tilleyite

Tilleyite is a rare and distinctive mineral belonging to the sorosilicate group, characterized by its unique chemical composition and striking appearance. The mineral was first identified in 1925 by British mineralogist George Frederick Kullerud and named in honor of British geologist Alfred Harker’s student, Harold Tilley, who studied its occurrence in the Isle of Skye, Scotland. Tilleyite has the chemical formula Ca₄Mn₄⁺²Si₆O₂₀(OH)₂·3H₂O, indicating its composition of calcium, manganese, silicon, oxygen, and water.

Tilleyite typically forms as small, dark brown to black, tabular or prismatic crystals, often exhibiting a vitreous to resinous luster. It is usually found in low-temperature hydrothermal veins associated with other manganese-rich minerals such as rhodonite, rhodochrosite, and calcite. Notable occurrences include the Isle of Skye, Scotland; the Franklin Furnace district in New Jersey, USA; and the Urals region in Russia. The mineral is relatively soft, with a Mohs hardness of approximately 4 to 4.5, and has a specific gravity around 3.1 to 3.3.

Due to its scarcity and distinctive properties, tilleyite is of interest to mineral collectors and researchers studying manganese-bearing silicates. Its formation is linked to hydrothermal processes where manganese-rich fluids interact with calcium-rich host rocks. While not economically significant, tilleyite serves as a valuable indicator mineral in geological studies, particularly in understanding the conditions of its deposition. The mineral is stable under normal conditions but may decompose upon prolonged exposure to heat or strong acids.