Stephanite

Stephanite is a carbonate mineral belonging to the calcite group, with the chemical formula CaCO₃. It forms as a pseudomorphic replacement of the older mineral calcite, preserving the external crystal shape while mineralizing into a substituted lattice. The mineral is named after German mineralogist Alexander Stephani (1824–1854), who first described it in 1853. Stephanite crystallises in the trigonal crystal system and typically appears as fibrous, radiating masses or as elongated prismatic crystals, often exhibiting a milky to translucent appearance.

The mineral’s colour ranges from white and colourless to pale cream or yellowish‑brown, and it may display coloured inclusions due to iron or manganese. Its Mohs hardness of 3–3.5 places it among the softer members of the carbonate family, and it displays a specific gravity of about 2.71. Stephanite exhibits conchoidal fracture and may show a faint twinning. Optical properties indicate an isotropic nature with a refractive index of 1.644–1.711, and it can exhibit weak pleochroism when viewed under polarized light.

Stephanite is commonly found in hot spring travertine deposits, tufa, and speleothems, where it replaces calcite through hydrothermal alteration. The mineral often occurs in association with scallop, vauxite, and other secondary carbonates, and may be found in mineralized veins within limestone and dolomite formations worldwide. Notable localities include the Galápagos Islands, Iceland, the Carlsbad Caverns in the United States, and the Kızıldağ dolomitic formations of Turkey. In many of these settings, its fibrous fibric texture and subtle colouration distinguish it from more common calcite.

From a geological perspective, Stephanite is of interest because its ductile fibrous nature can provide clues to the fluid chemistry and temperature history of travertine formations. Although it is comparatively rare, it is valued by mineral collectors for its unique textural qualities and for the insight it offers into secondary carbonate formation processes.