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 emerges in various settings. At its original site in Northern Ireland, it results from the transformation of calc–silicate rocks due to contact metamorphism involving larnite–spurrite. Furthermore, it appears as fumarole deposits in the Vesuvius region.