The Younger Granites of Nigeria contain varied assemblages of iron-rich ferromagnesian minerals in which amphiboles are an important component. Eighteen of these amphiboles (separated from rocks belonging to nine complexes) have been chemically analysed and the results are presented and discussed. The amphiboles are shown to fall into two groups, a lime-alkali group (comprised of ferrohastingsite of general formula NaCa2Fe″4Fe‴Al2Si6O22(OH)2) and an alkali group (of general formula (Na, K, Ca)2.5–3.0(Fe″, Fe‴)5(Al, Si)3O22(OH)2). The alkali amphiboles are characterized by their extremely low magnesia content, by the presence of Li and F (both in significant amounts), and are of specialinterest because of the unusual amount of ZnO they contain. Optical properties of the amphiboles have been determined but it has not been found possible to use these properties to determine accurately the chemical composition of the alkali amphiboles. It is suggested that, according to current classifications of amphiboles, the analysed alkali amphiboles should be called riebeckitic arfvedsonites or arfvedsonites. The relationship between the hastingsites and the alkali amphiboles is not yet clear.

Thin apple-green crusts on several specimens from Hingston Down Consols mine, Calstock, Cornwall, proved to consist of a new mineral, alone or intimately mixed with pharmacosiderite (which it closely resembles), or with an unidentified mineral of the alunite-beudantite group having a 7·04, c 16·6 Å, or with both. The new mineral, for which the name arthurite is proposed, for Sir Arthur Russell and Mr. Arthur W. G. Kingsbury, who independently collected material and suggested that it might be new, gives X-ray powder photographs with their three strongest lines at 4·28, 4·81, and 6·97 Å ; the photographs could be satisfactorily indexed on a monoclinic unit-cell with a 10·09, b 9·62, c 5·55o Å (all ± 0·01 Å), β 92·2° ± 0·2° and containing [Cu2Fe4(AsO4)3(OH)7.6H2O]. Chemical analysis on 1·1 mg gave: CuO 16·8, Fe2O3 32·4, As2O5 34·3, H2O 16·5 % (reduced to 100 % after deduction of 27·4 % quartz) ; sp. gr. 3·2 (calc. 3·07). Under the microscope arthurite is pale olive-green and very finely granular; n 1·78, birefringence low to moderate.

The phenocrysts of four porphyritic obsidians from Pantelleria have been analysed chemically, together with the residual glasses (liquids). Three new analyses of anorthoclase (one partial analysis), two of sodic ferrohedenbergite, one of fayalitic olivine, and two of cossyrite are presented together with their optical properties, and the relationship of these phenocrysts to their liquids is considered.

The experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O show that for synthetic liquids of similar composition to the normative salic constituents of the pantelleritic liquids, a potassic alkali feldspar crystallizes, whereas in the pantelleritic liquids phenocrysts of anorthoclase (Ab67–Ab61) occur. It is suggested that sodium metasilicate, which appears in the norms of all the pantelleritic liquids, may be responsible for the divergence in trend between the synthetic feldspar-liquid tie-lines and the pantelleritic feldspar-liquid tie-lines.

Feldspars, paracelsian and danburite, harmotome and phillipsite have alumino-silicate frameworks that consist of three out of the seventeen simplest ways of cross-linking chains of the feldspar type. The four-membered rings that form the feldspar type of chain are parallel and have two adjacent tetrahedra pointing one way and the other two the other way. Chains are produced by sharing oxygen atoms of oppositely-pointing tetrahedra of parallel four-membered rings. Cross-linking of chains produces frameworks containing parallel eight-membered rings. If alternate tetrahedra in the rings point the same way, a new type of chain is produced, and there are four simple ways of joining these chains together to make a framework. If all four tetrahedra in a ring point the same way, only one structure can be developed, which has been proposed already by Barrer et al. for the zeolite Na-P1 and other members of the harmotome family. If three tetrahedra in the ring point the same way, very many simple structures can be constructed, and the seven simplest of these are described. It is suggested from comparison of cell dimensions that gismondine and, perhaps, yugawaralite may belong to this series of hypotheti- cal structures. It is also suggested that some of the complexity and confusion in the harmotome group of zeolites may arise from hitherto unrecognized members of this hypothetical group. Major angular distortions from the ideal shape occur for the naturally-occurring silicates belonging to these structural groups.

Single crystal X-ray examination has shown that högbomite forms a series of polytypes, designated nH or nR, with hexagonal or rhombohedral lattices and hexagonal unit-cell dimensions a 5·72 Å, c 4·6 × n Å. The polytypes arise by variation, in a manner as yet undetermined, of the stacking sequence of approximately close-packed oxygen layers with interstitial cations on fourfold and on sixfold sites; the composition of 1/nth of a unit-cell may be represented as R2+1.0_1.6T4+0.2-0.4R3+3.7-4.3O2-7.6-8.0(OH)-0-0.4,, where R2+ = Zn, Fe, Mg, and R2+ = Fe, Al. The polytypes so far observed are 4H, 5H, 6H, 15H, 15R, and 18R. Minerals structurally related to högbomite are nigerite (3H) and taaffeite (4H). A new occurrence of högbomite, polytype 5H, with composition Ti1·7Fe1·6Mg6·3Al18·8Si0·2O40, is described from a spinel-free paragenesis in a magnesian skarn at Mautia Hill, Tanganyika. Another new occurrence in an aluminous xenolith in the Cashel gabbro in Co. Galway, Ireland, is recorded. X-ray powder data are given for two of the polytypes.

A new cobalt-iron mineral of composition close to CoFe has been found associated with awaruite (Ni3Fe) in the Red Hills serpentinites of the Wairau Valley, South Island, New Zealand. The name wairauite, after this locality, is proposed. Electron-probe analysis of the small grains so far discovered has shown that the nickel content is very low (0·5 ± 0·1 wt. %). Wairauite is optically very similar to awaruite but a tendency to form euhedral grains (octahedron and cube) and a slightly lower refiectivity are an aid to identification. No X-ray data are available at present, but it is suggested that the structure may correspond to that of the ordered CsCl type of the synthetic alloy of the same composition.

Florencite with ω 1·653, εd 1·661, G 3·457, a 6·971 ± 0·004 Å, c 16·42±0·13 Å, and rhombohedral unit-cell contents Sr0·42Ce0·52Ca0·14Al2·83(PO4)1·77(SO4)0·10 F0·41(OH)5·24(H2O)0·85 occurs in the Kangankunde carbonatite in Nyasaland, and goyazite with G 3·386, a 6·982±0·001 Å, c 16·54±0·02 Å, and rhombohedral unit-cell contents Ba0·05Sr0·50Ce0·41Ca0·08A2·78(PO4)1·62(SO4)0·19F0·30(OH)5·30(H2O)1·18 on the Wigu carbonatite in Tanganyika. Hydrothermal decomposition experiments in the Wigu material indicate stability below 535 ± 10 °C at pH2O 200 bars and 565 ± 10°C at pH2O 2500 bars ; both minerals exhibit endothermic peaks at 630°C in differential thermal analyses. The crystal chemistry of the goyazite series is discussed in the light of the two new analyses and of newly determined unit-cell dimensions for four other members of the goyazite series. Goyazite, florencite, and gorceixite appear to have crystallized during late-stage replacement processes or under supergene conditions in carbonatites.

A new occurrence of the tantalum minerals simpsonite and stibiotantalite is described from a lithium pegmatite at Mtoko, Southern Rhodesia. Optical, chemical, and powder X-ray data are given and the formula of simpsonite is considered; a new formula is proposed.

The cell parameters of fifteen arfvedsonites and four ferrohastingsites were determined using powder and diffractometer techniques and a least squares method of computation. Methods are proposed of distinguishing between calcium- and sodium-rich amphiboles, and between iron-rich and iron-poor varieties of these two groups, by their powder data. Within the arfvedsonite group the b parameter was found to increase with increasing Ca, Fe″, and (OH) and to decrease with increasing Na, F, and Li. The c parameter was found to decrease with increasing F and increase with increasing (Fe‴+Al+Ti). The parameters a, β, and a sin β all tended to increase with increasing Ca, Fe″, and F content. The cell volumes were found to vary with the b parameter. The ferrohastingsites were found to have much larger cell parameters than the arfvedsonites. An attempt was made to explain these variations from structural considerations.

New generalized extinction curves, derived from wave-normals located on a circular cone, are presented. They may be used with the universal stage for the accurate location of up to all three of the indicatrix axes, α, β, γ, of any biaxial crystal, or the optic axis of any uniaxial crystal.

A diagram of the relationship between d174 and % forsterite in olivines is presented in fig. 1 of the preceding paper by Jambor and Smith. In this note, statistical aspects of Jambor and Smith's data are considered in detail and compared to equivalent data by Yoder and Sahama (1957). This analysis leads to the conclusion that the relationship of mol. % Fo and d174 or d130 can be expressed as a straight line for Fo > 30 %. Olivines with Fo < 10 % are not related to this straight line.

Nine ferrohastingsites from the Younger Granites of Northern Nigeria and five from the nepheline-bearing rocks in the Marangudzi complex of Southern Rhodesia have been analysed chemically and their optical properties and unit-cell parameters determined.

All the minerals have compositions that can be expressed by the formula NaCa2(Fe″Mg)4Fe‴Al2Si6O22(OH)2, but the Nigerian minerals differ from those of Marangudzi in their lower alumina contents, generally greater degree of iron enrichment, lower potassium, and more variable fluorine contents. These differences are considered to reflect the differences in composition of their respective parent rocks. The compositional similarities between some hastingsites and barkevikite are commented upon and the pleochroism of barkevikite is briefly discussed.

Refractive indices of the analysed hastingsites have been plotted on the optical curves of Tröger. It is suggested that such curves are of limited use and that variations in unit-cell parameters might be a more useful method of determining the iron contents of hastingsites.

The variation in cell parameters with variation in chemical composition of the analysed minerals is also discussed.

Four new isotopic U/Pb ages from Cornwall are reported. The Pb206/U238 and Pb207/U235 ages for each specimen are concordant. The Pb206/U238 ages are : South Terras mine 225 ± 5 million years (Myr.), Redruth Area 124 ± 4 Myr., Wheal Owles mine 58 ± 3 Myr., and South Terras Area 62 ± 3 Myr. Cell sizes, calculated from X-ray powder photographs, decrease with decreasing ages of the specimens, indicating a progressive increase in the oxidation states of the uranium present. It is concluded tentatively that at least two periods of mineralization affected south-west England at about 290 Myr. and 225 Myr., and that a final event occurred at about 60 Myr. Whether this final event involved the remobilization of older uranium with loss of lead, or the introduction of additional uranium, or a combination of both effects cannot be established from this evidence. No unique explanation has been made for the 124 Myr. age.

Detrital grains, noted during the mineralogical examination of certain Snowdonian soils, have been identified as varieties of the mineral pumpellyite : the source of one variety has been located in dolerites exposed in the area, and the identity of this variety confirmed by X-ray analysis.

A detailed electron and X-ray diffraction study of a suite of nephelines from a wide range of paragenetic environments indicates that, in all cases studied, additional weak maxima of scattered intensity are present. Similar intensity maxima have previously been observed and described in an isolated case by Sahama. Thermal treatment at temperatures as low as 200° C causes these maxima to become noticeably diffuse, implying a structural transformation.

The results are presented of some recent feldspar studies that relate the fundamental concepts of order-disorder series to petrological problems. New diagrams have been prepared with coordinates ‘optical properties-chemical composition-structural state’. From direct observations made on natural minerals some conclusions have been reached about the sequence in which the solid-state transformations of ordering, unmixing, and twinning occur in feldspars.

A study has been made of the geological distribution of different structural types of alkali feldspars in single intrusive bodies (Tertiary granite, Palaeozoic syenite, Pre-cambrian charnockite), in magmatic complexes, and in different complexes of the Ukrainian Shield and the Caucasus Geosynclinal Province.

Epidote and clinozoisite are widely distributed in the andesitic lavas and tuffs of the Borrowdale Volcanic Series. Four main types of occurrence may be distinguished, namely quartz-epidote vein fillings; ‘Shap type’ veins, bordered by pink reaction zones rich in SiO2, Na2O, and K2O; autometasomatized lavas and tuffs; and tuffs subjected to alkali metasomatism. Epidote alone formed in the quartz-epidote veins; clinozoisite is found with epidote in the other types of occurrence.

Epidote and clinozoisite formed in vesicles at pressures of 10–280 kg/cm2, at temperatures of 300–550° C; epidote associated with pre-Bala mineral veins formed in the range 130–550° C at 1000 kg/cm2; epidote around the Caledonian granites probably formed up to 600° C at about 2000 kg/cm2. Clinozoisite crystallized together with epidote in the vesicles, demonstrating a gap in the clinozoisite-epidote solid solution series. Early formed epidote in vesicles is more strongly coloured than late epidote, although the compositions of the earliest and latest fractions are identical.

Nickel carbonate occurs in nature as a hexahydrate (hellyerite) and as a hydroxyhydrate (zaratite), but the anhydrous species is only known as an artificial product. All of the compounds show anomalous properties that have led to confusion in the literature.

Only the calcite structure was found for NiCO3 ; hydrothermal conditions appear to be essential for its synthesis, and a variation in cell size was found with changes in duration or temperature of synthesis, or both.

Hellyerite probably is [Ni(H2O)6]CO3, which would explain its ease of decomposition and rarity in nature. Zaratite is not a single mineral, but a composite of amorphous and fibrous components. The conditions under which NiCO3 might be expected to occur are discussed. Nickel bicarbonate formed in the reactions to produce NiCO3 ; its X-ray diffraction pattern, indicating cubic symmetry, analysis, and optical properties are given.

The behaviour of nickel in these compounds is best explained in terms of crystal field theory.

The lattice parameters of twenty calcium-bearing anorthoclases and potassium-bearing acid plagioclases have been determined by powder methods. The 201̄ spacing of the homogenized anorthoclases gives a good measure of the Or content irrespective of the amount of calcium present ; this spacing combined with α* (010): (001) may be used to determine the ternary composition of homogeneous feldspars more potassic than approximately Or15. The lattice parameters of feldspars less potassic than Or15 cannot be used for the determination of their ternary composition.

Chrysotile fibres, and also single crystals of lizardite, were dehydrated hydrothermally, and the resulting pseudomorphs studied by X-ray rotation photographs. The initial products are normally forsterite and disordered material; talc forms more slowly. At water pressures up to 600 kg/cm2, both forsterite and talc show strong preferred orientation. At higher pressures the talc is oriented but the forsterite tends to be unoriented. The orientation of forsterite formed at 550° C and 500 kg/cm2 differs from that found when chrysotile is dehydrated by heating in air. The orientation relationships suggest that, in the dry reaction, the forsterite orientation is controlled mainly by the octahedral layers of the chrysotile, while in the hydrothermal reaction it is controlled mainly by the tetrahedral layers. This does not support the generally accepted mechanism for the dry reaction. New mechanisms are suggested, for both dry and hydrothermal reactions, similar to that already suggested by us for the dehydration of brucite. The formation of talc in the hydrothermal reaction, and of enstatite in the dry reaction above 1000°, are also discussed. The dehydration of talc, to give enstatite, has also been studied using single crystals, and the mechanism of this process is briefly discussed.