Whilst doing some tidying on the website I came across the mineral postite recently listed. I needed to check out the composition the relation to other minerals. It was not long before I was looking into the range of minerals containing the decavanadate ion, generically expressed V10O28.
The wonderfully symmetrical structure of the ion comprises two central VO6 octahedrons each with 4 tetragonally arranged VO5 groups.
3D Model of Decavanadate Ion - (Wikpedia)
The oxidation state for each vandium atom within a decavanadate polyanion is +5, or V(v) and the overall charge balance of the V10O28 ion is -6.
However within the decavanadate chemistry the vanadium can hold a reduced charge and exist as +4, in a ratio of 1:9, or 2:8, or indeed the ion can be protonated, by 1,2 or 3 hydrogens (protons), the one proton version being currently only known as synthetic.
This chemistry and the unique structural unit of the decavanadate ion gives rise to some interesting mineralogy where small groups of cations with water form repeating symmetrical structures through hydrogen bonds.
Most of these minerals are pretty new to mineral science by comparison, however synthetic compounds have been understood for a while.
The different mineral types are apparently found post-mining through the decomposition/oxidation of vanadium ore minerals (eg corvusite and montrosite), often leached out of near-surface vanadium oxide residue as efflorescences on mine walls. Generally these minerals are water soluble, with an yellow/orange colour (down to the decavanadate ion). Bluestreakite and nashite colour are exceptions being dark blue-green - due to the V4+ component?
Here below is a list of the minerals currently classified, it is interesting that that different data sources have different presentations of the chemistry within the list and also the membership of the Pascoite Group.
On e-Rocks Database?
You can see example photos below - click to expand.