The mineral here described was exhibited at the meeting of the Iron and Steel Institute held on May 6, 1920, in illustration of a paper by Mr. J. F. Wilson, who found that slags yielding these crystals had a composition approximating to the orthosilicate ratio. The slag was obtained from a metal-mixer ladle of about ten tons capacity, and consisted of a network of interlacing rhombie prisms, from which the mother-liquor had withdrawn, leaving material on which it has been possible to make fairly accurate measurements. Mr. Wilson very kindly arranged to place at the disposal of the present authors the specimen on which this account is based.