A new mineral, nukundamite, from a Fijian kuroko deposit is described. It has been referred to as idaite, Cu5FeS6, or Cu5.5xFexS6.5x. Electron-probe analyses gave Cu 56.51, Fe 9.64, and S 33.51, total with traces of Ag and As is 99.79%. Hexagonal with a 3.782, c 11.187 Å, Z = 1. Empirical formula Cu3.37, Fe0.66, S3.97 on the basis of eight atoms. Copper-coloured, metallic lustre, cleavage {0001} perfect. Density (g cm−3) 4.53 (calc), 4.30 (meas). Strongly pleochroic from reddish orange (Ro) to pale grey (Re). Parallel extinction. Very strong anisotropy with pale green-grey rotation colours. Reflectance values and quantitative colour values in air and oil are given. Similarities with the covelline structure are indicated.

An attempt to fully describe idaite from the Ida mine was unsuccessful. The description of the type material therefore remains inadequate.

The Lower Cambrian Weymouth Formation at Nahant, Massachusetts, consisting of interbedded pelitic and calcareous rocks, was intruded by the Nahant Gabbro. The predominant metapelitic mineral assemblage of the contact aureole is quartz-muscovite-chlorite-magnetite-ilmenite. The calcareous hornfelses exhibit a varied mineral assemblage, and in some cases the variation can be spatially related to the intrusive. A number of cross-cutting thin mineral veins, many containing prehnite, are characteristically associated with these calcsilicate rocks.

The minerals have been analysed by electron microprobe and this work indicates the presence of a possible solvus in the Fe3+-Al epidote solid solution series. At the physicochemical conditions of the formation of the Nahant hornfelses, the ferric mole fractions of coexisting epidotes are 0.49 and 0.98.

Comparison with experimental work shows that the conditions of the contact metamorphism were T ≃ 500°C, Ptotal ≃ 2 kb, and XCO2 ≃ 0.2. However, the present assemblages are the result of a later low-grade regional metamorphism, the ultimate product of which was prehnite.

Three groups of amphibolites within the Moine rocks of Sutherland have been analysed for twenty-six elements by XRF analysis. (1) Very rare epidotic hornblende schists within pelite, possibly derived from volcanic ash deposited penecontemporaneously with the pelite (2 analyses). (2) Major and minor bodies of ‘early Moine’ schistose garnetiferous amphibolite intruded early in the deformational sequence (41 analyses). (3) Small bodies of ‘Loch a' Mhòid metadolerite’, intruded later in the deformational sequence (23 analyses). Rare Moine ultrabasites are associated with these metadolerites and probably formed from the same magma as olivine cumulates. These ultrabasites (5 analyses) are chemically distinct from Lewisian ultrabasites (both in the basement sheets within the Moine rocks of Sutherland and in the foreland Lewisian rocks) and from the Glen Urquhart ultrabasic body (3 analyses).

Using variation diagrams it is inferred that chemical variation in these amphibolites is due largely to pyroxene and plagioclase crystallization in the early amphibolites and olivine, pyroxene, and plagioclase crystallization in the Loch a' Mhòid metadolerites and ultrabasites, followed in both cases by high-level fractionation. This has led to high values of incompatible elements in many of the early amphibolites. Although geochemical discrimination diagrams tend to give ambiguous results for many of these rocks, the early amphibolites are shown to be tholeiitic in nature with a chemistry transitional between within-plate and island-arc type basaltic magma. The Loch a' Mhòid metadolerites are mildly alkaline within-plate type basaltic magma.

A nomenclature is proposed for the alluaudite and wyllieite complex series which is based on sequentially distributing the cations in the cell according to increasing polyhedral size, matching that size with increasing ionic radii of the cations. For oxidized members, the largest site may be partly occupied to empty after all the cations have been distributed. This is supported by structural study.

For alluaudites, the cell formula is X(2)4X(I)4 M(I)4M(2)8(PO4)12 and is written according to decreasing size of the discrete sites. The X(I) and X(2) sites are appended as suffixes in the trivial nomenclature, that is specific name—X(I)X(2).

For wyllieites, the cell formula is X(2)4X (1a)2X(1b)2M(I)4M(2a)4A14(PO4)12. The X(1a), X(1b), and X(2) sites are appended as suffixes in the trivial nomenclature, that is specific name--X(la) X(Ib)X(2).

The nomenclature proposed is:

Seventeen analyses are discussed (of which five are new) for alluaudite and four analyses (of which three are new) for wyllieites. Their distribution is given parenthetically above. One analysis revealed predominant Mg2+ in M(2). It is named maghagen-dortite.

Six new analyses are presented for the arrojadite family of minerals including re-examination of dickinsonite from Branchville, Connecticut. Al3+ is always present. We propose X1 Al (OH,F)(PO4)12, Z = 4, where X = large cations (K+, Ba2+, Pb2+, etc.), Y = Na1+, Ca2+, and M = Fe2+ Mn2+, Mg2+. A range of cations X, Y, M, and Al between 76.8 and 85.9 in the cell (84 for proposed formula) suggests the likelihood of some vacancies in the structure.

A study has been made of topaz crystals showing a sectoral texture related to the growth of the crystal, and showing anomalous optical properties within the sectors. The growth surfaces responsible for the development of most of the sectors have been identified. The fluorine/hydroxyl sites in topaz are symmetrically equivalent in the solid crystal, but at a growth surface this equivalence may be lost, resulting in a reduction in the crystal symmetry and the ordering of fluorine and hydroxyl, the ordering scheme being retained once the crystal has formed. The reduction in symmetry expected to be produced by this ordering is in general agreement with the actual reduction in symmetry indicated by the optical properties. Heating of the topaz to about 950 °C results in the almost complete disappearance of the optical anomalies due to disordering, which may be related to the loss of hydroxyl from the crystal. Anomalous pyro- and piezoelectric phenomena may also be the result of ordering. Published optical and X-ray determinative curves for fluorine in topaz may give erroneous results for ordered topaz.

From the microscopic examination of lightly etched, cleaved (111) surfaces of banded blue fluorite (Blue John, from Castleton, Derbyshire) it is concluded that the coloured lamellae are not necessarily or exclusively associated with lattice-line imperfections. Elemental analyses, by electron dispersive methods, of fresh cleavage (111) surfaces in the immediate vicinity of small inclusions (c. 10 µm diameter) possessing associated coloured haloes detected no appreciable concentrations of impurities. From the evidence available, it is suggested that the zones of blue colour consist of colloidal calcium resulting from radiation damage caused by the intermittent deposition of radioactive material on the surfaces of fluorite during crystal development. The dispersion of colloidal calcium produced is particularly stable as a consequence of the close correspondence between lattice spacings in calcium fluoride and in calcium metal.

The mineral deerite is restricted in occurrence to blueschist facies meta-ironstones. From the ideal formula of Si12O40(OH)10 there are a limited range of substitutions: Ti and V for Fe3+ and Mn on the Fe2+ site. In the present survey of deerite compositions from the majority of deerite localities the maximum substitutions have been 2Ti and 3Mn in the above formula. There is an at present unexplained anomaly in the totals of a number of the analyses indicating that there may be variations away from the ideal formula towards increased hydroxyl and decreased cation contents.

From differences between the mineral compatibilities of deerites from the higher-grade Franciscan exotic block blueschist localities and those from more conformable Alpine blueschists, in conjunction with previous work on the experimental stabilities of the low-pressure/low-temperature hydrous iron silicate minerals, it has been possible to map the form of the low-pressure deerite stability field involving reactions of greenalite or minnesotaite or grunerite with magnetite and quartz. Within the deerite stability field, further deerite-forming reactions involve the breakdown of riebeckite and the hydrous incompatibility of magnetite with quartz; both are very dependent on the activity of sodium. A low-pressure stability to 4 kb at 200 °C and 6 kb at 300 °C are estimated from these low-temperature breakdown reactions. This fits in well with the high-pressure deerite stability determinations of Langer et al. (1977).

Classical clay mineralogy determinations and electron microprobe analyses of weathering minerals developed in altered two-mica granites indicate that the chemical forces that produce new minerals are often constrained to small volumes, frequently on the scale of a mineral grain or contact between two grains in the granite.

Chemical potentials such as pH, alkali and alkaline earth and silica activity in the altering aqueous solutions provoke a destabilization of pre-existing minerals, which recrystallize locally to give a new multimineral product. The chemical composition of the new phases is largely governed by the relative concentrations of the elements present in the former minerals.

Three mineral facies were observed in the weathered granites: initially a sericite-beidellitic type, then a beidellite-kaolinitic type, and finally a last stage kaolinite-oxide facies assemblage. The position of each facies is not restricted to a given depth in the profile but the relative proportions of each facies found in a thin-section size sample change towards the kaolinite-oxide facies.

The global rock chemistry reflects the type facies predominant in each sample. The first two facies are roughly silica conservative while the kaolinite-oxide facies loses silica as well as alkali and alkaline earths.

Geochemical and clay mineral studies of rock alteration should consider problems of mineral genesis at very localized sites.

Compositions of glaucophanes and actinolite-hornblende solid solutions occurring in chemically similar metabasaltic rocks from blueschist terranes in east-central Shikoku, W. California, Valtournanche (W. Alps), and W. Liguria are compared. Chemical contrasts among coexisting Na and Ca amphibole pairs, which reflect disparate P-T histories under the presumed attendance of local equilibrium, include: Na contents are rather high among barroisitic hornblendes from the western and Ligurian Alps, as well as among high-grade tectonic blocks from California; in contrast, actinolitic amphiboles from both lower-grade Franciscan tectonic blocks and in situ schists and the blueschists of Shikoku are impoverished in Na relative to blue-green hornblendes. Sodic amphiboles contain less than 0. 5 Aliv per formula unit, whereas Alvi is very high; a situation reversed among calcic amphiboles. The Na + Ca contents ofglaucophanes are strongly clustered around the sum of 2.0 (i.e. A site vacant) whereas calcic amphiboles have a wider range with the A site variably occupied. No solvus has been detected within either sodic or calcic amphiboles under blueschist facies conditions. For low-grade metabasaltic parageneses, a miscibility gap separates these two amphibole groups; at relatively high grade such compositions have sodic calcic amphiboles of barroisitic type; this may mean that glaucophane + hornblende assemblages are metastable, accounting for textural relations indicating that the sodic amphibole typically did not grow at the same time as the barroisite. Ti, Mn, and K appear to be concentrated in calcic amphibole compared to coexisting glaucophane, probably in the M2, M4, and A sites, respectively.

Contrasts in coexisting amphibole tie lines are thought to be a consequence of the fact that the parageneses of Shikoku and California reflect high P and very high P prograde P-T paths respectively, whereas those from Valtournache and W. Liguria show evidence of decompression recrystallization (or back-reaction) of high P (i.e. eclogitic) protoliths. Comparison of the inferred physical conditions operating during the production of these four contrasting paragenetic sequences allows the provisional assignment of a P-T stability region for barroisitic amphibole in metabasaltic rocks as: P 4–5 kb at c. 350°; P 5–7 kb at c. 450 °C.

57Fe Mössbauer spectroscopy has been used to determine the Mg2+-Fe2+ distribution between the two crystallographically inequivalent cation sites in hypersthenes from a stratigraphic tephra sequence of the Taupo Pumice formation, North Island, New Zealand. From these distributions and X-ray fluorescence analytical data, cooling histories were constructed using previously determined Mg2+-Fe2+ equilibrium isotherms for orthopyroxenes. The results showed that this cooling history geothermometer could be used to enable airfall and airflow tephra to be distinguished. In addition, they showed that the cooling rate of the above deposits correlated better with the deposit thickness, than with the distance of the deposit from the source.

Previous interpretations relating to precipitation of anorthoclase megacrysts from alkaline basaltic magmas under high-pressure conditions are contradicted. It is proposed that these feldspars were crystallized above the solvus from hydrous melts at shallow depths, under low-pressure equilibrium conditions. These melts had been considerably enriched with volatiles of sodium, strontium, and barium, and to some extent also of rubidium.

Electron-microscope observations of the oxidation products of olivine suggest that oxidation results in the formation ofplatelets of an ‘oxidized olivine’ phase, which has a superstructure of the olivine structure. Subsequently this intermediate phase breaks down by a cellular decomposition process to a eutectoidal intergrowth of magnetite and pyroxene. This intergrowth is responsible for the dendritic magnetite inclusions commonly observed in some olivines from Rhum.

Specimens of layered baryte containing a range of pyrite forms, including framboids, framboidal aggregates, spherulites, polyhedra, and zoned euhedral crystals from Kempfield, New South Wales, Australia, have been examined by mineragraphy, X-ray diffraction (XRD), X-ray fluorescence (XRF), and electron-probe microanalysis (EPMA). These investigations showed that the pyrite occurred in two forms—an arsenical variety containing approximately 5% arsenic in solid solution within the pyrite and a non-arsenical variety. EPMA studies indicate that the framboids and the cores of the zoned euhedral crystals have similar levels of arsenic and a genetic relationship is deduced to exist between these forms.

Electron-microscope observations have been used to follow the low-temperature dehydration of boehmite, and show that a coherent skeleton of γ-Al2O3 with a porous texture is produced. For material fired in air the development of a number of pore systems is described. Of these the most important, as far as the dehydration process is concerned, is a lamellar pore system parallel to (001)γ and having a regular spacing of the order of 35–40. Å. A much faster dehydration process is induced by exposure to the electron beam and results in the formation of a more randomly oriented pore structure. The observed microstructure and the calculated internal surface area are compared with the results of previous adsorption studies.

A statistical study on the modal analysis data of eight minerals of the Meldon aplite is made with a view to determining their distribution patterns in the rock. Various parameters and statistics have been calculated and the results are tabulated.

Topaz and elbaite are considered as censored samples; ‘normalization’ in the case of these two minerals is obtained by double probit transforms. Findings of the statistical study are correlated with the modes of formation of the minerals.

Analyses are given of four pairs of coexisting pyroxenes; their Kp ranges from 1.282 to 1.369, suggesting equilibration temperatures as high as 1200 °C and pressures of the order of 8 to 9 kb.

CALC-GRANULITES at Garividi are intruded by microcline granites and pegmatites. At the contacts fluorapatite, fluorian allanite, ferroan sahlite, and epidote occur; rarely, the epidote is associated with a partly metamict allanite, an analysis of which is given, also X-ray data for the unheated and heated mineral.