Cation substitution in synthetic meridianiite (MgSO4·11H2O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates

Abstract

Meridianiite, MgSO 4 Á11H 2 O, is the most highly hydrated phase in the binary MgSO 4 -H 2 O system. Lower hydrates in the MgSO 4 -H 2 O system have end-member analogues containing alternative divalent metal cations (Ni 2? , Zn 2? , Mn 2? , Cu 2? , Fe 2? , and Co 2? ) and exhibit extensive solid solution with MgSO 4 and with one another, but no other undecahydrate is known. We have prepared aqueous MgSO 4 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 4 forms an undecahydrate; ZnSO 4 forms an orthorhombic heptahydrate (synthetic goslarite), MnSO 4 , FeSO 4 , and CoSO 4 form monoclinic heptahydrates (syn. mallardite, melanterite, bieberite, respectively), and CuSO 4 crystallises as the well-known triclinic pentahydrate (syn. chalcanthite). NiSO 4 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 , a = 92.904(2)°, b = 97.678(2)°, and c = 96.618(2)°. The unit-cell volume of this crystal, V = 521.74(1) Å 3 , is consistent with it being an octahydrate, NiSO 4 Á8H 2 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 2? or Mn 2? , 20-30 mol. % Ni 2? or Zn 2? , but less than 10 mol. % of Cu 2? or Fe 2? . 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 4 , we observed a melanterite-structured heptahydrate at Cu/ (Cu ? Mg) = 0.5, which we identify as synthetic alpersite [(Mg 0.5 Cu 0.5 )SO 4 Á7H 2 O)]. In the NiSO 4 -and ZnSO 4 -doped systems we characterised an entirely new hydrate which could also be identified to a lesser degree in the CuSO 4 -and the FeSO 4 -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 b = 95.047(3)°, systematic absences being indicative of space-group P2 1 /c with Z = 4. The unit-cell volume, V = 1,176.59(5) Å 3 , is consistent with it being an enneahydrate [i.e. (Mg 0.5 Ni 0.5 )SO 4 Á9H 2 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 , b = 95.020(4)°, V = 1,182.77(7) Å 3 , and the new Febearing enneahydrate has refined unit cell dimensions of a = 6.7726(3) Å , b = 12.0077(3) Å , c = 14.6920(5) Å , b = 95.037(3)°, and V = 1,190.20(6) Å 3 . The observation that synthetic meridianiite can form in the pres...

Cation substitution in synthetic meridianiite (MgSO4·11H2O) II: variation in unit-cell parameters determined from X-ray powder diffraction data

“…%, up to 30 mol. % Ni, at which concentration DV/V & -0.5 %, comparable to the shrinkage obtained on cooling Mg11 from 250 to 150 K (Fortes et al 2008). The effects of Co 2?…”

“…%, up to 60 mol. % Mn, at which concentration we observe DV/V & ?1.3 %, which is equivalent (given the measured elastic properties of Mg11 measured under compression: K 0 & 195 kbar at 240 K, Fortes et al 2011) to a negative pressure of *3.25 kbar, a substantial stress for such a relatively soft material. In contrast, incorporation of Ni 2?…”

“…8). It is noteworthy that this axis also corresponds to the direction of maximum thermal expansion (Fortes et al 2008), suggesting that it is a particularly easily strained direction in the structure. The direction of greatest expansion (or least contraction) is always e 3 , and the intermediate-value axis is always e 1 ; these fall in the xz plane of the orthogonal reference axes, typically with e 3 closer to the z axis (= c crystallographic axis) than the x axis.…”

MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data

“…As described by Fortes et al (2012a), we used observations of thermal plateaux in the warming curves of quenched samples to determine the eutectic temperatures in both the protonated and deuterated MgSO 4 -water systems (269.05 and 272.15 K, respectively). New measurements made with nitrogen-quenched MgCrO 4 solutions show that the eutectic temperature in the binary MgCrO 4 -H 2 O system is 262.05 K. Hence, our crystals were grown from a slightly undercooled metastable solution.…”

X-ray powder diffraction analysis of two new magnesium selenate hydrates, MgSeO4·9H2O and MgSeO4·11H2O

“…In a series of papers, my colleagues and I reported the results of work to form structural analogues of MgSO 4 •11H 2 O in which the cation was replaced by Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ (Fortes et al, 2012a(Fortes et al, , 2012b and the oxyanion was replaced by CrO 4 2− : Fortes et al, 2013.…”