It is shown here that dyke rocks of type localities for ‘soda-minettes’ are not minettes (= lamprophyres dominated by phlogopite and K-rich feldspar). It has long been suggested that ‘soda-minette’ also be applied to otherwise normal minettes from elsewhere that carry modally variable amounts of groundmass aegirine and/or, more commonly, arfvedsonitic to riebeckitic amphibole. Despite the presence of sodic pyriboles, these rocks are poorer in Na2O then minettes lacking these phases. ‘Soda-minette’ is thus misleading and self-contradictory to the many petrologists to whom ‘soda-’ legitimately connotes that the rock in question either has K/Na < 1 and/or is rich in albitic feldspar. To eliminate this ambiguity it is recommended, with the approval of twenty-six other petrologists, that ‘soda-minette’ no longer be used as a rock name. As minettes with Na-pyribole(s) are markedly K-rich and most are both ultrapotassic and peralkaline, independent of the modal abundance of the Na-pyrobile, it is suggested that one of these three chemical characteristics be utilized adjectivally in naming these minettes.

Titanian aegirine in a pegmatoid differentiate from leucitite sensu stricto (i.e. without modal plagioclase) was found to contain significant K (up to 0.07K atoms per formula unit; 0.11 wt.% K2O) by electron probe microanalysis. A transmission electron microscope study showed that this pyroxene contained a lamellar amorphous phase and other more irregular, amorphous domains where K and Al are concentrated. The microstructures suggest that the aegirine-amorphous phase system did not achieve textural equilibrium. Also, the chemistry of this phase is variable, particularly in Ca and Mg, suggesting lack of chemical equilibrium within the liquid from which the phase derived. The average composition of the latter approaches that of the coexisting K-richterite and is interpreted as trapped residual liquid, i.e. glass, from pyroxene crystallization which failed to develop a crystalline amphibole. In view of the extensive stability of richterite coexisting with pyroxene to high pressure, the reported occurrence of ‘K-bearing pyroxene’ may not be considered as a homogeneous K-bearing phase and a predictable potassium carrier in the petrogenesis of K-rich rocks like leucitites and lamproites.

Thirty pyrite samples from a wide range of localities were analysed using relative comparator and k0 neutron activation analysis (NAA) techniques at the University of Missouri Research Reactor, Columbia, Missouri, USA (MURR) and the Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia (ANSTO), respectively. Statistical analyses of the trace-element data produced by the two methods showed a generally good correlation, with the majority of elemental concentrations of paired data reported by MURR and ANSTO being indistinguishable at a 0.05 significance level. Trace-element analyses of pyrite from Navajún in Spain by both techniques compare well with published data. There is evidence for contamination by Al, Na and Ti in one set of samples, this is likely to have been introduced by contact with a plastic used in sample preparation.

A Bi-Ni-Co vein in the contact zone of a granite near Pozoblanco, Córdoba, Spain, shows a zoned structure with mm-thin quartz wall zones, up to 20 cm thick Bi-rich quartz zones, and a few cm thick quartz-bismuth transition zones grading into a central quartz vug zone. The Bi-rich zones show textures indicating a metacolloidal origin from a silica gel enriched in adsorbed Bi: (1) globular and chain aggregates of fine-granular allotriomorphic quartz represent morphological relics of a globulated silica gel; (2) quartz rosettes, chain rosettes and stringer-combs radiating from the quartz globules and chains represent crystallised silica gel matrix; (3) crack fillings of quartz, gersdorffite and platy bismuth dendrites represent precipitates of metal-saturated dilute hydrosols filling shrinkage cracks in the desiccating and crystallising gel; (4) quartz spherocrystals in rosettes and combs are clouded with colloidal particles inherited from the silica gel; (5) unsupported wall rock inclusions represent inclusions in a viscous silica gel precursor covering fissure walls. The thin wall zones are quartz fillings of cracks between shrinking gel and walls. The transition and vug zones are recrystallised colloidal silica crusts precipitated by fissure-filling rest-solutions. Tree-like bismuth-gersdorffite dendrites in the transition zones are precipitates of metal-rich solutions infiltrating from the Bi-rich zones into the recrystallising silica crusts. The pre-concentration of metals in silica gels explains the dendrite formation without assuming abnormally high metal concentrations in transporting solutions or long range diffusion processes.

The Coire Dhuinnid fault zone contains emerald green chromian illite-ankerite-quartz rocks that are similar in appearance to the fuchsite(or mariposite)-carbonate-quartz parageneses that are commonplace in Archaean greenstone belts but which are rather rare in Phanerozoic rocks. The chromian illite contains 2.3 wt.% Cr2O3, low K2O (7.1–7.6 wt.%) and high H2O+ (5.7 wt.%), and it is a 1M polytype with ≤10% of an illite/smectite interstratification indicative of a formation temperature of c. 175–200°C. The host rocks contain high concentrations of Ni and Cr, and show low concentrations of Ti, Nb, Y and Zr, suggesting a former primitive mafic protolith (boninitic magma?); they are considered to be retrograde remnants of Lewisian rocks. The latter, and the associated rocks of the Moine Series, have been affected by CO2 metasomatism that was accompanied by the addition of Ca(+Sr), Fe and Mg, and by the removal of Na from, and the addition of H2O to the Moine metasediments. Radiogenic isotope studies of mineral separates and whole rock from sample no. 43 yielded ages of 483±2 Ma (Ar-Ar dating on Cr illite), 413±12 Ma (K-Ar dating on Cr illite), and 322±9 Ma (Rb-Sr dating on minerals and whole rock); the significance of this discrepant pattern is discussed.

Historical Introduction.—As given in modern text-books, the holohedral-cubic silver haloids occurring as minerals include the following species: the pure chloride, chlorargyrite, AgCl. ; the pure bromide, bromargyrite, AgBr; the intermediate embolite, Ag(Cl,Br), containing chlorine and bromine in varying proportions ; and iodobromite, containing all three halogens in a definite ratio corresponding to the formula 2AgCl.2AgBr.AgI.

The terms hornsilver (used by Gesner as far back as 1565) and cerargyrite (Beudant, 1832) are usually taken as referring only to the pure chloride. As regards their signification, however, they are equally applicable to all of the above species, and in lack of chemical analyses were certainly so applied in the past. Thus Breithaupt, after the discovery of the bromide and chloro-bromide of silver by Berthier in 1841 and 1842, remarks that many of the specimens of ‘Hornerz’ in the Werner Museum having a greenish colour probably contained bromine.

This paper forms part of a communication recently made to the Geologists Association, with some additional matter relative to certain criticisms passed upon Mr. Dunn's hypothesis as to the origin of the diamonds.

It is taken for granted on this occasion that the leading geological facts are known, I will merely, therefore, draw attention to a generalised section of Kimberley Mine, where one side represents the wall or reef bounding the pipe itself, and is thus constituted:—

During the investigation of a zeolite it became necessary to separate small quantities of zeolitic material from a mixture of calcite and other heavier minerals; this could be done most easily by using a liquid of such density that the zeolite just floated, while all the other minerals sank to the bottom. A solution of potassium-mercuric iodide could not be used, as this reacts chemically with zeolites and alters their composition ; acetylene tetrabromide diluted with benzene was therefore chosen, and in consequence the apparatus had to be made entirely of glass. Certain dittlculties were found in using the ordinary form of glass separating apparatus, and to overcome these the form here described was devised.

The existence of rotatory polarization is comparatively easy to detect in isotropic crystals and in uniaxial crystals in the direction of the optic axis ; but in a biaxial crystal the problem presents far greater difficulties, since in general the rotatory polarization is obscured by the double refraction. The possibility of its existence in a biaxial crystal was denied by É. Mallard (Traité de Cristallographie, 1884, vol. 2, p. 818). However, the persistence of rotatory polarization in a quartz crystal , made biaxial by pressure, brought this conclusion into doubt. Later work, both experimental and theoretical, by H. C. Pocklington (Phil. Mag., 1901, ser. 6, vol. 2, p. 861) established its presence in certain biaxial crystals.