Portlandite occurs in a variety of environments. At the type location in Northern Ireland it occurs as an alteration of calc–silicate rocks by contact metamorphism of larnite–spurrite. It occurs as fumarole deposits in the Vesuvius area.
Portlandite appears in various settings. At its type site in Northern Ireland, it forms as a result of contact metamorphism altering calc–silicate rocks to larnite–spurrite. It also occurs as fumarole deposits in the Vesuvius region.