Abstract
Many hydroxy-(hydrated) Cu 2+ oxysalt minerals, particularly Cu 2+ sulfates, are based on structural units that are variants of the [ M ϕ 2 ] N sheets of edge-sharing octahedra, where M is an octahedrally coordinated cation (including vacancies) and ϕ is an anion. Where ϕ = O 2− ,OH,Cl,H 2 O, the sheet can be considered as a simple sheet; where ϕ = (SO 4 ) 2− , the sheet may be considered as a decorated sheet. In such structures, Cu 2+ ϕ 6 octahedra show strong axial elongation; Cu 2+ –ϕ(equatorial) bonds have a bond-valence of ~0.42 vu , and Cu 2+ –ϕ(apical) bonds have a bond-valence of ~0.17 vu. Local bond-valence requirements dictate that an (SO 4 ) 2− group can only link to a [Cu 2+ ϕ 2 ] N sheet if three Cu 2+ –ϕ(apical) bonds are incident to the linking anion. This constraint provides considerable restrictions on possible structural arrangements with small unit-cells ( 2+ atoms/plane). These may be derived by two-coloring the 6 3 net. There are currently 20 minerals known that correspond to this structural theme, and it is almost certain that many others await discovery.
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