A giant chondrule, nearly 4 cm in diameter, has been found in the Parnallee meteorite, and is described, with analysis. An estimate of the composition of the partly devitrified groundmass falls within the known compositional range of chondrule glasses. The bulk composition of the chondrule is very close to the bulk composition of the meteorite excluding metal and sulphide.

In the south-eastern part of the Khetri copper belt, actinolite occurs in association with alteration assemblages resulting from the Fe-Mg metasomatism that accompanied sulphide mineralization, and more commonly with albite-bearing rocks formed by albitization of quartzites and schists near granitic rocks. Within the latter occur many coarse, massive, and unoriented aggregates of actinolite crystals, individuals being commonly more than 10 cm long. Locally fluorite-bearing veins oecur within granitic and albite-quartz rocks. The actinolite is pleochroic from pale pink to green; γ: [001] = 26°; γ = 1·642 ± 0·003; 2Vα = 80°. The composition of the analysed actinolite closely compares with those published in the literature excepting in (OH), which is low. The mineral assemblages, formed by metasomatic replacement of pre-existing rocks, are equivalent to those of albite-epidote-amphibolite facies. The metasomatic fluid was apparently rich in Ca, F (indicated by fluorite), and oxygen (indicated by magnetite, ilmenite, and hematite).

Myrmekite-like intergrowths in the Lewisian gneiss and the igneous complex of Loch Borolan are described. The intergrowths are of diverse mineral composition but have many features in common with myrmekite and it is shown that they result from the constriction of a previously existing mineral during the growth, in the solid state, of the host mineral. More than one stage of intergrowth is often found and these stages can be correlated with geological events.

Diaboleïte has been synthesized between 25 and 100° C at atmospheric pressure and approximate water vapour partial pressures of 14·7 lb/in2. Under similar conditions at 170° C cumengéite appears to be the stable phase produced from a diaboleïte composition. Synthetic diaboleïte is much simpler morphologically than the natural mineral but the hemimorphic symmetry is more clearly demonstrated morphologically in the artificial specimens. A comparison of X-ray powder data for natural and synthetic diaboleïte shows almost exact detailed correspondence in line position and intensity between 0 and 180° 2θ. The cell dimensions obtained from X-ray powder data are a 5·869 ± 0·002 Å and c 5·495 ± 0·003 Å.

An enstatite-cordierite-sillimanite-quartz rock from a pyroxenegranulite facies terrain in southern Rhodesia gives evidence of at least two stages of recrystallization : an early, high-pressure recrystallization to form the assemblage enstatite-kyanite-quartz, and a later, ‘retrogressive’ recrystallization during which kyanite inverted to sillimanite, and cordierite was formed by reaction. The rock itself has ratios of R2+ : Al : Si that are virtually those of cordierite ; the recent data of Schreyer (1967) may thus be applied to the occurrence. The early kyanite-enstatite recrystallization occurred under pressures in excess of 10 kb, corresponding to depths greater than those of the continental Moho.

Olivine crystals showing {100} deformation lamellae between crossed polars show spots streaked out transversely to 00l layer lines in X-ray oscillation photographs. Compositional inhomogeneity is ruled out by optics and electron microprobe analysis. Structurally controlled gliding on {100} (plane of greatest atom density) in the direction [001] (close-packing direction) is deduced on analogy with deformed metals. Mechanism of lamella formation is discussed in terms of superincumbent load; but it is concluded that pressures due to crystal growth are mainly responsible.

The observed order of importance of the forms of cerussite has been determined by the methods introduced by Donnay and Harker, used in a modified form. The space group derived by morphological analysis is Pmcn, as determined by X-ray diffraction. For the more common forms good agreement is found between the observed and theoretical orders of importance. The rank distribution of the more important forms is given.

A hypothesis for the mechanism of oxidation in titanomagnetites is first put forward on the basis of new oxidation data for pure magnetite and the new model for cation distribution in titanomagnetites. Contrary to previous claims by other workers, it is shown that single-phase spinel oxidation products of titanomagnetites can only be produced under very exacting circumstances. Lowtemperature magnetic measurements on such synthetic, oxidized materials were made to show the formation of magnetite by the decomposition of the metastable oxidation products of titanomagnetites on prolonged heating at moderate (∼ 390° C) temperatures. This magnetic method of detection is more sensitive than that of normal X-ray diffraction. It is concluded that previous claims of an increase in magnetization due to the formation of synthetic single-phase, highly oxidized titanomagnetites must be due to the formation of undetected magnetite.

The classification of framework structures built from chains of four-membered rings of type UUDD is incomplete: there are two basic ways in which the chains can be cross-linked to form eight-membered rings. The first type of frame-work (already described) is extremely flexible, permitting the cavities to be either large or small; the second type is inflexible and the cavities are small. Feldspar belongs to the second type and had been incorrectly ascribed to the first type: paracelsian, harmotome, and gismondine had been correctly placed in the first group.

An interpretation of part of the hydroxyl-stretching region of the infra-red absorption spectrum of biotite is proposed. A consideration of band shapes enables the interpretation to be quantified and tested against a series of natural biotites. The equation derived may be used to determine the atomic ratio Mg2+/(Mg2++Fe2+) in biotites but the necessary assumptions and simplifications make it unlikely that the method will be very useful in practice.

The crystal structure of gersdorffite (III) has been examined with three-dimensional Weissenberg X-ray diffraction data. The unit cell is isometric with a 5·6849 ± 0·0003 Å, space group PI, and four formula units per cell. This structure has the sulphur and arsenic atoms equally distributed over the non-metal atom sites of pyrite. All atoms show significant random displacements from the ideal pyrite positions to produce triclinic symmetry, which serves to distinguish this mineral from a disordered cubic gersdorffite (II) and a partially ordered cubic gersdorffite (I). Factors responsible for the atomic distortions are discussed.

The results of the electron-probe microanalysis of the three minerals from orthopyroxene-amphibole-spinel coronas between olivine and plagioclase in a troctolite from Sulitjelma, Norway, are presented. They are shown to be incompatible with a late-magmatic origin for the coronas, but compatible with an origin by diffusion of material in solution in an aqueous medium across the olivine-plagioclase interface. This theory is discussed on a quantitive basis in the light of the microanalytical data.

New chemical analyses, cell-sizes, and specific gravities are given for 12 garnets, and TiO2 values, cell-sizes, and refractive indices are presented for a further 33 garnets all belonging to the andradite-melanite-schorlomite series. The increase of refractive index, cell-size, and specific gravity with increasing titania is shown graphically. A plot of Ti against Si atoms in the unit cell suggests that Ti replaces Si rather than Fe3+ in the structure, while electron-probe results indicate that Si or (Si+Fe) may be replaced by Ti. The chemical and infra-red absorption data are considered in a discussion of the structural role of titanium in these garnets : both Ti4+ and Ti3+ may be present.

The weathering of biotite in some freely drained soils developed from a biotite-rich quartz gabbro near Strathdon, Aberdeenshire, has been studied. Using a combination of X-ray, chemical, differential thermal, infra-red, and optical techniques, it was found that the biotite weathers to an intergrade aluminous vermiculite-chlorite together with zones of kaolinite and gibbsite. The crystallographic axes of biotite and included kaolinite are common and parallel and it is proposed that both kaolinite and gibbsite zones crystallized epitaxially within opened out cleavage spaces of the host biotite. The development of kaolinite and gibbsite in this manner could have occurred relatively quickly and the significance of this with regard to generalized pedogenic weathering reactions of biotite is discussed.

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.

Measurements have been made of the magnetic susceptibility and of the polarized absorption spectra of epidotes of the Al-Fe, Al-Fe-Mn, and Al-Cr series. Magnetic susceptibility measurements at room temperature by the Guoy method indicate that Fe3+, Mn3+, and Cr3+ are in ‘high-spin’ electronic states in the epidote structure, and the electronic transitions observed in the absorption spectra have been assigned on this basis. The polarized spectra, measured over the range 4000 to 22 000 Å by a polarizing microscope using a universal stage technique, showed each epidote group to be distinctly pleochroic. This pleochroism correlates with the presence of ions in distorted coordination sites.

Extinction coefficients of Fe3+ in epidote are higher than those for other Fe (III) compounds, indicating that Fe3+ ions occupy the non-centrosymmetric (Al, Fe) site. The low value (13200 cm−1 at 0·155 Fe3+ and 12700 cm−1 at 0·915 Fe3+) of the crystal-field splitting parameter, Δ, implies that the site is easily expanded to accommodate ions larger than Al3+ There is also evidence that increasing Fe3+ content causes distortion of the (Al,Fe)O6 octahedron, suggesting a possible mechanism for the zoisite ⇌ clinozoisite transition.

The spectra of Al-Mu-Fe epidotes have been interpreted in terms of intense contributions from Mn3+ ions in the non-centrosymmetric (Al, Fe) site, and weak contributions from Mn3+ ions in the centrosymmetrie AlOH site. Values of Δ vary from 13730 cm−1 (0·082 Mn3+) to 13400 cm−1 (0·747 Mn3+). Both (Al, Fc) and AlOH sites have strong tetragonal distortions (c/a ~ 0·95), leading to the observed large Jahn-Teller splitting of 12000 cm−1.

The Al-Cr epidote spectra show weak absorption bands, attributed to Cr3+ ions in a centrosymmetric site of approximately orthorhombic symmetry (i.e. the AlO site). The value of Δ is about 16300 cm−1.

Review of all available data on the distribution of Fe3+, Mn3+, and Cr3+ between the sites gives the following site preferences: Fe3+, Al > AlO > AlOH; Mn3+, Al ≥ AlOH ≫ AlO ; Cr3+, AlO ≫ Al, AlOH.

Forty-eight samples of tourmaline from the granitic rocks of south-west England have been analysed by emission spectrography for Fe, Mn, Mg, Ca, Ti, Na, F, Li, Sc, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Zr, Sn, and Sc. The tourmaline reflects the fractionation trends of the rocks from which it comes in the proportion of Mg, Ca, Ti, and certain trace elements it contains and is considered likely to have been a primary product of the magma. Textural evidence that indicates a later development is interpreted as recrystallization, possibly at low temperature, with little change in chemical composition. Hydrothermal tourmaline has a distinctive chemical composition, being higher in Mg, Ca, Sr, and Sn and lower in Fe, Mn, and F and also tending to have a higher Cr, V, Ni, and Sc content. The chemical composition of the tourmalines is used to show that some quartz-tourmaline rocks are the product of extreme fractionation and are not completely tourmalinized granites. Parti-coloured tourmaline is shown to be the result of hydrothermal growth.

A hypothesis has previously been advanced to account for the termination of the dehydrogenation process for the oxidation of South African crocidolite : it was postulated that the presence of magnesium ions would block the electronic migration required for complete oxidation of iron(II) in the temperature range 400° to 500° C. In view of criticism of the validity of the evidence for this hypothesis, new data for the oxidation of Bolivian crocidolite, which has a much higher magnesium content than the South African material, is presented; this is more satisfactory evidence of the validity of the hypothesis.

Gersdorffite belongs to space group P213 in the cubic system with cell dimension a 5·6885 ± 0·0003 Å and four formula units per cell. The hk0 data were measured from precession photographs obtained with Mo-Kα radiation. The structure was solved by Patterson methods and refined by least-squares techniques. This structure has complete ordering of arsenic and sulphur similar to ullmannite.

Some sillimanite-kyanite quartzites, which may grade into sillimanite-kyanite topazfels, from NE. Tanzania are described. In view of the suggestion that a solid-solution series might exist between topaz and sillimanite, the cell dimensions and some optical properties of both minerals were investigated. No abnormal results, suggesting solid solution, were obtained and the coexistence of topaz, sillimanite, and kyanite is ascribed to particular physico-chemical conditions.