An analysed magnetite from Bisperg, Säter, Dalecarlia, Sweden, gave the formula , which corresponds closely to the ideal composition. A precise determination of the cell dimensions gave a 8·3963±0·0005 Å. at 18° C. Gentle heating of the powdered magnetite in evacuated silica glass tubes, before X-ray examination, resulted in sharply defined lines with high Bragg angles; an improvement which enabled precision data to be obtained. Four other new analyses of magnetites are also given and their lattice parameters are determined in the same way and are found to range from 8·3960 Å. to 8·3970 Å. The Bisperg magnetite being the purest sample examined, the value of its cell edge is taken as representative of pure magnetite.

The effect of the different ionic substitutions on the cell dimensions of natural magnetites is discussed. An attempt is also made to explain the great variations among the published values of cell dimensions of artificial preparations; one main reason being the presence of defect structures with varying oxygen contents in excess of the formula requirements.

Mineragraphic, chemical, and X-ray data are presented on the titanomagnetites1 from a differentiated teschenite sill. The minerals are generally homogeneous. The earliest titanomagnetites are Fe3O4.Fe2TiO4.FeTiO3 a solid solutions; the titanomagnetite crystallization trend is one of enrichment in Fe3O4 and FeTiO3 at the expense of Fe2TiO4. The relations of the titanomagnetites to the parent rocks and associated minerals, and their role in basaltic magma iron enrichment are also discussed.

An X-ray study of a number of rare-earth silicates has been carried out; single-crystal and powder data are presented.

It is found that cerites can contain up to at least 6 % CaO without disruption of the structure, which is trigonal with probable space-group P321 etc. The cell dimensions are a ca. 10·8 Å., c ca. 37–38 Å., although there is a strong pseudo-cell with halved c-axis. An ideal formula (Ca,Ln†)3Si2(O,OH,F)9 is proposed for this series.

Lessingite and beckelite can be regarded chemically as lime-rich members of this cerite series. They do not, however, have the same crystal structure as cerite; their diffraction patterns are very similar to that of britholite, and indicate a structure dimensionally comparable with apatite. For a hexagonal cell, the dimensions are a ca. 9·7 Å., c ca. 7·1 Å., with probable space-group P63, &c. An ideal formula (Ca,Ln)2(Si,Al,P)(O,OH,F)5 is proposed for this series. Marked biaxial optical properties suggest that the structure may be truly orthorhombic, with a very close dimensional approximation to a hexagonal cell.

The new mineral, stillwellite, is unrelated to either of these series. It is trigonal with cell dimensions a ca. 6·9 Å., c ca. 6·7 Å., with probable space-group P3112. An ideal formula (Ca,Ln)(Si,Al,P)B(O,OH,F)5 suggested by previous work is confirmed.

Chloritoids from the Saxa Vord schists in the Shetland Isles and the Dalradian schists north of Stonehaven have been studied optically and chemically. In both eases monoclinic and triclinic crystals occur with α = b in the monoclinic varieties. Twinning is common with twin axes [110] and [230]. The formula of chloritoid is considered in relation to the structure recently suggested by Brindley and Harrison (1952).

A simple photoelectric-cell attachment for use with an ordinary oremicroscope is described. Stabilization of the light source and accurate calibration of the photometer enable reflectivity values to be determined within an accuracy of 1% of the measured value. The choice of a universally acceptable standard is discussed. The photometer can be used with white light or with light of different wave-lengths. Bireflection, dispersion, and dispersion of the bireflection can readily be measured.

A visual microphotometer depending on three polarizing screens is used to provide the primary scale for measuring the ratio of intensity between the incident and reflected beams for a prepared standard surface, which is intended for subsequent use in the ordinary procedure with the ore-microscope. A reversing mirror on the microscope stage gives reflection at quasi-normal incidence. Direct measurements on a surface of diamond confirm the validity of the Fresnel formula for a polished surface of this substance, and there is general agreement for a series of transparent bodies ranging up to 17 % reflectance. The standard is a glass-covered metallic surface with a reflectance near 50 %. For pyrite RD = 54·5 %.