MUSHERALS: HYPER-HYDROUS MINERALS

"Musheral" is a tongue in cheek label for minerals that have as many water molecules in their formulas as all other atoms combined. Carnallite KMgCl₃•6(H₂O), Mirabilite Na₂SO₄•10(H₂O), Melanterite FeSO₄•7(H₂O)and Epsomite MgSO₄•7(H₂O) are possibly the most familiar examples. Hyper-hydrous minerals, predictably enough, occur in extremely wet settings such as mines and caves, fumaroles, and frozen hypersaline brines. They are of interest also as possible ingredients in the Martian subsurface, and indeed one, Meridianiite MgSO₄•11H₂O was actually named for Sinus Meridianii on Mars. Although water of hydration is commonly considered to be nonessential, in every case, the water in hyper-hydrous minerals is integral to the structure. That is, removal of the water results in a complete transformation of the structure, and generally, collapse to a denser structure. Hyper-hydrous minerals show an interesting and surprising variety of structures. In some cases, water molecules cluster around a central cation, with the oxygen anions facing inward and the hydrogen cations outward. The overall cluster has the charge of the central cation and a positively charged surface, and effectively behaves as a complex cation, with anions joining neighboring clusters. In many hyper-hydrous sulfates, the central cations are octahedrally coordinated, but two sites are occupied by oxygen atoms. Chains of complex cations and sulfates make up the structure and are cross-linked by hydrogen bonds, additional water molecules, or other ions. In some cases, water-cation clusters share oxygen ions with carbonate ions, and the resulting module is neutral. The modules are linked by hydrogen bonding. Hydrohalite, NaCl•2(H₂O) has a dioctahedral sheet structure in which the sodium ions are enclosed by close packed sheets of water molecules and chlorine atoms. In each sheet, two zigzag rows of water molecules alternate with a row of chlorine ions, and hydrogen bonds link the sheets. Apart from methane hydrates and related materials, none of the hyper-hydrous minerals are clathrates.