Feldspar polymorphs: diversity, complexity, stability

Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 4. P. 16-66

https://doi.org/10.31857/S0869605520040036

Language: English

Abstract

The various aspects of polymorphism in the feldspar family of minerals are considered with special emphasis upon their structural diversity and complexity. The feldspar family is defined as consisting of valid minerals and unnamed or conditionally named mineral phases with the general formula Mn+[ ], where n is the average charge of the Mⁿ⁺ cation (n = 1–2; Mⁿ⁺ = Na⁺, K⁺, Rb⁺, (NH₄)⁺, Ca²⁺, Sr²⁺, Ba²⁺), k is the average charge of the T^k⁺ cation (k = 4 – n/4; T^k⁺ = Be²⁺, Zn²⁺, Al³⁺, B³⁺, Fe³⁺, Si⁴⁺, As⁵⁺, P⁵⁺). There are twenty-nine valid mineral species known to date that can be assigned to the feldspar family. Maskelynite is the natural X-ray amorphous feldspar polymorph (glass) with the plagioclase composition. All feldspar polymorphs can be classified into two groups: those containing T atoms in tetrahedral coordination only and those containing T atoms in non-tetrahedral coordination. There are four basic topologies of the feldspar-family tetrahedral networks: fsp (3D; feldspar sensu stricto; eleven mineral species), pcl (3D; paracelsian; seven mineral species), bct (3D, svyatoslavite; two mineral species), and dms (2D; dmisteinbergite; six mineral species). There are three minerals that contain T atoms in exclusively octahedral (sixfold) coordination and crystallize in the hollandite structure type. The high-pressure polymorphism for the structures with the fsp and pcl topologies is controlled by the distinction of these topologies as flexible and inflexible, respectively. The analysis of structural complexity by means of the Shannon information theory indicates the following general trends: (i) structural complexity decreases with the increasing temperature; (ii) kinetically stabilized metastable feldspar polymorphs are topologically simpler than the thermodynamically stable phases; (iii) the high-pressure behavior of feldspar-family structures does not show any obvious trends in the evolution of structural complexity. The feldspar polymorphism includes a number of structural phenomena: (i) coordination changes of intra- and extraframework cations; (ii) topological reconstructions, including changes in dimensionality; (iii) cation ordering, including Al/Si and M-cation ordering in solid solutions, resulting in the chemical stabilization of particular structure types and the formation of incommensurately modulated structures (in plagioclases); (iv) displacive distortions involving tilting of tetrahedra and rotations of crankshaft chains; (v) amorphization. The observed structural phenomena are controlled by temperature, pressure (including shock-induced transformations) and crystallization kinetics that may stabilize metastable phases with unique crystal structures.

Keywords: feldspar, crystal structure, polymorphism, structural complexity, phase transition, metastability, structural topology, high pressure, high temperature.