MINERALS WITH METAL-ORGANIC FRAMEWORK STRUCTURES (Invited Presentation)

Coevolution of the geosphere and biosphere in the history of the Earth results in the appearance of a number of minerals that combine organic and inorganic building blocks, thus representing links between natural organic and inorganic structural chemistries. Such materials are of current interest in material science and attract considerable attention due to their interesting physical and chemical processes. In particular, metal-organic frameworks (MOFs) are an increasingly important family of advanced materials based on open, nanometer-scale metal-organic architectures, whose design and synthesis are based on the directed assembly of carefully designed subunits. It has always been thought that MOFs are typically artificial materials, but recently we have been able to demonstrate that coordination-driven self-assembly used to create advanced materials in the laboratory can also play a role in geological processes [I. Huskić et al. Science Advances, in press]. Stepanovite, (NaMgFe(C₂O₄)₃·8-9H₂O), and zhemchuzhnikovite, (NaMg(Fe_0.4Al_0.6)(C₂O₄)₃·8-9H₂O), are two minerals first discovered near the estuary of Lena river (Sakha-Yakutia, Siberia, Russia) in 1964. Both minerals are water-soluble and were found in drillcores at depths up to 230 m below the day surface, in the permafrost zone, within lignite saturated by natural acetic acid. Our X-ray diffraction study demonstrated that stepanovite and zhemchuzhnikovite exhibit structures found in well-established magnetic and proton-conducting metal oxalate MOFs, consisting of open frameworks with monovalent (Na⁺) and trivalent cations (Al³⁺ or Fe³⁺) as nodes, and are templated by divalent cations retained in framework cavities. Structures of stepanovite and zhemchuzhnikovite, exhibiting almost nanometer-wide and guest-filled apertures and channels, respectively, change the perspective of MOFs as exclusively artificial materials, and represent so far unique examples of open framework architectures in organic minerals.