During the last few years large collections of rock specimens from the Toro-Ankole volcanic fields, east and south-east of Ruwenzori, have been sent to Durham by the Geological Survey of Uganda for petrological investigation. The specimens recently studied include representatives of a suite of highly potassic ultrabasic lavas. Some of these resemble melanocratic olivine-nephelinites or olivine-rich ankaratrites, but the nepheline-like felspathoid which they contain, instead of being nepheline, is a polymorph of KAlSiO4. For chemical reasons this mineral was at first thought to be kaliophilite. Other types, closely related, contain leucite, biotite, or melilite, in addition to the supposed kaliophilite.

Professor Arthur Holmes has described in the preceding paper a suite of rocks from Mafuru, Uganda. During his study he observed in thin section a mineral which in optical properties is indistinguishable from nepheline. He gives chemical evidence, however, for the presumption that a potassium aluminosilicate is present in mafurite which would suggest either that the nepheline-like mineral is kaliophilite or a polymorphous form of KAlSiO4. Kaliophilite is not isomorphous with nepheline, but has similar optical properties, and the two minerals could be easily confused in thin section. They may, however, be rapidly distinguished by X-ray methods. Even minute grains insufficient for chemical analysis can be determined with certainty; so it seemed probable that an X-ray study of the nepheline-like mineral from Uganda would give additional information and decide whether or no a separation and chemical analysis of the mineral should be attempted. Professor Holmes accordingly invited me to examine his specimens and rock-sections.

In the investigation of the system CaO–MgO–SiO2 by Ferguson and Merwin the orthositicate 3CaO.MgO.2SiO2 was not observed, and the existence of this compound was first proved in 1921 by Larsen and Foshag who detected it in a contact-altered limestone at Crestmore, California, and gave to it the name merwinite. Since then this compound has been found in contact-altered limestones at Scawt Hill, Antrim, near Neihart in the Little Belt Mts. of Montana, and at Velardeña, Mexico. The history of the recognition of merwinite as an artificial mineral is less easy to trace, but it seems that research workers on refractory bricks were the first to recognize its existence. Thus U.S.A. Patents applied for in the years 1934 and 1935 and British Patents applied for on 11.10.1935 and 19.5.1936 by Non-Metallic Minerals Inc. specify merwinite and periclase as the essential constituents of a refractory brick produced by firing mixtures of dolomite and serpentine, dolomite and sand, or impure magnesite and dolomite at temperatures varying between 2700° and 2900° F.