Meridianiite(?): structure determination

Reference:
• Fortes, A.D., Wood, I.G., Knight, K.S. (2008): The crystal structure and thermal expansion tensor of MgSO4–11D2O (meridianiite) determined by neutron powder diffraction. Physics and Chemistry of Minerals, 35, 207-221.

Abstract:
We have collected high-resolution neutron powder diffraction patterns from MgSO4·11D2O over the temperature range 4.2–250 K. The crystal is triclinic, space-group (Z = 2) with a = 6.72746(6) Å, b = 6.78141(6) Å, c = 17.31803(13) Å, α = 88.2062(6)°, β = 89.4473(8)°, γ = 62.6075(5)°, and V = 701.140(6) Å3 at 4.2 K, and a = 6.75081(3) Å, b = 6.81463(3) Å, c = 17.29241(6) Å, α = 88.1183(3)°, β = 89.4808(3)°, γ = 62.6891(3)°, and V = 706.450(3) Å3 at 250 K. Structures were refined to wRp = 3.99 and 2.84% at 4.2 and 250 K, respectively. The temperature dependence of the lattice parameters over the intervening range have been fitted with a modified Einstein oscillator model which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion, αV, is considerably smaller than ice Ih at all temperatures, and smaller even than MgSO4·7D2O (although ∂αV/∂T is very similar for both sulfates); MgSO4·11D2O exhibits negative αV below 55 K (compared to 70 K in D2O ice Ih and 20 K in MgSO4·7D2O) The relationship between the magnitude and orientation of the principal axes of the expansion tensor and the main structural elements are discussed.
Electronic supplementary material The online version of this article (doi:10.1007/s00269-008-0214-x) contains supplementary material, which is available to authorized users.



Edited 1 time(s). Last edit at 04/07/2008 12:26PM by Marco E. Ciriotti.

Reference added.

It's actually the deuterium analogue of meridianiite. (The authors shouldn't have used the mineral name in the paper title in the way they had used it.)

Yes, I agree.

I think it actually is the synthetic analogue of meriidianite. Wether the hydrogen has a natural isotopic composition or is enriched in one isotope shouldn't change the mineral name... But it still is a synthetic compound and not a mineral.

Yes, it is the synthetic (deuterium) phase as the previous structural determination:
Fortes A.D., Wood I.G., Vocadlo L., Brand H.E.A., Knight K.S. (2007)
* J.Applied Crystallogr., 40, 761-770
[synthetic]

The deuteration is largely irrelevant, except in the sense that it allowed us to make neutron diffraction measurements without an enormous background. Natural water contains a small amount of deuterium anyway, so you might think of this as a very extreme 'mineral' end member (which is no reason, unless it is not isostructural, to adopt a new name). You'll find plenty of examples in the literature of deuterated substances referred to by their mineralogical name; for example, brushite, gypsum, chalcanthite... to name a few.
Besides, the mineralogical journal and the reviewers had ample opportunity to request that the name be removed from the title.

"... (which is no reason, unless it is not isostructural, ..."

For anyone interested: there are cases where deuteration changes the hydrogen-bond scheme (including associated phase transitions) and - probably (not aware of an example right now) - the space-group symmetry.