Delaware State Mineral – Sillimanite

Sillimanite – Al₂SiO₅

Delaware State Mineral, Sillimanite

In 1977, the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical Society, established sillimanite as the Delaware State Mineral. This act recognizes the geological and mineralogical significance of the large masses of this mineral found as boulders at Brandywine Springs, an occurrence that was recognized as important in the 6th (1892) edition of Dana’s System of Mineralogy. The Brandywine Springs boulders are remarkable for their size and purity. The sillimanite has a fiberous texture reminiscent of wood and could potentially be cut into cabochon gems showing a chatoyant (“cat’s eye”) effect. Sillimanite has no industrial value and is not mined as an ore or raw material.

Sillimanite, a white to tan to green aluminum silicate, (Al2SiO5) occurs in high temperature, aluminum-rich metamorphic rocks. In Delaware, it is found in the Hoopes Reservoir and Brandywine Springs areas.

Use and importance: Sillimanite is one of the most important minerals in the refractory process for brick lining all types of furnaces. In the ceramic industry, Sillimanite may be added to kaolin and clay for making certain types of ceramic ware.

Source: The Geology of New Hampshire