 6/m
Lattice parameters: a = b = 9.41 Å, c = 7.64 Å, α = β = 90°, γ= 120°
Picture: Christian Rewitzer – CC BY-SA 3.0
Crystal structure (click on the pictures to download the VESTA file):
 J.A. Mandarino, E. Matzat, S. J. Wiliams, Zemannite, a zinc tellurite from Moctezuma, Sonora, Mexico.Canadian Mineralogist1976, 14, 387–390.
 R. Miletich, Crystal chemistry of the microporous tellurite minerals zemannite and kinichilite, Mg0.5 [(Me2+Fe3+) (TeO3)3] · 4.5 H2O, (Me2+ = Zn, Mn). European Journal of Mineralogy1995, 7, 509–523.
In a very interesting article by Giese and Seppelt from 1994  the question of the preferred coordination geometry for the coordination number 7 is explored. Until then, it has been shown that the pentagonal bipyramid is preferred for main group elements (for instance in the compound IF7), although this geometry results in an overall slightly higher ligand repulsion than the alternative arrangements according to a capped octahedron or capped tigonal prism, respectively.
In this article they raised the question if this is also valid for transition elements, and the ideal candidate to answer this question would be the homoleptic compound ReF7. Unfortunately, for various reasons it is very difficult to determine the crystal structure of ReF7.
Alternatively, they synthesized several ionic compounds comprising of anions with transition elements with the coordination number 7 and varied the nature of the cation to exclude lattice energy effects that might influence the arrangement of the ligands.
The results were as follows:
The anions in Cs+MoF7– (see Fig. 1), Cs+WF7–, NO2+MoF7– ∙ CH3CN, and C11H24N+MoF7– (C11H24N+ = 1,1,3,3,5,5-hexamethylpiperidinium) (all belong to the cubic crystal system) as well as (H3C)4N+MoF7– (tetragonal, space group P4/nmm, ) form a capped octahedron coordination polyhedron. The latter result is of special interest as the 7 fluorine atoms around Tellurium in (H3C)4N+TeF7– (again space group P4/nmm) form a pentagonal bipyramid instead.
Fig. 1: The crystal structure of Cs+MoF7– (space group Pa-3, ICSD deposition number = 78390); Cs = purple, Mo = gray, F = green. (Image made with VESTA ).
According to the authors, it can therefore be concluded that neither the lattice type nor the crystal packing have an influence on the different structures of the anions.
Now, what will happen, if one of the 7 fluorine ligands is exchanged with a larger atom? As expected, a pentagonal bipyramid is then realised, as in Cs+ReOF6– (space group P21/a), where the larger ligand occupies one of the two axial positions (see Fig. 2).
Fig. 2: The crystal structure of Cs+ReOF6– (space group P21/a, ICSD deposition number = 78392); Cs = purple, Re = gray, O = red, F = green. (Image made with VESTA ).
I have to admit that I was too lazy to check if the structure of ReF7 is known by now. In fact – thanks to Robert McMeeking (from The Chemical Database Service/CrystalWorks, @cds_daresbury) for the hint – the crystal structure of ReF7 was published in Science only a single(!) day after the publication of the Angewandte paper by Giese & Seppelt: ReF7 builds a (slightly distorted) pentagonal bipyramid , see Fig. 3!
Fig. 3: The crystal structure of ReF7 (space group C-1, ICSD deposition number = 78311); Re = gray, F = green. (Image made with VESTA ).
 W. Krause, H.-J. Bernhardt, R.S.W. Braithwaite, U. Kolitsch, R. Pritchard Kapellasite, Cu3Zn(OH)6Cl2, a new mineral from Lavrion, Greece, and its crystal structure Mineralogical Magazine, 2006, 70, 329-340 DOI: 10.1180/0026461067030336
Can be formed from liquid water at 11 kbar by lowering the temperature to approx. -3 °C
Density: 1.31 g/cm3
Ice VI is a proton-disordered phase
it is composed of two independent interpenetrating networks of H-bonded water molecules (shown above in blue and red, respectively)
the main structural motif is a tricyclic, cage-like water hexamer, similar as in liquid water
This motif is also found for the silicon atoms in the zeolite edingtonite, see here for comparison.
The respective topology of the underlying net is edi, a binodal (4,4)-c net with transitivity pqrs = 2343
Space group: P42/nmc (No. 137)
Crystal system: Tetragonal
a = b = 6.116(1) Å, c = 5.689(1) Å
α = β = γ = 90°
 W. F. Kuhs, J. L. Finney, C. Vettier and D. V. Bliss, Structure and hydrogen ordering in ices VI, VII and VIII by neutron powder diffraction. J. Chem. Phys.1984, 81, 3612-3623. DOI: 10.1063/1.448109
For both metal cation positions there is a complete disorder between Ni and Fe.
There are two distinct coordination environments; octahedrally coordinated metals at the center and all edge centers and tetrahedrally coordinated metals for the others.
Eight tetrahedra each form edge-connected Fe/Ni8(µ-S)6S8 motifs, that means cubes of metal ions with six face-capping and eight terminal S atom. If we take now these cubes and octahedra as building blocks they form a NaCl-like structure.
Fe/NiS4 tetrahedra (blue)
Fe/NiS6 octahedra (orange)
For a 3D interactive version on sketchfab, see here:
 Tenailleau, C., Etschmann, B., Ibberson, R. M. & Pring, A.
A neutron powder diffraction study of Fe and Ni distributions in synthetic pentlandite and violarite using 60Ni isotope. Am. Mineral. 91, 1442–1447 (2006)
Named in 1971 by George T. Faust and Waldemar T. Schaller in honor of Arthur Moritz Schoenflies ( 17 April 1853 – 27 May 1928) Professor of Mathematics, University of Frankfurt. Schoenflies’ researches in group theory and topology resulted in his proof of the 230 space groups.
Space group: Pn-3 (No. 201)
Crystal system: cubic
Crystal class: m-3
Lattice parameters: a = b = c = 7.7449(4) Å, α = β = γ = 90°
Crystal structure (click on the picture to download the VESTA file):
Description of Schoenfliesite, MgSn(OH)6, and Roxbyite, Cu1.72S, from a 1375 BC Shipwreck, and Rietveld Neutron-diffraction Refinement of Synthetic Schoenfliesite, Wickmanite, MnSn(OH)6, and Burtite, CaSn(OH)6 L.C. Basciano, R.C. Peterson, P.L. Roeder The Canadian Mineralogist1998,36, 1203-1210.