Recent optical and structural investigations have shown that ; oth high- and low-temperature forms of plagioclase can exist naturally over the range An0-70, and in the more basic plagioclases (An70_90) Gay (1954) has shown that a structural distinction can be effected although no definite optical differences have yet been established. In the anorthite range slight optical and distinct structural differences have been observed between synthetic plagioelases and crystal lapilli on the one hand and all other natural plagioclases on the other (Gay, 1953).

The chlorites form an extensive isostructural series with a high degree of isomorphous substitution, within which it has always been difficult to define or delimit species; until recently, owing to the lack of adequate X-ray studies; several structurally distinct species were included with the group, further complicating the problem. The group comprises aluminosilicates of magnesium and iron (ferrous and ferric); a few contain appreciable amounts of chromium, nickel, or manganese, and one (pennantite) contains manganese as a major constituent.

The first step towards the understanding of the relations of the chlorites was taken by G. Tschermak (1890, 1891), who divided them into two groups: the orthochlorites, with compositions between (Mg,Fe")2Al2SiO5(OH)4 and (Mg,Fe")2Si2O5(OH)4, and the leptochlorites, with compositions not explicable on this basis, and in general richer in trivalent ions (often including considerable Fe"′) relative to silicon and divalent ions. The two orthochlorite end-members have the composition of amesite and serpentine respectively, but it is now known that neither of these two minerals has the chlorite structure.

The mineral, for which the name isokite is now proposed, occurs in the carbonatite plug which forms Nkumbwa Hill Oat. 10° 10' S. long. 32° 51' E.), 15 miles east of Isoka (pronounced Iso̅ka) in Northern Rhodesia. A preliminary account of this interesting mineral locality has already been published (Reeve and Deans, 1954) and a more detailed account is now in preparation, so that only the main features of the occurrence need be mentioned here. The carbonatite plug or pipe, 0.6 by 1 mile across and rising to a height of 1000 feet above the surrounding country, pierces granitic and micaceous gneisses which are fenitized to a distance of 500 yards from the walls of the pipe, but, unlike the classic carbonatite complexes, no intrusive alkalic rocks are exposed.

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. 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, 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 R2O3 324, essentially a mixture of cerium earths with but little yttria, but did not adduce any evidence for this assumption.

These two minerals were described by Eakle from the crystalline limestone at Crestmore, Riverside County, California. Both were shown to be hydrated monocalcium silicates, containing small amounts of P2O5, SO3, and CO2, occurred with a fibrous habit, and had closely similar optical properties. They were distinguished chiefly by the higher water content (approx. 1.3 mol.) of crestmoreite as compared with riversideite (0.5 mol.), and by the fact that the former was included directly in the calcite, whereas the latter formed veins in masses of idocrase which were contained in it. The minerals had been produced under hydrothermal conditions caused by igneous intrusions. In both cases, the primary material was believed to have been wilkeite, which is an apatite mineral with much of the phosphorus replaced by sulphur, silicon, and possibly carbon.

In 1916 Kôzu recorded the fact that a specimen of yellow Madagascar orthoclase, subsequently shown by Seto (1923) to contain 2-93% Fe203, had an optic axial angle of 20° 17' (NaD light) with the optic axial plane parallel to (010). This observation is in apparent conflict with the useful curves relating 2V to composition for alkali-felspars compiled by Dr. O. F. Turtle (1951, 1952). These curves were designed to show how the felspars fall into several more or less distinct series, rather than as an aid to the precise determination of composition, which in fact must be decided by other means. The possibility of transitional forms between the series was clearly recognized (e.g. 1952, p. 565).

Suppose that in the study of a mineral we have determined the cell-dimensions and the density, and made a ehemienl analysis. From these we can readily calculate the empirical unit-cell contents, which will usually approach integral values and so suggest a probable formula.

But the calculated empirical unit-cell contents, being based on chemical and physical data subjeet to experimental error, will be subjeet to numerous sources of error, and in any serious study it is very desirable to have some rough idea of the probable limits of error. Unfortunately, this is a matter of considerable complexity, but the following suggestions may be of assistance, though they will doubtless be considerably improved on.

Although detailed experimental investigations have been carried out on the lime, magnesia, and ferrous pyroxenes, relatively little attention has been devoted to synthetic studies of the group involving replacements by ions of higher valencies. In the present study, determinations have been carried out on the limits of solid solution of alumina in pyroxenes in the system CaSiO3-MgSiO3-A12O3. Determinations of the solubility of ferric oxide and titania in diopside were also made.

Pure calcium carbonate, magnesia, silica (crushed and washed rockcrystal), alumina, ferric oxide, and titania were used for the preparation of the mixtures. The constituents in their requisite proportions were fused together four or five times with intervening crushing of the quenched glass. The product was an homogeneous glass free from crystalline material.

Bassetite and uranospathite Were described as new uranium minerals from Cornwall in 1915 by A. F. Hallimond, to whose paper reference should be made to supplement the descriptions given here. The position of both minerals as relatives of autunite was recognized by Hallimond. Unfortunately, chemical analyses could not be obtained and the composition of these minerals has remained uncertain. No further information has been published about uranospathite. In 1940 Meixner reported that microchemical tests of an apparently authentic specimen of bassetite from Cornwall indicated that the mineral was a hydrated iron uranyl phosphate.

In view of the important uses to which the metal titanium has been applied in recent years, it seems fitting that some tribute should be paid to its Cornish discoverer, the Rev. William Gregor, and with this end in view I have collected the following facts concerning this now almost forgotten distinguished early chemist.

The Rev. William Gregor, M.A., chemist and mineralogist, was the younger son of Francis Gregor, member of an old Cornish family of Trewarthenic, Cornelly, CornwalI. Born on December 25, 1761, he was educated at Bristol Grammar School, and it was while there that he first developed a taste for chemistry.

A Green mineral akin to monazite, derived from the State of Travancore in southern India, was investigated chemically during the first world war by the Imperial Institute, London, and was found to be exceptionally rich in thorium and uranium. No description of this unusual 'monazite' has hitherto been published.

The high content of radioactive elements in this mineral led the State Geologist, Mr. Venkitarama Mahadevan, to have a deposit opened up in 1945, in readiness for an assessment of the radioactive mineral resources of Travancore. At the invitation of the former Dewan, Sir C. P. Ramaswami Aiyar, K.C.S.I., K.C.I.E., this assessment was carried out by Dr. C. F. Davidson early in 1946.

As part of a study of the structure of whitlockite (β-Ca3(PO4)2) attempts were made to prepare single crystals of the mineral synthetically. At 1180° there is a polymorphic transition from the rhombohedral β-form to a high-temperature monoclinie form α-Ca3(PO4)2. The density of the α-form is 12% less than that of the β-form and large crystals were not found to transform without powdering. CaCl2 was therefore added as a flux to see whether crystals of Ca3(PO4)2 could be produced from a melt below 1180°. The melting point of pure Ca3(PO4)2 is about 1800°. In addition to chlorapatite (Ca5(PO4)3C1), a phase of composition Ca2PO4Cl was produced. This compound had been earlier observed by Cameron and McCaughey, who identified the crystals as being closely related to the mineral spodiosite (Ca2PO4F).

C . W. Bunn and H. Emmett have observed the spreading, of multi-molecular layers on growing crystals of artificial compounds, and have suggested that deposition takes place on the high-index surfaces at the edges of the layers. Their work led the author to examine crystals of several mineral species for evidence of the mechanism of growth, and this account is the result of a preliminary survey of the growth structures found.

So many growth structures could be seen with the naked eye or with a hand-lens that it was decided to use low magnifications in order that comparatively large areas of the crystal faces could be examined and photographed at one time. A metallurgical microscope with a prism reflector and 1-inch objective was used for the observations and most of the photography. This work is qualitative rather than quantitative, but it is intended to carry out a quantitative examination of the same specimens later.

Bultfonteinite from Bultfontein mine, Kimberley, South Africa, is a hydrated fluorine-containing calcium silicate, of approximate composition Ca4Si2O10H6F2. It was first described in 1932 by Parry, Williams, and Wright,1 who gave an account of its morphology, optical properties, composition, and chemical relationships. It is triclinic but markedly pseudo-orthorhombic. The present work consists primarily of a determination of the unit-cell dimensions, involving a more accurate evaluation of the interaxial angles than the earlier one; this is followed by some tentative suggestions about the structure.

During the study of hydrothermal alteration of cores recovered Dfrom holes drilled at Wairakei during the exploration for geothermal steam, an unusual zeolite mineral was found. Optical, chemical, and X-ray data indicate that this zeolite is a new mineral, the calcium analogue of analcime. The name wairakite is given to the new mineral after the locality Wairakei in New Zealand. Wairakei is situated in the central part of the North Island, approximately 4 miles north-north-east from Lake Taupo, in an active volcanic region. The material was submitted to Dr. D. S. Coombs, Department of Geology, University of Otago, Dunedin, for an investigation of its X-ray crystallography.

This work has been primarily concerned with plagioclase felspars within the composition range Ab30An70 to Ab0An100, i.e. felspars with the anorthite-type structure. Some results for less basic specimens will be quoted where appropriate ; a fuller description of the effects of heat-treatments on intermediate plagioclase felspars will be published later.