Fine radiating groups of columnar aragonite crystals occur in a basaltic pipe at Kulnura, New South Wales, associated with calcite in habits varying from simple rhombohedra to complex spherulitic groups. It is proposed that the presence of impurities, especially magnesium, induced the crystallization of aragonite and was also responsible for the variety of calcite crystal habits present. Two distinct growth environments are indicated: steam-explosion breccia pipes containing aragonite and calcite showing an advanced stage of spherulitic growth; and vesicles with calcite in varying stages of spherulitic growth. This paper presents a preliminary study based on available material and on observations made in situ while access to the locality was still possible.