In the absence of published experimental data on phase equilibria in the ternary system Fe-Sb-S and on the stability relations of gudmundite (FeSbS) and berthierite (FeSb2S4) , some preliminary investigations on the upper stability limit of natural gudmundite are briefly described. On heating in vacuo, As-free gudmundite broke down between 269° and 288°C, an approximate upper stability limit of 280±10°C conforming to the stability relations of the associated phases.

Zoning in plagioclase is reported from the charnockitic rocks of Kondapalli, Krishna district, Andhra Pradesh, India, for the first time. Electronprobe studies indicate that the zoned plagioclases consist of homogeneous sodic cores mantled by thin calcic shells, which show continuous zoning of the inverse type. Partial chemical analyses of twelve density fractions of the zoned plagioclase from a basic charnockite exhibit a wide variation in An content from 54·80 to 72·39 %; X-ray studies suggest that they possess ‘fixed’ transitional structural state.

Several restricted outcrops of potash-rich volcanic rocks occur in the Spanish provinces of Murcia and Almeria, where they form intrusive pitchstones or surface lavas among country rocks consisting of Tertiary and Quaternary sediments: the volcanic rocks include jumillites and verites from the type localities of Jumilla and Vera and it is these rocks that have been investigated. Jumillites are basic lavas, sometimes coarse-grained, rich in MgO, CaO, K2O, TiO2, and P2O8, poor in SiO2, Al2O3, and Na2O and they contain important, if minor, concentrations of Cr2O3, BaO, SrO, and ZrO2. Their mineralogy is distinctive: chrome diopside, forsterite, titaniferous phlogopite, kataphorite, and sanidine, with rare leucite. Verites, typically, are black pitchstones or pitchstone breccias that range from basic to acid in composition ; they are more variable in their chemistry than jumillites, but in part this variation might be due to introduction of silicate and carbonate into some of the verites during extensive hydrothermal alteration, which has undoubtedly affected their initial chemistry. Mineralogically, verites are simpler than jumillites and have fewer phenocrysts: forsterite, some diopside, phlogopite, and, occasionally, leucite. Certain mineralogical and compositional similarities of jumillites and verites suggest they may have originated from the same type of parental magma. Their mineralogy and chemistry also relate the jumillites and verites to phlogopite-bearing potash-rich volcanics from East Africa, America, and Australia and it seems possible that all these rocks might have a common parent in mica peridotite or mica pyroxenite.

Cell dimensions obtained from X-ray powder photographs of twenty-four natural olivines and one synthetic forsterite have been related to the cation contents by a new statistical procedure developed by M. H. Hey. Unlike conventional statistical analysis, which assumes error only in one parameter, this procedure simultaneously considers the estimated errors in all variables.

Linear terms in Ca, Mn, Mg (or Fe(≡ 1-Mg-Ca-Mn)) satisfactorily account for the cell dimensions, and there was no need to invoke a term in Mg2. Since there are three cell dimensions and three chemical unknowns, it is theoretically possible to estimate the major cations in olivine just from the cell dimensions. However, the coefficients for Mn and Fe are so similar and the ranges of Ca and Mn are so small that the technique would be impractical.

Data for ideal (Mg,Fe) olivines were estimated by adjusting the cell dimensions by linear extrapolation from the cell dimensions of tephroite and monticellite. A simple chemical term was sufficient to account for each cell dimension. The linear relation between cell volume and Mg is consistent with the complete disorder of Mg and Fe revealed by parallel single-crystal X-ray studies.

The cell dimensions of a synthetic fayalite deviate significantly from those of natural fayalites after correction for the effects of Ca and Mn substitution in the latter: some structural or chemical difference, probably involving ferric iron, is indicated.

No superstructure lines have been found, in contradiction to Eliséev's data.

The orientation of the boundaries of out-of-step domains in ‘transitional’ anorthite has been deduced from the direction of elongation of the diffuse X-ray reflections h + h + l = 2n + 1 in selected precession photographs. The reflections are elongated normal to which is in the zone and contains the body-centring vector (a+b−c)/2. The structure is presumed to be perfect parallel to . Atoms in essentially identical, primitive anorthite-like domains are related across the out-of-step domain boundaries by body-centring vectors. Increasing diffuseness of the h + k + l = 2n + 1 reflections indicates progressively smaller domains (more frequent domain boundaries), and when thesereflections disappear, the structure has become body-centred on the average.

The humites are structurally analogous to olivine wherein the replacement of four oxygen by four (F,OH) anions in the slightly distorted, hexagonal close-packed array is balanced by the replacement of one tetrahedrally coordinated Si by a tetrahedral void, according to the general formula Mg2xSix-1O4x-4(F,OH)4 where x = 3, 5, 7, 9. In humites the key structural units are not ‘olivine and sellaite (or brucite) layers’, as previously assumed, but are zigzag chains of edge-sharing octahedra, just as in olivines. It is shown that for humites and olivines alike the unit cell parameters a, b, and d001/n and the cell volume (normalized to one-half the mean anion-anion distance along the normal to (001)) vary linearly with the average radius of the octahedrally coordinated cation in the chain.

Substitutions of (F,OH) for O and vacancies for Si have second-order effects on the unit cell parameters, causing a linear decrease of the normalized cell volume with increase in F/O ratio in the synthetic series forsterite-humite-norbergite. Comparison of the crystal structures of forsterite and norbergite shows that the polyhedral distortions in norbergite are smaller than in forsterite in accord with the decreased number of shared edges: the fluorines in norbergite are bonded to three Mg atoms whereas all anions in forsterite are bonded to three Mg and one Si atom.

Contact metamorphic and metasomatic effects are described in the black radiolarian cherts of Lower Carboniferous age occurring at varying distances from the margin of the Dartmoor granite in the neighbourhood of Meldon, near Okehampton, Devon.

Development of biotite by contact metamorphism is over-printed by metasomatic conversion of chert to coarse-grained calc-silicate hornfels. Changes are localized and controlled by structural planes including faults and joints. Earliest effects are scapolitization giving black-and-white calc-flinta by local addition of calcium and subtraction of silicon. The same trend in transfer of elements is maintained in later phases: wollastonite-diopside reaction zones may enclose cores of black-and-white cale-flinta and are in turn replaced by wollastonite-grossular and grossular reaction zones marking the sites of main channels for migration of mineralizing fluids. Finally, veins with datolite-diopside, datolite-grossular-idocrase, and datolite-grossular-zeolite assemblages were formed in the altered cherts. Tin was available with aluminium and ferric iron during metasomatic changes in which garnet developed.

Some chemical transfer may be reciprocal changes with interbedded limestones, but appeal is made to external sources for the later metasomatizing agents.

A zinc-rich helvine (genthelvite) has been found with bertrandite in miarolitic cavities in adamellite at Coire an Lochain in the Cairngorm Mountains. The genthelvite occurs as green or brown tetrahedral crystals of sp. gr 3·58±0·01. It is isotropic, n 1·742±0·003, a 8·133±0·005 Å. Bertrandite crystals are pale brown with sp. gr. 2·60±0·01; biaxial negative with 2V 75°, and α 1·590, β 1·603, γ 1·613, all ±0·003.

A garnet-olivine metaperidotite and a garnet-amphibole pyroxenite are described. Chemical analyses are presented for six rocks and optical properties and chemical analyses are tabulated for clinopyroxene, almandine-pyrope garnet, and hastingsitie amphibole from the garnet-amphibole pyroxenite. A possible origin for the garnet peridotite and chemically similar granulite facies ultramafic rocks is suggested. The eclogite facies in South Harris is reinstated in the light of the data presented.

Phase equilibria involving solids, liquids, and vapours on the join Ca(OH)2-CaF2-Ca3(PO4)2-H2O were determined at 1000 bars. The temperatures and liquid compositions (weight %) of the isobaric invariant reactions encountered were: portlandite+fluorite → liquid, 687° C, 70 % Ca(OH)2 30 % CaF2; portlandite+fluorite+vapour → liquid, 675° C, 69% Ca(OH)2 26 % CaF2 5 % H2O; portlandite+fluorite+apatite → liquid, 675°C 73 % Ca(OH)2 24 % CaF2 3 % Ca3(PO4)2; portlandite+fluorite+apatite+vapour → liquid, 665° C, 70 % Ca(OH)2 22 % CaF2 3 % Ca3(PO4)2, 5 % H2O. The temperatures of these reactions were determined from over 400 experimental runs at selected pressures from 250 to 4000 bars. The composition of the apatite involved in the last two reactions was 60 % hydroxyapatite 40 % fluorapatite and in subsolidus regions in the presence of water, fluorapatite did not crystallize but fluorhydroxyapatite solid solutions coexisted with vapours containing up to 2·5 % hydrogen fuoride. These observations suggest that hydroxyapatite is more stable than fluorapatite at elevated temperatures and pressures and would be the composition expected to crystallize under igneous and metamorphic conditions.

The distribution of trace elements is studied in different acid and basic rocks, adjacent to the copper lodes in the Mosaboni mines, Singhbhum, and adjoining the thrust zone, as well as of areas far away from the mineralized zones. Specific differences in composition and trace constituents in these rocks especially as regards S, Cu, Ni, and Co, suggest that the relationship of the Singhbhum granite with the soda granite is most unlikely, and that there is a connection between the soda granite and the sulphide ore-fluid ; a genetic relationship of sulphide mineralization with the epidiorite is doubtful. The possibility of the existence of a common source for the Chotanagpur granite and the Singhbhum granite is inferred.

Group theoretical analysis of the normal vibrations of isolated molecules and of macroscopic crystals indicates that the principal effect of symmetry reduction is to remove the degeneracy of the normal vibrations, thus reducing the number of microscopic complexions of the phonon distribution, and reducing the entropy. This mechanism is effective only in point groups containing a rotation axis of order 3 or greater, and may play a part in the high-low cordierite transition.

Two other symmetry-related entropy contributions are discussed : the first is the loss of positional degeneracy in systems in which the number of available positions exceeds the number of atoms, as in high-low quartz, vlasovite, and many ferroelectrics. The second is the loss of positional degeneracy in a solid solution, which leads to unmixing, as in high-low albite, microcline-sanidine, and high-low cordierite transitions. These mechanisms are effective in all point groups down to 1 = C1, and can proceed even within C1 by a change in the unit cell volume.

Consideration of entropy-volume relationships suggests that the glaucophane I–II transition has less order-disorder character than previously supposed. It is suggested that zoisite-clinozoisite, anthophyllite-cummingtonite and enstatite-clinoenstatite relationships are polytypic rather than polymorphic in the usual sense, and the mechanisms responsible for producing the stacking order are discussed.

The implications of the differences between the behaviour of the classical double oscillator (often used as a model for displacive transitions) and its quantized analogue are discussed.

The subaerial decomposition of struvite has been investigated at room temperature (21±2° C), from which it has been concluded that the decomposition product is newberyite. X-ray power patterns of newberyite, both from a diffractometer and Debye-Scherrer cameras, are given. Comparison of these patterns with other newberyite patterns suggest that the camera pattern is more reproducible; this may be due to grain orientation in the diffraetometer mounts.

Pyrolusite pseudomorphous after manganite occurs at Serro do Navio, Amapa, Brazil. It consists of wedges, flattened on (100) and elongated along [010], twinned on (011) and showing forms d{210}, , b{010}, and f{021} (all indices referred to manganite, in the setting of Dana, 7th edn). The difficulty of distinguishing between pyrolusite pseudomorphous after manganite and after groutite is discussed and also the possible existence of the latter. The distinction was made here by measuring the twinning angle between the pyrolusite lattices using (0kl) precession photographs. Specimens of manganite intergrown with pyrolusite are described and their relative orientations determined. On heating, manganite partly decomposes to a monoclinic phase, a 9·9 Å, b 5·73 Å, c 4·89 Å, β 98°, space group I2, Im, or I2/m.

Extensive studies on oriented reactions in the system MnO-O-H2O have revealed surprising symmetry changes, which we have been unable to explain. Orientation relationships are often complicated, but where interpretation is possible, the results support the idea that orientations are controlled by the oxygen frame-work and that considerable migration of manganese occurs.

Microprobe analysis of leached and unleached varieties of a charnockitic rock from Madras supports the contention of Howie (1964) that the characteristic dark greenish colour of such rocks is contained mainly in fine (chloritic ?) veinlets cutting the feldspar and quartz, though a certain amount appears to be contained in exsolved sodium feldspar. Leaching of the darkish colour of the charnockitic rock has not affected the crystallographic state of the potassium feldspar.

Though the transitional members between the pyralspite and grandite series occur only very sporadically, there exists under favourable conditions a perfect miscibility between grossular on the one hand, and spessartine or almandine or pyrope on the other. The conditions favouring this miscibility are discussed.

A heulandite from Nelson Creek, Skamania County, Washington, has 30 H2O per unit cell instead of the normal 24; the excess water is lost on gentle heating. The b and c cell-dimensions are also rather larger than normal. Normal heulandite also occurs in the same area.