Shirokshinite, K(NaMg2)Si4O10F2, a new mica with octahedral Na from Khibiny massif, Kola Peninsula: descriptive data and structural disorder

Abstract

Shirokshinite, K(NaMg 2 )Si 4 O 10 F 2 , is the analogue of tainiolite, K(LiMg 2 )Si 4 O 10 F 2 , with the M 1 octahedron fully occupied by Na instead of Li. It was found in the Kirovskii underground apatite mine (Kukisvumchorr Mountain, Khibiny massif, Kola Peninsula, Russia) as a late hydrothermal mineral in a small hyperalkaline pegmatite embedded in ristschorrite. Shirokshinite is associated with microcline, kupletskite, aegirine, natrolite, lorenzenite, calcite, remondite-(Ce), donnayite-(Y), mckelveyite-(Y) and galena. Crystals are usually skeletal and coarse hexagonal [001] prismatic. Shirokshinite is transparent to translucent, colourless to pale greyish, hardness Mohs9 ∼2.5; D(calc) = 2.922 g/cm 3 . Optically biaxial (-), α = 1.526(1), β = 1.553(2), γ = 1.553(2); 2V meas = −5(5)°, 2V calc = −0°; Y = b , Z ∼ a , Xc = 3(2)°. The IR spectrum of shirokshinite is unique even if close to that of tainiolite: in particular, the presence of Na + instead of Li + shifts some bands towards low-frequencies. Single-crystal diffraction data (Mo K α-radiation) gave a = 5.269(2), b = 9.092(9), c = 10.198(3) A, β = 100.12(7)°, Z = 2, 1 M -polytype, space group C 2/ m. Structure anisotropic refinement converged R = 0.13 for 715 observed reflections. Evidence of stacking faults in the structure is discussed and compared with the so called Ďurovic effect. The very little ditrigonal distortion in spite of the large dimension of the Na octahedron is discussed in comparison with tainiolite. A critical revision of old published data indicating octahedral Na in micas shows that this hypothesis was biased by the low quality of the chemical analyses.