Osumilite of composition closely approaching that of the K,Mg end-member occurs as a significant constituent of tridymite-enstatite-feldspar-cordierite-hematite-magnetite buchites at the contact of the Tertiary dolerite plug of Tieveragh, Co. Antrim; it is proposed that the mineral be known as osumilite-(K,Mg). Despite laboratory demonstration of the metastability of osumilite-(K,Mg) over a wide pressure-temperature range, the unaltered character and apparently constant composition of the Tieveragh osumilites raises the possibility of an osumilite stability field within the system K2O-MgO-FeO-Al2O3-SiO2.

The behaviour of 25 major and trace elements, together with mineral changes, has been examined in a deep weathering profile on the Carnmenellis granite, Cornwall. The bulk chemical and mineralogical composition of the regolith shows only minor changes from the parent rock indicating a low degree of chemical weathering. However, within the regolith marked differences occur between the fine fractions from the various horizons. Chlorite and degraded muscovite characterize the fines of horizons A+B and C whereas gibbsite and kaolinite characterize horizon D. Secondary minerals becoming base deficient with depth is the reverse of normal behaviour and two possible theories are advanced to explain this anomaly. Most of the trace elements are concentrated in the < 10 µm fraction of the various horizons, in part by an association with degraded muscovite. Tin and Zr occur in resistates and Be, Sr, and Co are mainly lost during weathering.

Several uncommon phosphate minerals including tarbuttite, parahopeite, scholzite, and collinsite occur in near-surface gossans at the Reaphook Hill zinc prospect. Electron-probe microanalyses of the tarbuttite and scholzite compare well with published analyses of specimens of these minerals from the type localities in Zambia and Bavaria respectively. However, microanalyses of the parahopeite are significantly different from published analyses of parahopeite, and suggest that it may be one member of an isomorphous series in which six-coordinated Zn can be replaced by other cations, including Fe, Mg, and Mn. Furthermore, crystals of the Reaphook Hill parahopeite are compositionally zoned with their centres enriched in Fe and Mn relative to the margins, which are enriched in Mg.

Zincian collinsite occurs at Reaphook Hill as concentric crusts composed of very fine grained radially oriented plates. Electron-probe microanalyses of this material are significantly different from published analyses of collinsite from the type locality in British Columbia: Zn instead of Fe occupies the divalent cation sites; this material may be part of an isomorphous series that includes fairfieldite. Microanalyses also indicate that the Reaphook Hill collinsite crusts are compositionally zoned such that Zn:Mg ratios vary from the base to the outer edges of the crusts. This zoning is interpreted as the result of variation in the availability of Zn and Mg during growth. Zoning of similar type, but due to variation in the Fe: Mg ratio, was observed in concentric crusts of collinsite from the type locality in British Columbia.

Hematophanite, ideally Pb4Fe3O8Cl, is tetragonal, space group P4mm, with a = 3·92 and c = 15·31 Å. The crystal structure, which has been refined to an R-factor of 9·1 %, may be described as an interlayering of perovskite-like units with a cesium chloride or, alternatively, a nadoritelike unit. The nadorite-like part consists of a double layer of PbO4Cl4 distorted square antiprisms. Iron atoms are in 6-fold (tetragonal bipyramidal) and 5-fold (square pyramidal) coordination.

Twenty-six compositions in the system MgO-Al2O3-SiO2-H2O were investigated under conditions of 1 and 2 Kb water pressure and temperatures between 300 and 700°C. The solid solution for 7 Å and 14 Å chlorites has been delimited as well as that of the expanding phases (tri- and dioctahedral montmorillonites and expanding chlorites). Negative slopes were found for the transformation montmorillonite → expanding chlorite, and expanding chlorite → chlorite+quartz and a positive slope for 7Å → 4 Å transformation. The relative positions of the reactions chlorite+quartz → cordierite+talc, chlorite+andalusite → cordierite and chlorite+corundum → cordierite+spinel are located between 500 and 65°C.

Cell dimensions of the synthetic chlorites can be correlated with their chemical composition. Solid solution in synthetic minerals compares well with 325 analysis of natural minerals from the literature, indicating that the chemiographic relations between phases in the simplified synthetic system are applicable to natural mineral assemblages. The phase relations indicate that at low temperature the 7Å aluminous chlorite is not stable with quartz or another silica phase.

Killalaite occurs in a thermally metamorphosed limestone at Killala Bay, near Inishcrone, Co. Sligo. It is a secondary mineral and occurs in cavities and veins with calcite or afwillite in spurrite-wollastonite rocks. It is found in well-formed colourless crystals, up to 2 mm long, with a characteristic penetrative twinning observed under crossed nicols as a cross ‘bow-tie’, cleavage (100) perfect, (010) good, (001) poor. The crystals are monoclinic, biaxial, 2Vα26°, refractive index α 1·635 and γ 1·642, optic axial plane (010), β = [010] (elongation), γ: [001] ≈ 16°, density calculated from RI ∼ 2·88, provisional approximate cell dimensions a 9·3, b 9·9, c 7·7 (±0.1 Å), β ∼105° and Z = 2. Electron microprobe analysis gave CaO 57·o, SiO2 39·8; H2O (by difference) 3·2; MgO, Al2O3, and Fe2O3 together < 0·3 weight %, composition 2Ca3Si2O7.H2O. The stronger lines of the X-ray powder pattern are: 3·03, 2·824, 2·724, 2·275, 2·224, 1·688, 1·673, and 1·413 Å. It probably formed in a CO2-deficient hydrothermal environment in the PT range of 350° to 500 °C and 500 to 3500 bars. The mineral is named after the name of the locality, Killala Bay, and should be pronounced as killalahite. Type material is preserved at the Ulster Museum and at the British Museum (Natural History).

Electron microprobe and textural data are used to deduce a crystallization history for plagioclase in the Grand Manan Island (Canada) diabase sheet. The plagioclase crystals consist of a prominent core (An79-67) surrounded by a distinctly more calcic zone (An82-76), which grades into a normally zoned mantle (An73-40). This zoning pattern is interpreted as reflecting the elevation of a dry diabase magma from at least 10 to 20 km to near surface conditions.

The formation of porphyroblasts by replacement and displacement of pre-existing S surfaces is examined in the light of textural evidence displayed by sediments hornfelsed by the Bushveld Igneous Complex. Helicitic biotite, staurolite, and cordierite all show replacement and displacement textures, the former being more common. The evidence indicates that porphyroblasts arc capable of creating local pressure differences that disturb the pre-existing banding, even in hornfelsed rocks that have suffered neither regional metamorphism nor orogenic movement.

Seven manganese minerals have been observed in geodes from Chihuahua, Mexico. Ramsdellite occurs in a number of different habits, but the morphology and precession X-ray patterns suggest that it is pseudomorphous after groutite. Pyrolusite is found only in association with rams-dellite and is believed to be an alteration product of that mineral. Todorokite generally occurs as a late-stage fibrous overgrowth in many of the geodes. Ranciéite, birnessite, hollandite, and cryptomelane are present in trace amounts. This assemblage of manganese minerals is typical of supergene mineralization.

Peridotitic rocks in the Massif du Sud, southern New Caledonia, have suffered about 30 % serpentinization. Optical studies combined with X-ray and chemical analyses indicate that chrysotile and lizardite are the dominant serpentine species present. Serpentinization probably occurred during emplacement of the Massifs in their present overthrust position.

An unusual paragenesis of chromite, nickeline, cordierite, and red-brown mica occurs in veins, pods, and schlieren in serpentinized ultramafic rocks at La Gallega and Los Jarales in Máilaga province, Spain. The chromites in this paragenesis are low-ferric, ferrous-rich aluminous chromites with exceptionally high vanadium content ranging between 1·8 and 2·9 wt % V2O3 and high zinc content ranging between 0·5 and 1·0 wt % ZnO. A comparison of chromite composition with available experimental data, together with paragenetical and textural evidence, suggests that the chromite, cordierite, and red-brown mica crystallized at magmatic temperatures between approximately 1300° and 1000°C at low oxygen fugacities and maximum pressures below 10 kb, probably near 5 kb.

The carbonate phase of impure sideritic concretions from a sequence of argillaceous sediments from the Westphalian of Yorkshire has been analysed using an acid-dissolution technique and corrections allowing for the decomposition of non-carbonate minerals applied to obtain accurate compositional formulae.

The siderites show considerable compositional variation, some containing Ca in excess of anticipated substitutional limits. Whilst these may be metastable it is suggested that coupled Mg-Ca substitution could provide an explanation.

The behaviour of iron in the weathering of biotite in a sedentary soil profile developed on appinite has been investigated using the Msbauer effect. Both Fe2+ and Fe3+ in sites with cis and trans hydroxyl groups were observed at various stages in the weathering sequence. The results indicate that there is little or no ordering of the octahedral cations in the fresh biotite and hydrobiotite, but in the more weathered interstratified vermiculite-chlorite samples Fe3+ concentrates in the more distorted sites. No evidence was obtained for the presence of an anisotropic recoil-free fraction.

A new example of the Nevada twin from the Mont-Dore (Massif Central, France) is described. A structural interpretation of the twin is given assuming a (01) composition plane parallel with the chains of the feldspar structure.

A detailed petrographic and microprobe study was carried out on atoll-shaped garnets from the aureole of the Ardara pluton, in which regionally metamorphosed rocks have been affected by thermal metamorphism. Textural evidence showed that the atoll garnets are formed by replacement mainly by biotite, some plagioclase, quartz, and opaques, and may proceed from the garnet interior towards the margins, or from the margins into the core. Core replacement occurs mainly in those garnets enclosed by quartz or quartz-rich matrix.

Microprobe analyses are presented of garnets in different stages of atoll formation. Compositional profiles at the garnet margins show abnormal (reversed) zoning with increase in MnO and Fe2O32, marked decrease of CaO, and decrease of MgO towards the extreme edge. With decreasing width of the atoll rims, there is generally an increase in MnO and to a lesser extent Fe2O3, a decrease in CaO; MgO is variable. The microprobe data support the textural observations and a model for atoll formation involving prograde resorption (i.e. garnet replacement during temperature increase) is presented.

Clinopyroxene crystals with complex lamellar intergrowths of orthopyroxene and garnet are described. The orthopyroxene lamellae are irrationally orientated within the clinopyroxene host although the optical relationships of these minerals are consistent with (100) exsolution in clinopyroxene. The intergrowth is nevertheless considered to be the result of exsolution processes. Possible modes of origin are reviewed together with the general significance of exsolved pyroxenes in eclogite- and granulite-facies associations.

A 1 M solution of potassium permanganate was sealed in a gold capsule and was treated under conditions of temperature and pressure up to 800 °C and 2·5 kb. Different kinds of products were produced under different conditions of synthesis, and potassium manganate, birnessite, cryptomelane, hausmannite, manganosite, and a new phase appeared. The chemical composition of the new phase was determined as K2Mn4O9 and its X-ray powder diffraction pattern was indexed on a hexagonal cell with a 11·295 and c 21·870 Å.

In contrast with some other hydrous aluminum phosphates, vashegyite is crystalline. Simultaneous application of several assumptions leads to the hypothesis that it has a structure analogous to that of kaolinite, and thus contains tetrahedral sheets, n[X2O5], where X is both P and H3 (in lieu of Si). Thus, the theory involving [HXO4] is extended from phosphates with isolated tetrahedral groups [XO4] and framework linkages [XO2] to sheet linkages [X2O5]. Kingite also may have a kaolinite-like structure.

Occurrence and major- and minor-element compositions of chlorites from plutonic rocks of the central Sierra Nevada batholith have been studied in detail.

Chlorite has formed largely as an alteration product of biotite, presumably late in the magmatie history of the granitic rocks as a result of subsolidus reaction. Major-element composition of chlorite is strongly influenced by composition of coexisting biotite. Minor-element differences between the two minerals can, for the most part, be attributed to structural differences. The minor-element data, particularly for Cu, indicate that the process of chloritization is not a likely mechanism for release of ore metals from primary minerals.

Barea and Emery are typical mesosiderites, whereas Dyarrl Island is unique in having the lowest metal content and the highest FeO/(FeO + MgO) ratio of any meteorite of this class. Average pyroxene compositions have been determined for the individual mesosiderites, and show a sequential increase in FeSiO3 content, ranging from a mean of 21 mole % for Veramin to 45 mole % for Dyarrl Island. Steinbach may possibly be classed as a mesosiderite, since its mineralogy is identical except for the absence of plagioclase. Dalgaranga and Mt. Padbury may be a paired fall. The chemical and mineralogical composition of the silicate material of the mesosiderites is similar to, but not identical with the howardites; the overall range of composition is greater in the mesosiderites.