Sillimanite

Here’s a palm-sized specimen of sillimanite, displaying coarse, strongly lineated crystals embedded in a schist matrix. Also known as “fibrolite” due to its distinct fibrous nature, sillimanite is an aluminum silicate that forms under high-temperature and high-pressure conditions in high-grade regional metamorphic rocks, and less frequently in pegmatites. This piece features lustrous, parallel-growth crystals with dark olive-green to dark brown hues. The specimen originates from Lac-Sainte-Marie, Quebec, an obscure yet geologically significant locality within the Grenville Province. While the region is better known for its abundance of pegmatites and occurrences of fluorapatite and diopside, the intense deformation and high-grade metamorphism during the Grenvillian orogeny also created ideal conditions for the formation of sillimanite. This piece highlights the diverse mineralogy of the area and is a fine example from this relatively uncommon locality.