Two pebbles of good quality nephrite from Kohat District are described and the possible origin of the stones is discussed. The discovery is of significance in relation to the unsolved problem of the origin of the raw material of Indian carved jade.

The transformation was followed in Amelia albite powders by variation in 2θ131−2θ13̄1 with time under isothermal conditions at various temperatures in the range 920°–1080°C. The activation energy of about 74 kcal mole-1 is rather larger than that obtained for the ordering process in synthetic NaAlSi3O8 under hydrothermal conditions.

Various crystalline and X-ray amorphous forms of aluminium hydroxide prepared from sulphate and chloride solutions by varying the mole ratio of NaOH to Al3+ have been characterized by chemical analysis and X-ray diffraction. When the NaOH/Al ratio is 3 or 3·3, crystalline Al(OH)3 in the form of bayerite, nordstrandite, gibbsite, or mixtures is obtained in several hours. When the ratio is 2·75 or below, no Al(OH)3 is evident and the products are X-ray amorphous and remain so even after ageing for six months. Chemical analysis indicates that the amorphous precipitates are basic aluminium sulphates or chlorides. In the 0 to 2·1 range of NaOH/Al, the composition of the products is constant, being approximately Al(OH)2·2X0·8. In the 2·1 to 2·75 range, a continuous series of basic salts, of composition ranging from Al(OH)2·2X0·8 to Al(OH)2·75X0·25, is obtained.

The petrological and structural characteristics of four layered basic masses in Somalia are described. Sr contents of feldspars from three of the masses are similar for comparable An contents; Sr varies between 1000 and 1850 p.p.m. and tends to increase as An decreases from An68 to An48. In feldspars from the other mass Sr also increases with decreasing An content (An74 to An56), but is at a much higher general tenure (1650 to 2350 p.p.m.). It is concluded that the four masses were not all part of the same layered intrusion.

Eclogite occurring about 20 miles east of Queenstown, Tasmania, is the first example of this kind of rock to be described in detail from Australia. It has the chemical composition of a basic igneous rock and consists of garnet (A115.8Fe‴0.6Fe″10.0Mg7.3Ca5.7Si24O96) and pyroxene (Ca4.5Na2.5Mg3.7Fe″0.7Fe‴0.4Ti0.1Aliv3.7Aliv0.7Si15.3O48)with amphibole, muscovite, zoisite, quartz, rutile, and pyrite. The eclogite consists of lenticular bodies within Precambrian garnetiferous schists.

Evaluation of X-ray powder diffraction data shows that argon atoms (radius 1·9 Å) lie at the centres of the large holes in the cordierite structure. The reflection intensities for hydrous cordierite differ from those for anhydrous cordierite but to a lesser degree than those for argon-bearing cordierite. Although analysis is uncertain, it appears that the smaller water molecules (radius 1·4 Å) do not occupy either the centres of the six-membered rings or the centres of the large cavities. It is possible that they may be attached to the walls of the large cavities or occur partly as hydroxyl groups in the framework. Single-crystal X-ray analysis is necessary to resolve this problem.

Optical and other microproperties of owyheeite Pb5Ag2Sb6S15 are presented and details given of a new locality for this rare sulpho-salt. Apparent rotation of the plane of polarization (3·5°), microhardness (112 V.H.N.), and polarization effects are all very similar to those of the other feather ores, jamesonite and boulangerite. An intimate association with pyrargyrite is further noted.

An attempt is made to systematize the nomenclature of the calciferous and alkali amphiboles. The compositions of end-members can be plotted on a simple diagram, which can also be used to display the affinities of the naturally occurring minerals after deriving a basic atomic formula according to the rules given. The composition of an amphibole can in some cases be expressed quantitatively in terms of solid solution of two or three pure end-members.

Field, petrographic, and chemical data are presented which make it possible to follow the course of back-veining of felsite by a gabbro ring-dyke. The remelted felsite back-veined the chilled margin of the gabbro, then acidified the partly crystalline basic mush within the chilled zone and finally mixed with the mainly liquid basic magma farther inside the dyke to produce hybrids of marscoitic aspect. Spectrographically determined trace-element data, with data on some major elements, illustrate these changes. The veins are shown to have a composition nearer the ternary minimum in the system Ab-Or-Qz-H2O than the parent felsite, while culminations of soda and iron in the acidified gabbros and hybrids suggest that metasomatism has accompanied the mixing of the partly liquid end members. This study confirms the earlier suggestion that the gabbro post-dates the felsite.

Chemical analyses, optical data, and cell parameters are given for olivine (Fo90·3 and Fo91·4), orthopyroxene (Ca1·5Mg89·0Fe9·5 and Ca9·8Mg88·6Fe10·6), clinopyroxene (Ca39·6Mg55·8Fe4·6), and chromian spinel from olivine nodules from the Carboniferous basalt of Calton Hill, Derbyshire. Chemical and modal analyses of the nodules and of their host rock are also presented. The distribution of Mg and Fe in the coexisting pyroxenes is fairly similar to that observed by Ross, Foster, and Myers (1954) but differs slightly in that the extension of the pyroxene tie-line intersects the Ca-Mg side of the Ca-Mg-Fe triangle. The clinopyroxene is a normal augite rather than the hydrous augite previously reported (Tomkeieff, 1928). The high chromium content of the spinel and clinopyroxene contrasts with the low value for this element in the host rock, suggesting that these minerals are not differentiates of the basaltic magma: the nodules are considered to represent fragments from a deep-seated peridotite.

Tremolite and actinolite form rhythmic overgrowths over hornblende in appinites associated with pipes of explosion-breccia and represent growth under alternating physical conditions arising from increase of gas pressure and its explosive release. The number of overgrowths present in each appinite mass is related to the number of explosions required to breach the particular structural trap that hindered the upward movement of the volatiles.

It is possible to determine the directions of the optic axes of a biaxial crystal mounted on a spindle stage—and hence the angle 2V and the directions of α, β, γ—from only four measured extinction positions. The method is a generalization of the one proposed by Tocher (1962); but in the present procedure it is not necessary to determine first the optic axial plane.

From the extinction curve of a biaxial crystal mounted on a spindle stage (or one-axis stage goniometer), the related n0 curve can be derived and drawn on the stereogram. The n0 curve is the equivibration curve—or constant-refractive-index curve—that goes through the projection P0 of the spindle-stage axis. After the principal diameters (maximum 2η and minimum 2ζ) of the no curve have been measured, the angle 2V may be calculated by means of the formula cos V = sin ζ/sin η. This method can also be used for determining—or refining—the directions of the principal axes α, β, γ.

A zoned plagioclase crystal from a magmatic quartz-diorite shows two generations of pericline twinning: one growth twinning, the other glide twinning. Plotting the angle of the rhombic section against composition for each set of twins gives curves indicative of an intermediate structural state with respect to the theoretical curves determined by Smith (1958). The curve for the growth twins is closer to the high structural state than that for the glide twins and supports the primary origin of the former. This curve is believed to fix approximately the lowest structural state at which primary pericline twinning can develop in magmatic rocks. The lack of coincidence of the curves for the two generations of twinning indicates partial inversion prior to formation of the glide lamellae, while the position of the curve for these secondary lamellae suggests a genetic relation to inversion. Both rhombic sections are probably relict with respect to the present structural state. The form of the curves confirms the general accuracy of Smith's curves for the maximum high and low structural states.

The problem of magnesian pyroxenes in acid liquids is considered in the light of new analyses of pyroxene and magnetite phenocrysts. It is suggested that the early precipitation of titaniferous magnetite, which will strongly deplete the liquid in iron, will decrease the iron:magnesium ratio of the pyroxene components in the liquid. Accordingly the iron:magnesium ratio of a pyroxene precipitating from an acid liquid may vary over a wide range depending upon the presence or amount of early magnetite.

Chemical, optical, and X-ray data are presented on three analcimes from a differentiation sequence, olivine-theralite to analcime-tinguaite; compared with the ideal analcime composition, they reveal replacement of NaAl→Si. Optical and X-ray data on these minerals accord reasonably well with analcimes possessing similar compositions, and synthesized in the sub-solidus region in the NaAlSiO4-NaAlSi3O8-H2O system.

Sharp exothermic peaks occurring at about 90° C during D.T.A. treatment of natural volcanic glasses are described and are shown to be due to release of external strain. This is probably triggered by loss of surface water. No systematic variation in strain could be found in glass of different geological histories, but strain is found to decrease during natural weathering of volcanic glasses. Differences in weatherability of natural glasses may be related to degree of surface strain.

A new method of determining the optic axes from a few extinction measurements is presented. It is based on the progressive elimination of those parts of the stereogram in which it is obvious that the optic axes cannot lie. It has two applications: as a valid method in its own right, if applied systematically; and as a reconnaissance technique. The second application permits, for example, the recently presented trisogon technique to be rapidly and economically applied over a very small fraction of the stereogram instead of over its whole area.

Chemical analyses, cell parameters, and optical properties are presented for seven Scottish orthopyroxenes: four from Harris, two from Scourie, and one from Belhelvie, Aberdeenshire. The compositions range from En83·6 to En55·5. Six of the orthopyroxenes are from pyroxene granulites or related rocks and the other is from a plagioclase-hypersthene hornfels: the latter mineral is notable in having 7·21% Al2O3, causing a considerable contraction in the a and b cell parameters. The alumina content of orthopyroxenes is briefly discussed. Analyses are also given for olivine and diopsidic augite from a South Harris ultrabasic gneiss and for two enderbitic pyroxene gneisses from Scourie.

The Archean rocks of the Barrier Ranges, New South Wales, show zones of progressive regional metamorphism of which the highest is characterized by orthopyroxene-bearing basic granulites. Within this zone, further increase in metamorphic grade results in a detectable change in distribution of ferrous iron and magnesium between coexisting orthopyroxene and clinopyroxene, the latter becoming relatively richer in iron. Mutual solid solution between the two pyroxene phases also appears to increase. The value of coexisting pyroxenes as indicators of metamorphic grade is thus established. There are some points of disagreement between the Broken Hill results and previously advanced theories of cation distribution in pyroxenes.