TTuliite

TTuliite is a rare silicate mineral in the garnet group. It fits the general garnet formula X3Y2(SiO4)3, with the X site commonly occupied by Ca and/or Na and the Y site by Fe3+ and Al. The composition shows solid solution with related garnet members, and trace substitutions by Mn, Ti, and Cr have been reported in some specimens. Crystallography is isometric, and ttuliite forms well-shaped crystals commonly described as dodecahedra or trapezohedra. The mineral typically exhibits a vitreous luster and a pale olive-green to brown color; the streak is white.

Physical properties include Mohs hardness of 6.5 to 7, density around 3.9 to 4.2 g/cm3, and no cleavage; the fracture is uneven to conchoidal. It is transparent to translucent in thin crystals or very small grains, and it commonly shows a vitreous to adamantine sheen under reflected light.

Occurrence and formation: TTuliite is found in high-grade metamorphic rocks and occasionally in hydrothermal veins associated with other aluminosilicates. The type locality is described in published descriptions as a granulite-facies belt in eastern Norway, where ttuliite occurs together with grossular-andradite garnets and andradite-rich assemblages. Conditions implied by its associations point to elevated temperatures and pressures typical of granulite-facies environments.

Etymology and naming: The name ttuliite is derived from the personal name Tuuli, used in homage to the discoverer, together with the conventional mineral suffix -ite.

Use and significance: TTuliite is primarily of interest to mineral collectors and researchers studying garnet chemistry and metamorphic processes; it has no known industrial applications. See also: garnet group, silicate minerals.