Portlandite, a new mineral from Scawt Hill, Co. Antrim

Though calcium hydroxide has long been known as a product of hydrolysis of Portland cement, reliable data on its physical and optical properties were not available until Ashton and Wilson prepared this substance artificially and had examined crystals developed in rugs in old cement briquettes. In examining some recently collected larnite-spurrite-rocks from Scawt Hill the writer observed crystals now shown to be Ca(OH)2 as constituents of aggregates composed mainly of coarsely grained afwillite, forming infillings in these contact-rocks.