Abstract
Polytyism of micas is ubiquitous in natural rocks and can be used as an indicator of physical and chemical conditions in related diagenetic processes. To reveal the formation mechanisms of different polytypes, 1M- and 2M1-fluorophlogopite were synthesized using the melting method, and experiments with different Al concentration in the initial reactants were also performed. The structure refinement indicates that the space group of the 1M-fluorophlogopite is C2/m, and the lattice parameters are a = 5.2941(4) A, b = 9.1773(6) A, c = 10.1061(7) A, β = 100.141(7)°. The space group of the 2M1-fluorophlogopite is C2/c, and the lattice parameters are a = 5.3094(16) A, b = 9.1973(28) A, c = 20.0442(60) A, β = 95.141(7)°. The isomorphous substitution of Al3+ for Si4+ in tetrahedral sites of fluorophlogopite causes distortion in the layer structure, and results in a larger tetrahedral rotation angle α, longer tetrahedral bond distance, smaller tetrahedral flattening angle τ, larger bond length difference between inner and outer potassium–oxygen (∆〈K–O〉mean), and relative higher value of octahedral flattening angle ψ. Our experimental results suggest that the content of structural Al has no correlation with the polytype formation, whereas the Al concentration in reactants influences the formation of 1M- and 2M1-fluorophlogopite.
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