Aluminosugilite and norrishite from the Funakozawa mine, Iwate Prefecture, Japan

Abstract

Descriptive mineralogical studies, including single-crystal structure analysis, were conducted on aluminosugilite and norrishite, two rare minerals from the Funakozawa mine. Aluminosugilite occurs in the quartz layer spread along the schistosity of the slate as foliated reddish-light purple crystals up to 1 mm in diameter and 50 µm thick. The cleavage is indistinct, and the Mohs hardness is 6. Aluminosugilite coexists with aegirine and amphibole. In the outcrop, aluminosugilite often transforms into swinefordite with its texture retained. The representative chemical composition of aluminosugilite from a boulder sample was K_0.98Na_1.93(Al_1.31Fe_0.44Mn_0.17)_Σ1.91Li₃Si₁₂O_29.82, and single-crystal X-ray data yielded a hexagonal P6/mcc lattice with a = 9.9894(7) Å, c = 13.9527(9) Å, and V = 1205.77(14) ų. The structure refinement converged to R₁ = 2.67%. The structure may limit the accommodation of the Jahn-Teller cation, Mn³⁺, due to symmetry. Norrishite is one of the most abundant constituent minerals in slate and coexists with aegirine and amphibole. In thin sections, norrishite is observed to be surrounded by quartz. Norrishite is brown to black and is in the form of irregularly shaped foil-like to plate-like crystals that exhibit perfect cleavage at {001} planes. Norrishite is also pleochroic and highly polychromatic with color variation from yellow to brown to green. The representative chemical composition of norrishite from a boulder was K_0.99(Mn_1.97Mg_0.09Ti_0.03Li_0.92)_Σ3(Al_0.22Si_3.78)_Σ4O_11.95, which represents an absolutely Fe-free composition. Single-crystal X-ray data yielded a monoclinic C2/m lattice with a = 5.3024(2) Å, b = 8.9520(4) Å, c = 10.0812(5) Å, β = 98.191(4)°, and V = 473.65(4) ų as the 1M polytype. The structure refinement converged to R₁ = 4.30%. The structure is sufficiently flexible to accept the Jahn-Teller cation, Mn³⁺, but then its ability to form a solid solution with other components is limited.