Sugilite, a new silicate mineral from Iwagi Islet, Southwest Japan

Abstract

Sugilite occurs as an essential mineral amounting to 3–8 percent by volume in aegirine syenite from Iwagi Islet, Ehime Prefecture. It is associated with albite, aegirine, pectolite and a Ca-K-Ti silicate (a still to be identified mineral). Sugilite is light brownish yellow, luster vitreous, streak white. Symmetry hexagonal. Cleavage 0001 weak. Hardness 6–61/2. Specific gravity 2.802 (calc.), 2.74 (meas.).Under the microscope, it is uniaxial negative with e=1.607, ω=1.610, ω−e=0.003, and colorless.Estimated chemical composition is SiO2 71.38, TiO2 0.51, A12O3 2.97, Fe2O3 12.76, FeO 0.19, Li2O 3.14, Na2O 4.37, K2O 3.76, H2O(+) 0.81, H2O(−) 0.12, total 100.01%, corresponding ideally to (K, Na)((H2O), Na)2(Fe3+, Na, Ti, Fe2+)2(Li, Al, Fe3+)3(Si12O30) with Z=2.The unit cell dimensions are a0=10.007(2), c0=14.000(2)A. Space group D26h−P6⁄mcc. X-ray powder data resemble those for sodgianite, milarite, osumilite, merrihueite, roedderite and brannockite.It is clear from the above-cited data that sugilite has milarite-type structure and resembles sodgianite and brannockite in containing lithium in tetrahedral site, but it is distinguished by a high content of ferric iron in 6-coordination.The name is given in honour of the late Professor Ken-ichi Sugi (1901-1948), who first found the occurrence of this mineral with Mr. M. Kutsuna.