The Crystal Structure of Sergeysmirnovite, MgZn 2 (PO 4 ) 2 ·4H 2 O, and Complexity of the Hopeite Group and Related Structures

The crystal structure of sergeysmirnovite, MgZn 2 (PO 4 ) 2 ·4H 2 O (orthorhombic, Pnma , a = 10.6286(4), b = 18.3700(6), c = 5.02060(15) Å, V = 980.26(6) Å 3 , Z = 4), a new member of the hopeite group of minerals, was determined and refined to R 1 = 0.030 using crystals from the Këster mineral dep...

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Bibliographic Details
Published in:Crystals
Main Authors: Sergey V. Krivovichev, Taras L. Panikorovskii, Victor N. Yakovenchuk
Format: Article in Journal/Newspaper
Language:English
Published: MDPI AG 2022
Subjects:
sergeysmirnovite
hopeite group
crystal structure
polymorphism
structural complexity
mineralogical crystallography
Crystallography
QD901-999
Sakha
Këster
Yakutia
Online Access:https://doi.org/10.3390/cryst12081120
https://doaj.org/article/8c357e3b31524496a9cd2d8d85d50569
Description
Summary:The crystal structure of sergeysmirnovite, MgZn 2 (PO 4 ) 2 ·4H 2 O (orthorhombic, Pnma , a = 10.6286(4), b = 18.3700(6), c = 5.02060(15) Å, V = 980.26(6) Å 3 , Z = 4), a new member of the hopeite group of minerals, was determined and refined to R 1 = 0.030 using crystals from the Këster mineral deposit in Sakha-Yakutia, Russia. Similar to other members of the hopeite group, the crystal structure of sergeysmirnovite is based upon [Zn(PO 4 )] – layers interlinked via interstitial [ M O 2 (H 2 O) 4 ] 2– octahedra, where M = Mg 2+ . The layers are parallel to the (010) plane. Within the layer, the ZnO 4 tetrahedra share common corners to form chains running along [001]. Sergeysmirnovite is a dimorph of reaphookhillite, a mineral from the Reaphook Hill zinc deposit in South Australia. The relations between sergeysmirnovite and reaphookhillite are the same as those between hopeite and parahopeite. Topological and structural complexity analysis using information theory shows that the hopeite (sergeysmirnovite) structure type is more complex, both structurally and topologically, than the parahopeite (reaphookhillite) structure type. Such complexity relations contradict the general observation that more complex polymorphs possess higher physical density and higher stability, since parahopeite is denser than hopeite. It could be hypothesized that hopeite is metastable under ambient conditions and separated from parahopeite by a structural and topological reconstruction that requires an essential energy barrier that is difficult to overcome.

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