The formation of columbite has been observed in two carbonatites in south-west Tanganyika where it is due to autometasomatism following the emplacement of the bodies. Disoriented aggregates of columbite crystallites are formed as pseudomorphs after eubo-octahedra of pyrochlore in sövite. This alteration is accompanied by the addition of Fe, conversion of amphibole and biotite in the sövite to a ferruginous tale. the formation of an iron-bearing carbonate, and the introduction of colloform silica: Ca and Na are lost in part. The general alteration of the sövite can be ascribed to the influx of iron-bearing solutions at relatively low temperature and high pressure.

New chemical, optical, and X-ray data have been obtained for dumortierite. The unit cell. space group Pemn, contains 4[(Al,Fe)7BSi3O18]. Brown gem quality material from Ceylon has α 1·6860, black, [001], β 1·722, deep redbrown, and γ, 1·7229, brown, with 2Vα 13° , γ > υ. X-ray powder data are given.

Lead-zinc ore-bodies in dolomite at Broken Hill, Northern Rhodesia, consist of a sulphide core surrounded by an oxidized sheath. Oxidation has been proved to persist to a depth of at least 1150 ft. below surface. In the transitional zone between sulphide and oxidized ore galena forms a thin rim to relict masses of blende, and farther into the sulphide core it penetrates the blende along cracks and cleavages. Locally it comprises over 5 % of the ore. From its relations to smithsonite and willemite it is clearly supergene in origin and must have been deposited above the water table. This considerable migration of lead in the zone of weathering is believed to have occurred during a former period of aridity.

The position of the rhombic section of plagioclase felspars, as calculated from the measured cell dimensions, has been shown to vary with the structural state, the variation from high- to low-temperature forms being most pronounced for albite. The theoretical composition plane of pericline twins is the rhombic section. If the structural state or the composition of the felspar changes after twinning has occurred, the cell dimensions and hence the rhombic section will change; however, the composition plane of the pericline twin will probably remain unchanged unless recrystallization occurs, or perhaps if the felspar is subjected to strong stress. Thus the observed pericline composition plane need not coincide with the rhombic section. Comparison of observed composition planes with the corresponding calculated rhombie sections shows that some of them do indeed differ and the differences have been used to provide information on the changes occurring subsequent to the establishment of the pericline twins. The interpretation is complicated by the occurrence of the acline-A twin, whose composition plane is {001}, for the rhombie section of plagioclase felspars from An0 to An60 may coincide with {001} if the structural state of inversion has a particular value. When the observed composition would agree with both pericline and acline-A twinning, the new term b-axis twin has been used.

A closer understanding of the causes for the variation of refractive index with change of composition has been sought in terms of certain properties of ions. The study is confined to isostructural ionic crystals. It is assumed that polarization of the anions (usually O2− or F−) makes the principal contribution to the total refractive index effect, and that variation of refractive index is determined mainly by variable anion polarization. Polarization of the anion is controlled largely by the field in which it finds itself which, in turn, depends on the effective force of attraction of the cation: ionization potentials are used for the purpose of comparing such forces. For compounds involving pairs of 8- and 18-electron cations (Mg2+ and Zn2+; Al3+ and Ga3+; Si4+ and Ge4+; Ca2+ and Cd2+) or pairs of transition cations (Zr4+ and Hf4+; Nb5+ and Ta5+; Mo6+ and W6+) refractive indices are higher in the crystal that contains the cation of the element with the higher ionization potential. For larger suites of cations of various types (for example, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+, radii from 0·65 to 0·80 Å.) the predicted relationship between ionization potential and refractive index holds precisely for the compounds A2SiF6.6H2O (A2+, etc.), but only in part for other compounds; this may be due to the effect of the crystal field on certain 3d transition cations.

The standard thermal treatments proposed by Lima-de-Faria have been applied to metamict euxenites, polymignites, yttrotantalites, samarskites, pyrochlores, and allanites. In the case of these metamict niobotantalates a facecentred cubic phase is formed at 700° C., and at 1000°C. a more characteristic phase appears combined with the cubic one. In the case of allanites only a slight formation or strengthening of the original structure of allanite appears at 700°C., and at 1000°C. a cubic phase is formed together with other phases.

The space group and unit cell of natural syngenite have been redetermined. X-ray powder photographs of natural syngenite and synthetic CaSO4.K2SO4.H2O were found to be practically identical. Optical and d.t.a. data are also given.

Minium occurs at Broken Hill, New South Wales, as pseudomorphs after cerussite, formed during fires in the mine. It is tetragonal with a = 8·824 Å. and c = 6·564 Å.; sp. gr. 8·2 + 0·2. The Broken Hill minium appears identical in physical properties with synthetic Pb3O4 described by other workers.