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Publication Detail
Cation substitution in synthetic meridianiite (MgSO ·11H O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates
Meridianiite, MgSO ·11H O, is the most highly hydrated phase in the binary MgSO -H O system. Lower hydrates in the MgSO -H O system have end-member analogues containing alternative divalent metal cations (Ni , Zn , Mn , Cu , Fe , and Co ) and exhibit extensive solid solution with MgSO and with one another, but no other undecahydrate is known. We have prepared aqueous MgSO solutions doped with these other cations in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. The solid products have been characterised by X-ray powder diffraction, and the onset of partial melting has been quantified using a thermal probe. We have established that of the seven end-member metal sulfates studied, only MgSO forms an undecahydrate; ZnSO forms an orthorhombic heptahydrate (synthetic goslarite), MnSO , FeSO , and CoSO form monoclinic heptahydrates (syn. mallardite, melanterite, bieberite, respectively), and CuSO crystallises as the well-known triclinic pentahydrate (syn. chalcanthite). NiSO forms a new hydrate which has been indexed with a triclinic unit cell of dimensions a = 6.1275(1) Å, b = 6.8628(1) Å, c = 12.6318(2) Å, α = 92.904(2)°, β = 97.678(2)°, and γ = 96.618(2)°. The unit-cell volume of this crystal, V = 521.74(1) Å , is consistent with it being an octahydrate, NiSO ·8H O. Further analysis of doped specimens has shown that synthetic meridianiite is able to accommodate significant quantities of foreign cations in its structure; of the order 50 mol. % Co or Mn , 20-30 mol. % Ni or Zn , but less than 10 mol. % of Cu or Fe . In three of the systems we examined, an 'intermediate' phase occurred that differed in hydration state both from the Mg-bearing meridianiite end-member and the pure dopant end-member hydrate. In the case of CuSO , we observed a melanterite-structured heptahydrate at Cu/(Cu + Mg) = 0.5, which we identify as synthetic alpersite [(Mg Cu )SO ·7H O)]. In the NiSO - and ZnSO -doped systems we characterised an entirely new hydrate which could also be identified to a lesser degree in the CuSO - and the FeSO -doped systems. The Ni-doped substance has been indexed with a monoclinic unit-cell of dimensions a = 6.7488(2) Å, b = 11.9613(4) Å, c = 14.6321(5) Å, and β = 95.047(3)°, systematic absences being indicative of space-group P2 /c with Z = 4. The unit-cell volume, V = 1,176. 59(5) Å , is consistent with it being an enneahydrate [i. e. (Mg Ni )SO ·9H O)]. Similarly, the new Zn-bearing enneahydrate has refined unit cell dimensions of a = 6.7555(3) Å, b = 11.9834(5) Å, c = 14.6666(8) Å, β = 95.020(4)°, V = 1,182.77(7) Å , and the new Fe-bearing enneahydrate has refined unit cell dimensions of a = 6.7726(3) Å, b = 12.0077(3) Å, c = 14.6920(5) Å, β = 95.037(3)°, and V = 1,190. 20(6) Å . The observation that synthetic meridianiite can form in the presence of, and accommodate significant quantities of other ions increases the likelihood that this mineral will occur naturally on Mars-and elsewhere in the outer solar system-in metalliferous brines. © 2012 Springer-Verlag.
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