Abstract
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.
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