Since the mineral gyrolite was described and analysed by Anderson from Storr in Skye in 1851, it has been recorded from a considerable number of other places in that island. The writer observed it at the mouth of the burn called Allt Mòr, between Loch Brittle and Loch Eynort, and has now to record it from Rudha nan Clach at the mouth of Loch Bracadale, which brings the number of localities at present known in Skye up to fourteen. In fact, the mineral may be said to occur all along the south-western shore, all along the eastern escarpment, and at the more isolated localities of Lyndale and Portree. Similarly, it occurs in the other trap islands—the Treshnish group, Mull, Muck, Eigg, Canna, and Sanday, upon the last of which it has been observed by the writer at Stac nan Faoilean.

26 b-axis oscillation photographs of alkali feldspars of microsyenitic dykes of Southern Greenland are presented. The crystals are rather restricted in composition (Or42·4 to Or35·3) but are rather rich in Ca (up to 14 % An). The range of variation of the optic axial angles, 2 V 45° to 71° (exceptions up to 87°), puts the crystals between sanidine-anorthoclase cryptoperthite and orthoclase microperthite, as classified by Tuttle (1952).

Since the syenitic magma cooled down rather rapidly some peculiarities in the structural state of the alkali feldspars are to be expected. The potassium-rich phase is (nearly) monoclinic. According to the values of the reciprocal lattice angles α* and γ* the sodium-rich phases can be divided into three groups: high-temperature pericline-twinned and high-temperature albite-twinned phases, high-temperature pericline-twinned and low-temperature albite-twinned phases, and high-temperature pericline-twinned and low-temperature albite-twinned phases with nearly identical values of the reciprocal lattice angle γ*.

The water vapour released upon heating the specimen is driven through a water absorber. The intensity of the exothermic reaction produced by absorption of the water is recorded against the temperature of the specimen. A series of curves obtained with various clay minerals are presented to illustrate the method. The significance of results in the identification of the clay minerals and the determination of their structure is considered.

The Permo-Carboniferous quartz-dolerites of central Scotland are generally holocrystalline, and only two intrusions amongst them with a fresh vitreous base have been recorded—those of Dalmeny in West Lothian, andKinkellnear Kirkintilloch (northof Glasgow). Both rocks have already been described briefly, but in view of their many interesting features a more thorough treatment seems desirable, particularly as they have a bearing on some recent theories of magmatie differentiation advanced by Dr. C. N. Fenner.

Perhaps the most difficult question in the science of Geology at present, is the explanation of the origin of the metamorphic crystalline rocks which occupy large areas on the surface of the globe. These rocks have been generally regarded as having undergone a vast amount of change in lithological character, to which the term metamorphism has been applied. By local or contact metamorphism is meant the transformation which has been effected in ordinary sedimentary strata by the intrusion of igneous rocks, the extent of which depends on the relative size of the intrusive mass.

The mineral afwillite is one of the hydrated calcium silicates which are found in set cements. It has the empirical formula 3CaO.2SiO2.3H20; a recent determination of the structure by Megaw (in press) has shown that the structural formula is Cas(SiO3OH)2. 2H2O, the hydroxyl groups being linked directly to silicon atoms. When afwillite is heated it loses water. Parry and Wright observed that practically all the water could be driven off by heating for prolonged periods at 300° C., or for shorter periods at higher temperatures. They also found that a further reaction took place on heating to about 1300° C. The present work was undertaken to investigate these changes, and to identify the products formed, using-X-ray powder methods.

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.

Suppose that in the study of a mineral we have determined the cell-dimensions and the density, and made a ehemienl analysis. From these we can readily calculate the empirical unit-cell contents, which will usually approach integral values and so suggest a probable formula.

But the calculated empirical unit-cell contents, being based on chemical and physical data subjeet to experimental error, will be subjeet to numerous sources of error, and in any serious study it is very desirable to have some rough idea of the probable limits of error. Unfortunately, this is a matter of considerable complexity, but the following suggestions may be of assistance, though they will doubtless be considerably improved on.

As long ago as 1816, Brewster showed that many crystals have a lamellar structure developed in them by pressure, and his observations became the starting point of a long series of very valuable researches. Reuseh proposed that the planes along which such structures are developed by mechanical means should be called "Gleitflächen" (gliding-planes), and he, Pfaff, and other mineralogists have investigated a number of curious cases of the kind. Baumhauer in 1879 showed that the lamellar structure could be brought into existence, in the ease of calcite, by simply inserting s knife-blade in a definite direction into cleavage fragments of the mineral, and his observations were confirmed in the following year by Brezina.

Experimental data in the system CaO-MgO-FeO-SiO2 suggest that there may be a plateau on the liquidus and solidus of the multicomponent system basalt-peridotite. If this is so, fusion of peridotite would produce only basaltie magmas over a wide temperature range; when the temperature reached a value such that the liquid crossed the threshold of the plateau, there would be a rapid increase in the amount of fusion for small temperature increases, with the formation of picritic magmas; basaltic magmas containing suspended forsteritic olivine crystals could dissolve them if the temperature rose slightly above that of the plateau threshold; a high proportion of a picritic magma would crystallize in a small temperature interval, with the precipitation of forsteritic olivine that was only slightly zoned. These possibilities are compared with current theories, and it is concluded that several petrological axioms may require critical re-examination. An experimental procedure is outlined to determine the shape of the liquidus and solidus in the basalt-peridotite system.

It has been shown recently by Brindley and Robinson that anatase could be recognized by X-ray diffraction technique in fireclays from Scotland and Yorkshire which contained from one to two per cent. of titanium dioxide. These authors remark that the anatase appeared to be of a fineness comparable to the kaolin mineral which was the main constituent, and although kaolinites from various sources throughout the world were examined, in no case was either anatase or rutile detected.

The object of this note is to draw attention to the presence of anatase in the sedimentary kaolins of Georgia and perhaps South Carolina, U.S.A. According to Smith 115 kaolin samples from Georgia had on an average 1·13 % of TiO2 by chemical analysis. The values ranged from 0·5 to 2·2 % of TiO2. This is in marked contrast to most primary kaolins; those of Cornwall and Devon, for instance, have as a rule below 0·2 % of TiO2.

The object of this note is to place on record an interesting occurrence of vanadiferous nodules on the coast of South Devon. I have to thank Lord Clinton, the owner of the property on which the nodules occur, for the assistance and encouragement he has given me in this matter, and Lieut.-General Sir William Furse, the Director of the Imperial Institute, for kindly arranging to have chemical analyses carried out by Miss Hilda Bennett in the Institute Laboratory. I also have to thank Mr. Thomas Crook, of the Imperial Institute, for the trouble he has taken to write up this short paper on the basis of my notes and the laboratory work done at the Institute.

X-ray study of tuhualite from Mayor Island, New Zealand, has shown it to be orthorhombic, space-group either . Unit-cell dimensions are a = 14·31 Å, b = 17·28 Å., c = 10·11 Å., and a : b : c = 0·828 : 1 : 0·585. Goniometric measurements give a : b : c = 0·8243 : 1 : 0·5658. a{100} dominant with forms s{011}, p{021}, c{001}, b{010}, n{101}, m{110}, q{111}. Crystals with b{010} dominant are rare. Three good cleavages, a (100), b (010), c (001). α = 1·608, β = 1·612, γ = 1·621; γ-α = 0·013. , positive; β = b, γ = c, α = colourless to pale lavender, β = violet, lavender, γ = intense purplish-blue; γ > β > α. Sp. gr. 2·89 (measured), 2·86 (calculated).

An analysis of carefully purified material leads to the possible formula:

H9(Na,K,Mn)12Fe″6(Al,Fe‴,Mg,Ti)9[Si8O8]15.

Details of alteration phenomena and associated minerals are given. Occurrence is held to result by crystallization from magma quite impoverished in lime due to early fractionation of aegirine-hedenbergite, and perhaps aegirine-augite, in the intratelluric stage.

Thebaite-(NH4), (NH4,K)3Al(C2O4)(PO3OH)2(H2O), is a new mineral species (IMA2020-072) from the Rowley mine, Maricopa County, Arizona, USA. It occurs in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing NH4+. Other secondary minerals found in association with thebaite-(NH4) are antipinite, vanadinite and at least one other new mineral. Crystals of thebaite-(NH4) are colourless blades up to ~0.1 mm in length. The streak is white, lustre is vitreous, Mohs hardness is 1½–2, tenacity is brittle and fracture is splintery. There are two good cleavages in the [010] zone, probably {100} and {10$\bar{2}$}. The calculated density is 2.093 g⋅cm–3. Thebaite-(NH4) is optically biaxial (–) with α = 1.490(2), β = 1.534(2), γ = 1.570(2) (white light); 2V = 82.7(5)°; slight r > v dispersion; and orientation X = b, Y ^ c = 13° in obtuse β. Electron microprobe analysis gave the empirical formula [(NH4)2.12K0.69Na0.20]Σ3.01(Al0.84Fe3+0.11V3+0.04)Σ0.99(C2O4)[(P0.98Si0.02)O3OH]2(H2O), with the C, N and H contents constrained by the crystal structure. Raman spectroscopy confirmed the presence of NH4 and C2O4. Thebaite-(NH4) is monoclinic, P21/c, with a = 11.156(9), b = 6.234(6), c = 18.651(16) Å, β = 102.928(15)°, V = 1264.2(19) Å3 and Z = 4. The structural unit in the crystal structure of thebaite-(NH4) (R1 = 0.0612 for 863 Io > 2σI reflections) is a double-strand chain of corner-sharing AlO6 octahedra and PO3OH tetrahedra decorated by additional PO3OH tetrahedra and C2O4 groups. The decorated chains connect to one another through bonds to NH4+ and K+ and through hydrogen bonds.

Variations in the value of 2θ131 – 2θ13̄1 of two anorthoclase feldspars of compositions Ab95Or5 and Ab90Or10 were investigated by hydrothermal crystallization of gels for long periods in the temperature range 500–950 °C, at water vapour pressures up to 15 000 lb/in.2. The results are compared to those obtained by MacKenzie (1957) for pure albite. It is shown that the variation in 2θ131 – 2θ13̄1 is largely the result of Al-Si order-disorder and that at 700 °C and above the magnitude of the variation in the anorthoclases suggests that the degree of order is like that in albite, but at lower temperatures, particularly below 600 °C, the variation is less, and the anorthoclases are less ordered than albite at the same temperatures. At 600 °C and above the rate of attainment of equilibrium is the same for all three compositions. Below 600 °C Ab95Or5 behaves differently to albite. At low temperatures the equilibrium degree of Al-Si order in mixed Na-K feldspars is a function of composition as well as of temperature of crystallization.

Compositions of An61 and An69·5 have been determined for two domains in plagioclase from a gneiss. Optical properties indicate that the same compositions apply respectively for an intergrowth of ‘matrix’ and lamellae found in zones between the domains. The intergrowth appears to span the composition range between the Bøggild and Huttenlocher intergrowths.

We combined statistical analyses and GIS capabilities within the statistical environment R to create a semi-automated method for the assessment of As hazard risk in shallow groundwater in Cambodia. Arsenic concentration data for groundwaters of between 16 and 100 m depth were obtained from 1437 geo-referenced wells. We created a binary logistic regression model with these As measurements as the dependent variable and a number of raster maps (DEM-parameters, remote sensing images and geomorphology) as explanatory variables, and considering an As threshold of 10 ppb. This allowed us to make an As hazard map for groundwaters between 16—100 m depth: this can be used to help to identify populations vulnerable to exposure. The logistic regression analysis indicates a good correlation between topographic and geomorphologic environmental variables and the As hazard risk in groundwater. Ease of implementation, and the ability to update, along with objectivity and reproducibility are the main advantages related to this method of analysis.