The crystal structure of synthetic mica, KMg_2.75Si_3.5Al_0.5O₁₀F₂

Abstract

Crystals of KMg_2.75Si_3.5Al_0.5O₁₀F₂ were synthesized from a mixture of KF, MgF₂, MgO, Al₂O₃ and SiO₂. They have a structure of the 1M type mica with cell dimensions of a=5.292(1), b=9.164(5), c=10.143(1)Å and β=100.07(2)°. The structure was refined with the full-matrix least-squares procedure to the final R value of 0.029 for 1228 observed reflections. The observed distortion of the (Si, Al)O₄ tetrahedron is in good agreement with that predicted from the chemical formula of the crystal. The ditrigonal distortion of the tetrahedral sheet is intermediate between those of KMg_2.5Si₄O₁₀F₂ and KMg₃Si₃AlO₁₀F₂, having the tetrahedral rotation angle of 3.63°.