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.