Lithium in the Subsoil of Ukraine. Part 9. Genesis of lithium-bearing objects: geocrystallochemistry of rare-metal pegmatites of the Ukrainian Shield

For the first time, an attempt has been made to characterize pegmatites geocrystallochemically, that is, from the perspective of mineral chemistry polymerization, to clarify the composition, structure, and properties of mineral-forming environments. In general, within this theory, the leading reaction in silicates is of the type 2[SiO4] → [Si2O7] + O that is accompanied by the release of free oxygen. According to M.V. Belov, whose opinion has been confirmed by our research, the [SiO4] tetrahedron in the pegmatite melt is only a static unit, and the dynamic unit is the uncharged SiO2 molecule, in the form of which, silicon passes ("jumps"), by figurative expression of M.V. Belov, from one "lattice" to another, from the liquid phase to the crystalline phase. This feature of the pegmatite melt, complicated by mineralizers (volatile components), is laid as foundation for the geocrystallochemistry of rare-metal, partially chamber pegmatites of the Ukrainian Shield. In the pegmatite melt, lithium ions act as a depolymerizer, i.e. a "destroyer" of framework structures. The spodumene formation scheme is assumed as follows: SiO2 molecules attach to the oxygen bonds of pre-existing [SiO4]4– tetrahedra, forming a chain group [Si2O6]4–∞. This acidic group then "attracts" an alkaline lithium ion, which pulls aluminum and forms the structure of spodumene, LiAl[Si2O6].