Abstract: Ilvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke. Additional secondary minerals are ferroactinolite, cummingtonite, chlorite, biotite, prehnite, and epidote. Microprobe analyses demonstrate that the ilvaite is close to the ideal end-member composition [ CaFe 2 2+ Fe 3+ Si 2 O 8 (OH)] and that only cummingtonites, Mg/(Mg+Fe2+) 0.77–0.39, and carbonates (solid solutions between calcite and dolomite, dolomite and ankerite, magnesite and siderite) show significant compositional variations. It is estimated that ilvaite formed at temperatures < 470 °C and at pressures < 2 kbar, probably during a low-grade metamorphic event (or events) associated with the Caledonian orogeny. The fluids involved in the alteration process contained both CO2 and H2O, initial fluids probably being richer in CO2 than later ones. Ilvaite may be more common in hydrothermally altered igneous rocks than previously supposed.