The field relationships, petrography and mineralogy, including probe analyses of olivines, pyroxenes, and plagioclases, of the Karroo dolerite dike swarm of southern Malawi, are described. Rock analyses are given and define a typical tholeiitic trend of iron enrichment. The most magnesian rocks are thought to represent fairly picritic liquids. A number of authors have shown that there are significant geochemical differences between the basaltic rocks of the northern and southern parts of the Karroo outcrop in southern Africa, notably for elements such as Al, P, K, Ti, and Ba. The Malawi data presented here, however, show trends that are the reverse of those found hitherto, the Malawi dolerites being closely comparable with the rocks of the central Karroo basin of South Africa and Basutoland. The striking regional variation in the geochemistry of the Karroo basaltic rocks over southern Africa is independent of the tectonics or position vis-à-vis the central Karroo basin, and so probably reflects mantle heterogeneity.

Coexisting zoisite and clinozoisite in seventeen specimens from six localities in Japan have been studied with the electron-probe microanalyser. Zoisite and clinozoisite are commonly zoned, but compositional gaps between them are systematic. Referring to the metamorphic grade of the host rocks, a temporary and schematic phase-diagram for the system Ca2Al3Si3O12-(OH)-Ca2Al2Fe3+Si3O12(OH) is presented. With increasing temperature, in the range of low- to medium-grade metamorphism, the compositional gap between the two epidote-group minerals shifts towards higher Fe3+ compositions.

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.

The name hauchecornite has been applied in the literature to a group of minerals with similar crystal structures and the general formula (Ni,Co,Fe)9ABS8, where A and B are As, Sb, Bi, and Te in two nonequivalent structural sites. A classification of the group is proposed based on the relative abundance of the elements As, Sb, Bi, and Te in the two sites. Hauchecornite is redefined as Ni9BiSbS8, the end-member being closely approximated by the composition of the material from the type locality, and a new name, bismutohauchecornite, is proposed for the Ni9Bi2S8 end-member. The classification, redefinition, and the new name have been approved by the Commission on New Minerals and Mineral Names, IMA.

The coexistence of clinopyroxene (Wo18 En32 Fs44 Rh06) and pigeonite (W08 En35 Fs50 Rh07) in a meta-ironstone from Scourie is described. By comparison with experimental data, the temperature at which they were in equilibrium is estimated as at least 1000°C. The exsolution history of the two pyroxenes is documented. As the rock is an unambiguous metasediment, its presence among granulite facies rocks containing evidence of metamorphism at pressures greater than 10 kbar constrains at least part of the Scourie complex to a supracrustal origin and indicates that tectonic processes of burial were operative in the late Archaean.

Microprobe analyses of relict pyroxenes in meta-basalts from thrust slices of the Peel fault zone, Glenrock Station, NSW, demonstrate that the majority of these rocks have a tholeiitic affinity. Those of Ordovician to L. Devonian(?) age in the upper thrust slices show a greater Fe-enrichment, and lower wollastonite, Al and Ti contents than their Devonian counterparts in the lowest thrust sheet. The pyroxene data also indicate that some rocks are geochemically transitional between tholeiitic and alkaline magmas. The relict pyroxenes in metadolerites emplaced in the Devonian sequence show that these rocks are transitional to tholeiitic in character and that they appear not to be consanguineous with the metabasalts.

Arsenohauchecornite, Ni9BiAsS8, and tellurohauchecornite, Ni9BiTeS8 from the Sudbury area, Ontario, are now recognized as distinct mineral species in the Hauchecornite Group and not as varieties as described previously. Descriptions of these two minerals are given, summarized from Gait and Harris (1972).

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.

Losses of FeO, Na2O, and K2O from an alkali olivine basalt melted above the liquidus temperature in Pt80Rh20 wire loops are reported as a function of temperature, time, and PO2. Increasing temperature and decreasing PO2 increase the losses. Compared to open capsules the wire-loop container reduces FeO loss to a minimum but may exacerbate Na2O loss. Nonetheless for most types of experiment involving melt these losses are acceptable.

A new synthesis of Mg-Al double hydroxides is described, that gives products relatively low (0.8–1.0%) in CO2. The general formula [Mg1−xAlx (OH)2][(OH)x(H2O)0.81−x] with 0.23 ≤ x ≤ 0.33 is proposed. Cell parameters and thermal data are given. The c-axial length decreases with Al-content; this is attributed to increased electrostatic attraction between layers.

Analcime, occurring both as small, pale-brown trapezohedral crystals and pale-brown slaggy masses, is reported growing into and replacing aragonitic shell debris in a heavily faulted, 15-km long, coastal exposure of L. Jurassic marine sediments. Associated minerals include authigenic baryte, sphalerite, pyrite (some framboidal), celestine, aragonite, calcite, dolomite, quartz, and solid hydrocarbon. No other zeolites have been found. Its modes of occurrence, crystal morphology, and relationships to these minerals are described. The analcime appears to be restricted in occurrence to the W. Somerset coast. Partial chemical analysis of one sample is given. The brown colouration is suggested to be due to organic compounds. XRD analysis of five samples reveals progressive decrease in silica content towards western extremity of the exposure. No evidence of volcanic ash contamination of the sediments is found. A hydrothermal origin is discounted; formation of the analcime by purely diagenetic processes is suggested.

From the Buranga pegmatite, Rwanda, three occurrences of eosphorite-childrenite series minerals are described in assemblages of Li-Fe-Mn-bearing phosphates and in relationship with quartz-feldspar masses in close association with these assemblages.

Electron-microprobe analyses of these terms (Mn,Fe) AlPO4(OH)2 · H2O afford the following Fet/(Fet + Mn) ratios: 0.548, 0.341, and 0.200. However, thanks to an investigation of the refractive indices and pleochroism, the presence of ferric iron is demonstrated and an oxidation phenomenon, similar to that previously known in rockbridgeite, is suggested to explain the existence of oxidized eosphorite. The oxychildrenite problem is raised again.

The Fet/(Fet + Mn) ratios characterizing the eosphorite sensu lato would be correlated with the Fe and Mn content of the associated major phosphates, the chemical analyses of which are also provided. The investigation of the structural relationships shows that eosphorite might form a paragenesis with montebrasite and that these phosphates replace Na- or K-feldspars by a hydrothermal process. Owing to the apparent dependence of the chemical properties of the couple eosphorite-montebrasite on those of the earlier-formed phosphates, it seems likely that the formation period of eosphorite-childrenite series minerals was prior to the oxidation and ferrisickleriteheterosite formation.

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.

Wolfeite (Fe,Mn)2PO4(OH) and barbosalite FeIIFeIII2(PO4)2(OH)2 are the principal phases in phosphate pods from the core of a zoned altered granitic pegmatite at Thackaringa. Chemical analyses of the wolfeite show it to be Fe-rich and extends the triploiditewolfeite isomorphous series (Frondel, 1949) from ΣFe/Mn = 3·39 (this study) to greater than 100 (Clark and Couper, 1979).

A detailed electron probe study of irontitanium oxide intergrowths from slowly cooled granitic rocks from the granulite grade, Archaean Scourian complex of north-west Scotland has yielded a wealth of information about magmatic and metamorphic temperatures, subsolidus cooling, and the behaviour of the fluid phase during cooling. Five stages are documented in the cooling history of granites and trondhjemites which include: (i) magmatic-subsolidus cooling (1035 °C–890 °C); (ii) granulite facies metamorphism and the accompanied expulsion of a hydrous fluid phase (890 °C–830 °C); (iii) subsolidus cooling following the peak of the granulite facies metamorphism (830 °C–660 °C); (iv) the localized reintroduction of water into the rocks during retrogression (660 °C–530 °C) and (v) subsolidus cooling and re-equilibration in the presence of a finite amount of H2O (530 °C–320 °C).

Veins cutting the Brioverian sediments at Le Pulec, Jersey, were worked in the 1870s for silver and lead. Investigation of the mineralogy of both the sediments and cross-cutting veins has shown a complex mineralization history. Siltstones close to the contact with the North-West granite have been bleached, silicified, and dolomitized, and this has been accompanied by the introduction of pyrite, marcasite, and arsenopyrite. Later vein mineralization contains sphalerite and ferroan dolomite accompanied by lesser amounts of galena and chalcopyrite and minor quantities of argentian tetrahedrite, native antimony, bournonite, cubanite, plagionite, and stibnite. The mineral assemblage is unlike the other mineral occurrences of Jersey that have previously been described.

The symmetry of diamond is still disputed, and is usually regarded as 4/m3̄2/m or 4̄3m. The (rare) development of tetrahedral morphology has been cited in favour of the lower symmetry. A tetrahedral crystal of c. 4 mm edge-length was studied for evidence relevant to this controversy. Three similar quadrants have nearly plane tetrahedron faces, each surrounded by six curved surfaces belonging to forms {hkl}. The fourth (unique) quadrant, containing (IĪĪ), differs topographically. In the three similar quadrants the tetrahedron faces exhibit coplanar banding of trigons parallel to (IĪĪ), and the curved surfaces have striations parallel to it. The banding and striations are interpreted as stratigraphic etching of nitrogen-rich layers parallel to (IĪĪ) in a type I diamond. Slip, and apparently polygonization, occur in the unique quadrant. The three similar {III} faces were formed by cleavage, and the curved surfaces, banding, and striations by subsequent dissolution. It is probable that (IĪĪ) was part of an original octahedral surface, since tetrahedra of diamond are so rare. The tetrahedral morphology does not necessarily indicate that this crystal belongs to class 4̄3m. The doubt cast upon tetrahedral morphology (and the inferred twinning on {100} or about (100>) as evidence in favour of lower symmetry strengthens the case for assigning diamond to class 4/m3̄2/m.

Losses of Fe and Na from a typical basaltic liquid during experiments using the wire-loop method have been determined for both Pt and Ag40Pd60 alloy wire by electron-microprobe analyses of the quenched glasses. The amount of Fe lost is dependent on the duration of the experiment and is particularly sensitive to the ratio of sample to metal in direct contact with it. Fe losses can be minimized by using the thinnest wire possible in conjunction with the largest amount of sample that will adhere to the wire and, if Ag-Pd wire is used, can be kept below the 1% level even in long experiments. Much more serious in terms of changes in the bulk composition are losses of Na, which appear to be in direct linear proportion to the length of the experiment and in inverse proportion to the mass of the sample. Even when samples in excess of 200 mg are used the Na losses are unlikely to be less than 5% except in experiments of less than a few days' duration or on samples with very low initial Na contents.

The incorporation of impurities in tridymites taken from a used silica brick have been studied by means of a high-resolution microprobe, X-ray fluorescence, X-ray diffractometry, and optical microscopy. The bulk impurity content of the brick changes strongly from the hot zone (4.7 wt%) to the colder part of the brick (7.6 wt%), indicating material transport along the temperature gradient. The transport medium for migration processes is probably a melt occurring in narrow veins between large tridymite crystals at high temperatures. The average impurity contents of homogeneous tridymite single crystals are 0.49 wt% in the hot zone of the brick and 0.81 wt% in the cold zone. Al2O3, TiO2, and Na2O are main impurity constituents; the tridymites do not contain significant amounts of Fe2O3 or CaO. The a parameters of tridymites decrease by about 0.26% from 4.9837 Å to 4.9709 Å from the hot zone to the cold zone of the brick and the c dimension shows a smaller decrease (0.11 %) from 8.2023 Å to 8.1933 Å.

The chemical variation of clinopyroxene phenocrysts from the trachybasaltic lavas of Etna volcano is described. The phenocrysts show a limited, but distinct trend in chemical variation from calcic-augite in the hawaiites to augite in the benmoreites. The trend of this variation is unusual, being one of Mg-enrichment with differentiation of the magma. Ca shows a steady decrease in the clinopyroxenes from the hawaiites to the benmoreites. Na, however, shows little chemical variation in the pyroxenes. The trace element chemistry is briefly examined. The clinopyroxenes show well-developed oscillatory and sector zoning. The basal {11} sectors are enriched in Si and Mg and depleted in Ti, Al, and Fe relative to the {100}, {110}, and {010} prism sectors.