Aluminosugilite, KNa₂Al₂Li₃Si₁₂O₃₀, an Al analogue of sugilite, from the Cerchiara mine, Liguria, Italy

Abstract

Aluminosugilite (IMA2018-142), ideal formula KNa2Al2Li3Si12O30, is a new mineral that was found in the interstices in manganiferous metacherts from the Cerchiara mine, Liguria, Italy. Aluminosugilite is an Al analogue of sugilite belonging to the milarite group. It occurs as aggregates of small prismatic and/or granular crystals up to 1 mm in length, and it is pinkish-purple with a pale purple to white streak and a vitreous lustre. It has a Mohs hardness of 6–6.5. Its cleavage is indistinct, poor on (0001). Measured and calculated densities are Dmeas.=2.71–2.72 g cm−3 and Dcalc.=2.73 g cm−3, respectively. Aluminosugilite is optically uniaxial (–), with ω=1.577–1.586 and ε=1.575–1.585, with a weak pleochroism. The magnetic susceptibility is lower than that for sugilite. Aluminosugilite is insoluble in HCl, HNO3 and H2SO4, like sugilite. The empirical formula of aluminosugilite calculated on the basis of O =30 from the result obtained by electron microprobe analysis and laser-induced breakdown spectroscopy is K0.99Na1.99(Al1.38Mn0.313+Fe0.243+Ti0.05Mg0.01)Σ1.99Li3.06Si11.99O30. Structure refinement converged to R1=2.17 %. Its space group is hexagonal P6∕mcc, with unit-cell parameters a=9.9830(4) Å, c=13.9667(5) Å and V=1205.45(7) Å3. Based on the refined site occupancies, the ideal structural formula of aluminosugilite is CKBNa2AAl2T2Li3T1Si12O30. The variation of the A–O3 distance is governed by the cationic substitution at the A site. The a and c dimensions of aluminosugilite are shorter than those of sugilite due to Al substitution for Fe3+ at the A site.