Contact metamorphosed limestones and dolostones intruded by diabase.
Ref.: Tilley, C.E. (1929) On larnite (calcium orthosilicate, a new mineral) and its associated minerals from the limestone contact-zone of Scawt Hill, Co. Antrim. Mineralogical Magazine, 22, 77-86.
http://www.habitas.org.uk/escr/summary.asp?item=1130.