Thin sections of clays are not easily prepared by the routine petrographic methods. The chief difficulties are due to the extremely fine texture of the clays and to the soft and plastic nature of these materials. These difficulties are only too well known, and, although in some cases they have been partially overcome by impregnation in vacuo, it is safe to assume that really little work has been accomplished in the microscopic examination of clays on account of the difficulty of producing satisfactory thin sections. The writer has attempted to extend the scope of the microscopic investigation of clay rocLs by means of the technique outlined below.

The method has been devised in response to an appeal by a friend, who, working in a laboratory at a well-known pottery works and thus connected with the clay industry, desired a method that would make possible the examination of thin sections of clays under high powers with polarized light.

Scarbroite, a fine-grained but compact deposit obtained from fissures in the sandstone on the north Yorkshire coast, is shown by chemical analysis to have an idealized formula Al2(CO3)3·12Al(OH)3. X-ray and electron diffraction data indicate a triclinic cell with a 9·94 Å., b 14·88 Å., c 26·47 Å., α 98·7°, β 96·5°, and γ 89·0°. A layer structure consisting of gibbsite-type sheets of Al(OH)3 and sheets of Al2(CO3)3 is proposed. The structure is stable from room temperature to about 125° C.

A good chemical analysis of an amphibole will give cation numbers that can be correlated in detail with the available lattice positions. The substitions that must be recognized are described. A more rigorous check of the analytical results becomes possible and the sources of error may be indicated.

Two subsilicic magnesian potassium-hastingsites (4.55 and 4.34 wt.% K2O) and one magnesian potassium-hastingsite occur in calc-silicate pods in well-layered gneisses from the transitional amphibolite- and granulite-facies terrain of a Cambrian metamorphic complex, East Antarctica. Subsilicic magnesian potassium-hastingsite is the most K-rich Ca-amphibole yet reported:

A Devonian weathering profile was developed near the contact of Riphean dolostone containing disseminated carbonate-fluorapatite and intensively pyritized black shales. Samples from the profile were investigated by XRF, SEM, electron microprobe and ion microprobe. Rock-forming minerals are apatite, diaspore, chlorite and kaolinite, while accessory minerals are svanbergite, anatase, pyrite and zircon.

Primary marine carbonate-fluorapatite is represented by extremely weathered crystals concentrated near the footwall. Dissolution of apatite and pyrite provided an acidic environment that is expressed in the formation of S-rich apatite and svanbergite. The environment allowed Ti migration which formed anatase in weathered apatite grains. Coupled substitution Na+ + S6+ = Ca2+ + P5+ is suggested in S-rich apatite. A large volume of dissolved carbonate rocks was a source of Sr necessary for svanbergite formation. Apatite of the third generation formed at the final stage of weathering is represented by small (10—30 μm) very well shaped crystals.

The presence of structural OH in amphiboles in excess of the usual two OH per formula has been debated for over 40 years (Gier et ah,1964; Leake et ah,1968). However, the reality of the excess-OH phenomenon is still an open question, because accurate water analyses of amphiboles are rarely available. In this study, we review the data available on the chemically simple synthetic system Na2O—MgO-SiO2-H2O (NMSH) and present new results from NMR, infrared spectroscopy, and X-ray-diffraction that allow re-interpretation of previous studies of NMSH amphiboles along the pseudobinary join between the two end-member compositions Na2Mg6Si8O22(OH)2 and Na3Mg5Si8O21(OH)3.

We show that there is extensive solid solution involving excess H at 650—750°C, but also document the presence of a wide miscibility gap below 600°C. This miscibility gap is defined by amphiboles very close to the end-member composition Na3Mg5Si8O22(OH)3 coexisting with amphiboles with compositions near the ‘normal’ Na2Mg6Si8O22(OH)2 end member.

We also report the characterization of triple-chain silicates (TCS) in the NMSH system and their phase relations with NMSH amphiboles. The upper thermal stability field of the key TCS Na2Mg4Si6O16(OH)2relative to its decomposition to two NMSH amphiboles with a combined equivalent composition has been determined and a pronounced backbend of the transformation boundary documented. Phase relations observed in synthesis experiments suggest that at 550—650°C all TCSs have compositions close to Na2Mg4Si6Oi6(OH)2. Infrared spectroscopy indicates that the TCS synthesized on this composition, studied in detail here, vary from end-member Na2Mg4Si6Oi6(OH)2 to binary solid solutions with less than ∼6 mol.% clinojimthompsonite component. No clear spectroscopic evidence for a ‘Drits’ component NaMg4Si6Oi5(OH)3 (Drits et al.,1975) has been found. Analysis of H2O by vacuum extraction and Karl-Fischer titration indicates large excesses of H2O in all the TCSs studied here that clearly exceed the amounts expected from (OH) groups alone. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicate that this excess H2O is structural. We propose that the excess H2O is likely to be molecular H2O located in the ^4-site channels. The observed backbend of the triple-chain decomposition curve is in agreement with a reaction involving dehydration and loss of this molecular H2O. However, the absolute amount of analysed molecular H2O exceeds that expected from the change in Clapeyron slope alone.

While demonstrating the reality of excess OH in amphiboles, the evidence presented in this paper also points to interesting avenues for future research on both amphiboles and TCSs, such as understanding the dynamics and enhanced crystal chemistry of excess OH and molecular H2O in pyriboles.

Phase-contrast methods in microscopy appear to have been developed mainly with the intention of applying them to biological subjects, and their application to the examination of mineral substances has received only slight attention. Thin sections of rocks viewed under a phase-contrast microscope have a fascinatingly strange appearance and it does not take long to realize that some features are shown up more clearly and others less clearly than when viewed under an ordinary or a polarizing microscope. A considerable amount of work may be necessary before it is possible to assess the value of the new instrument for petrological purposes or to interpret all the phenomena observed by its aid. Nevertheless, it seems fitting to set out the following short account as a contribution towards this knowledge.

Amygdales found metamorphosed in the aureole of the volcanic plug at 'S Airde Beinn, northern Mull, may be divided into three classes by their behaviour on metamorphism. The first class, characterized by the original dominant presence of zeolites, forms feldspar readily. It is related to a type, composed to a greater degree or entirely of ferromagnesian minerals, which seems to have formed by the infilling of originally void amygdales during the recrystallisation of the basalts. The second class, originally containing dominant gyrolite, forms first reyerite and then wollastonite on metamorphism, the wollastonite-filled amygdales being enveloped in a rim of aegirine-augite from the earliest stages owing to reaction between the wollastonite and the basalt. The third class originally contained calcite, now represented by the anhydrous calcium silicates larnite, rankinite and wollastonite. These minerals form concentric monomineralic zones decreasing in Ca/Si ratio outwards. Later reaction has led to the replacement of both basalt and calcium silicates by melilite. Thin aegirine-augite rims surround the amygdales.

Mr. G. F. Herbert Smith has pointed out the advantages of the gnomonic projection for some special crystallographic problems. It seems to me that much of the labour in drawing the projections could be saved by a device similar to that employed by Fedorow, Wulff, Penfield, &c., in drawing stereographic projections.

Corresponding to the ‘stereographic net’ described by these authors (which is the stereographic projection of lines of latitude and longitude on a plane through a llne of longitude), we have the gnomonic net, which is the projection of the lines of latitude (l) and of longitude (m) on the tangent plane at the point whose latitude and longitude are zero. The lines m are parallel straight lines, the lines l are hyperbolae having double contact with each other at two fixed imaginary points.

Mode of Occurrence.—This new mineral was discovered at Gangapitiya, near Ambakotte and about 12 miles east of Kandy, Ceylon. In this locality quarries have been dug for the well-known Ceylon moonstone, which occurs in large porphyritic crystals in an acid granulite.

Bands of this granulite, which is composed mainly of quartz and felspar in finely granulitic and occasionally graphic structure, alternate with bands of limestone up to 18 inches wide. Between limestone and granulite occur contact-zones consisting, next to the limestone, almost entirely of a coiourless diopside, but near the granulite of a mixture of diopside with other minerals, viz. blue spinel, a little apatite, occasionally scapolite or plagioclase, and the blue mineral which is the subject of the present note.

Geophysical Laboratory work is especially applicable to quenched rocks, but does not lend itself to the solution of problems connected with the interpretation of structural relationships between the component minerals of deep-seated rocks. The detailed examination of a rock of simple mineral composition might be expected to yield a picture of the way in which phase-boundary adjustments are made in a rock crystallizing under such conditions. Some of the Bushveld basic rocks are of such a nature, and in the present paper an attempt is made to analyse and interpret the structural relationships between the two minerals bronzite and bytownite (An78-80), of which they are composed. The material studied was collected under the guidance of Dr. A. L. Hall during the International Geological Congress in 1929, supplemented by core specimens recently made available through the courtesy of an old student of the College.

The Tiree pink marble, a fine-grained, severely crushed limestone with evidence of earlier coarse crystallization, is exposed in several small areas up to 100 feet across on the farm of Balephetrish near the north coast of the island of Tiree in the Hebrides. It contains a remarkable quantity of dark silicate minerals and has discordant contacts with the adjacent Lewisian gneiss. The precise nature of its relation to the gneiss, and mode of emplacement, have been much discussed. The writer has been permitted to consult accounts of the literature, by Mr. V. A. Eyles, and of the petrography, by Sir Edward B. Bailey, and is also indebted to those authors for discussion of the problems. At the suggestion of Sir Edward Bailey the present work was undertaken as a contribution to the study of this problem from the mineralogical point of view. Most of the determinations were made in 1938, but it was not possible to complete publication at that time.

This zeolite, hitherto recorded only from Chaux de Bergonne, France, has been found as zoned spherules with thomsonite (var. faroelite), on specimens from Aci Castello and Aci Trezza, Sicily, and Capo di Bove, Rome, from Weilberg, Rhineland, and from Klöch, Styria. Three chemical analyses confirm previous results; optical data suggest some variation in composition within a spherule. There is a striking resemblance between X-ray fibre photographs of gonnardite and of thomsonite, and between X-ray powder photographs of gonnardire and of natrolite. The Klöch occurrence, where the gonnardite is accompanied by phillipsite, is described in detail.

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.

Flood basalts of the Isle of Mull were sampled in areas outside the metamorphic aureole of the central intrusions. Alteration products of olivine and pyroxene in these rocks are talc, chlorite, serpentine, iron oxides, brucite, calcite, and ‘iddingsite’. Plagioclase feldspar in the same rocks is unaltered. ‘Iddingsite’ is less common than mixtures of chlorite and serpentine.

In two flows the only alteration product near the bottom is ‘iddingsite’ but at higher levels in the same flow green ehloritic material replaces olivine. This variation is accompanied by an increase in the FeO/Fe2O3 ratio in the rock towards the flow top. Green alteration products may be converted to material optically identicaI with ‘iddingsite’ by heating in air at 600° C. Zoned patches of chloritic material have formed by crystallization of the late stage magmatic fluid, modified in composition by reactions with olivine and pyroxene.

The first occurrence of new mineral ardaite (a chlorine sulphosalt) has been discovered in the Madjarovo polymetallic ore deposit (Bulgaria). The average composition is Pb 56.50, Ag 0.04, Sb 22.48, S 15.56, Cl 3.78, total 98.36 wt.%. The formula is Pb20–18 Sb12–24S34–36.5Cl6–8. The mineral occurs as fine acicular aggregates associated with galena, nadorite, a Cl-bearing robinsonite and Cl-beating semseyite, pyrostilpnite, silver- bearing tetrahedrite and anglesite.

Textural relationships shown by oxide phases in olivine melilitites from Namaqualand-Bushmanland, South Africa, indicate early crystallization of picroilmenite from a relatively reduced magma. Evolution of a CO2-rich vapour during magma ascent led to an increase in oxygen activity and liquidus temperatures, causing rapid crystallization of titanomagnetite. Higher oxygen activities, together with the increase in calcium activity following CO2 loss from the magma, resulted in perovskite replacing ilmenite as the stable Ti phase.