The IUGS Nomenclature on Kalsilite-Bearing Volcanic Rocks: A Critical Appraisal and Recommendations

The current International Union of Geological Sciences (IUGS) recommendations for nomenclature of kalsilite-bearing volcanic rocks do not consider the possible coexistence of kalsilite (KAlSiO4) and nepheline (KNa3Al4Si4O16), nor do they address volcanic rocks containing kalsilite, other than the archetypal kamafugites. This review highlights the most important shortcomings in the existing IUGS nomenclature against the classical literature and the most recent reports on kalsilite-bearing volcanic rocks. In addition, a new modal classification system, more logical and complete, is proposed. This scheme follows the IUGS principles as far as possible and denotes an updated version of the most applied historical terminology. Based on the presence of the main light-coloured constituents of kalsilite-bearing volcanic rocks, only four rock names are required for petrographic classification: katungite (melilite dominant), ugandite (leucite dominant), mafurite (kalsilite dominant), and kalsilite nephelinite (nepheline dominant). Such terminology should only be applied for volcanic rocks that definitely contain kalsilite. This requires the use of advanced analytical methods (e.g. electron microprobe analysis coupled with back-scattered electron imaging), besides conventional transmitted-light microscopy, as kalsilite and nepheline are optically indistinguishable. The coexistence of these two minerals in K-rich volcanic rocks is much more common than previously recognised. Volcanic rocks with kalsilite can be separated into potassic and ultrapotassic groups. This highlights a major problem concerning the indiscriminate use of the term kamafugite and the existing confusion between the modal and chemical classifications of kalsilite-bearing volcanic rocks. Our recommendations are purely based on descriptive attributes.