Tobermorite, a hydrated silicate of calcium, was described by M. F. Heddle in 1880 from three Scottish localities: two near Tobermory in the island of Mull, and the third a quarry near the pier at Dunvegan in Skye. Further localities for the mineral are Sgfirr nam Boc, Loch Eynort in Skye, and Ardtornish Bay in Morven (Morvern), Argyllshire.Because of marked similarity in chemical composition it has been suggested that tobermorite might be identical with gyrolite. In view of this and of the current interest in hydrated calcium silicates in cement chemistry the species has been re-examined.

IN the course of a study of certain cerium minerals, a spectrogram of churchite was taken and it was noticed that the cerium lines were barely visible, whereas the yttrium lines were very strong. We therefore decided to re-investigate this species.Churchite was originally described by A. H. Church in 1865, and named by C. G. Williams in the same year. 1 The specimens were collected by R. Talling from an unnamed locality in Cornwall. It has only been analysed once, by Church, who showed it to be a hydrated phosphate of rare-earths and lime ; his quantitative data have been recalculated and included in table I. Church obtained qualitative reactions for the cerium group of rare-earths, and for cerium itself, 2 but made no attempt to determine the ratio of cerium to yttrium earths ; he assumed a mean molecular weight for the rare-earths which would correspond to R20 a 324, essentially a mixture of cerium earths with but little yttria, but did not adduce any evidence for this assumption. As will be shown below, churchite actually contains very little cerium, and is essentially a phosphate of yttrium with minor amounts of other rare-earths, principally neodymium, erbium, and cerium; but the standard textbooks 3 have listed churchite as ' a phosphate of cerium (didynfium)

SummaryA new copper arsenate, dimorphous with cornwallite, has been found on specimens from five localities in Cornwall, one in Devon, and one in Cumberland. The name cornubite (from Cornubia, the medieval Latin name for Cornwall) is proposed for the new mineral. Chemical analyses of cornubite and cornwallite agree well with Cu5(AsO4)2(OH)4; sp. gr. cornubite 4·64, cornwallite 4·52. X-ray study suggests that the unit-cell of cornubite has a volume of 228 Å.3 or a simple multiple thereof. New X-ray data for cornwallite (a 17·33, b 5·82, c 4·60 Å., β 92° 13′) agree well with L. G. Berry's except for a. Powder data for both are given.

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The investigation which has led to this description of a new mineral began because of a suggestion by Dr. W. F. Foshag, when on a Visit in 1951 to the Mineral Gallery of the British Museum (Natural History), that a brown cut gemstone exhibited as olivine had perhaps been incorrectly determined. More recently, Dr. Foshag has stated that the idea came from Dr. George Switzer, who as a result of an X-ray powder photograph taken in June 1950 of a similar specimen in the collection of the United States National Museum in Washington concluded that his material was not olivine and was likely to be a new species. The present work would not have been pursued had it been realized at the time that Dr. Switzer intended to continue the study when he had suitable material for analysis.