The habit and form of gypsum crystals in Irish mudstone aggregate affected by pyrite-induced swelling

In a recent court case in Ireland, pyrite-induced swelling of a mudstone aggregate backfill has been proven to have occurred and to have arisen from the growth of gypsum crystals in and around the aggregate fragments. There is a lack of published information on the subject and considerable debate exists regarding features associated with forceful crystallization. This paper describes the form and habit of gypsum crystals from petrological and SEM evaluations presented during the court case. Features associated with both forceful and passive crystallization of gypsum are illustrated. These illustrations should serve as a guide to the morphology of gypsum crystals where forceful crystallization under near-surface conditions has occurred. The paper is unique in that it describes a case of proven forceful crystallization of gypsum rather than attempting to infer forceful crystallization from theoretical concepts of crystal growth. This paper does not go into the detailed evidence and arguments put forward in the court case or attempt to explain the mechanisms involved in pyrite-induced swelling.