The published data on these three minerals, first found in the Laurion lead slags, is well summarized in Hintze (Handb. Min., 1915, vol. 1, pp. 2632-2641). In studying the original papers for the purpose of recalculating the elements of each, LaForge was impressed with their close crystallographic relationship in certain respects and with the curious complexity of the form series of fledlerite, which seemed to call for further observations. Accordingly, crystals of all three minerals from Laurion were studied on the goniometer, specimens loaned by the U.S. National Museum furnishing a welcome supplement to those found in the Harvard Mineralogical Museum. In the following pages the results of these studies will first be presented in brief form. Following this, the conclusion arrived at as to the most appropriate positions of the three minerals to bring out their mutual relationships will be stated.

It has recently been shown by W. Nieuwenkamp that matlockite is identical in chemical composition and crystal-structure with artificial lead fluochloride PbFC1. His conclusion is based upon powder photographs of the two substances and a fluorine determination of a specimen of matlockite from Matlock, Derbyshire. The present work was undertaken primarily to check Nieuwenkamp's interesting results. Access to an exceptionally fine suite of matlockite specimens in the British Museum collection made possible single crystal X-ray measurements, accurate optical determinations, and a new chemical analysis.

This interesting new mineral was discovered by the first author in February 1933 whilst in the employment of the British South Africa Company Limited as a field geologist to the Rhodesia Minerals Concession Limited under the direction of Dr. J. Austen Bancroft. The locality, longitude 29° 29′ E. and latitude 15° 27′ S., is about eighty miles east of Lusaka, the new capital of Northern Rhodesia, after which the mineral is named. The central African plateau has here been cut back from the northern escarpment of the Zambezi rift valley, about 10 miles to the south, producing a rugged topography varying from 1,000 feet to 4,000 feet in altitude.

In a further search for portlandite amongst the vug minerals of the Scawt Hill contact-zone, attention was drawn to some infillings containing a mineral which had not previously been observed. These infillings, which occur in a larnite-rock carrying melilitewollastonite aggregates of the hybrid zone of the contact, consist of afwillite, portlandite, ettringite, and aggregates of a mineral, the light-greenish colour of which was conspicuous.

With the exception of beryl, there are very few recorded occurrences of beryllium minerals in the whole of Africa. Phenakite is represented by a single small crystal from Tanganyika Territory which was described in this Magazine in 1906. Chrysoberyl has been found in small amount in the Somabula diamond field in Southern Rhodesia, and in Belgian Congo and Gold Coast; and gadolinite a in Gordonia in Cape Province.