Minium occurs at Broken Hill, New South Wales, as pseudomorphs after cerussite, formed during fires in the mine. It is tetragonal with a = 8·824 Å. and c = 6·564 Å.; sp. gr. 8·2 + 0·2. The Broken Hill minium appears identical in physical properties with synthetic Pb3O4 described by other workers.

When using many patterns of integrating stage the depth of focus is so great that it is not possible to distinguish between the upper and lower surfaces of a thin section; this may introduce serious errors into micrometric analyses. A procedure to meet this difficulty is suggested.

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.

Conclusions concerning the unit-cell contents and chemical formula of a mineral are all too often drawn from a small part of the available data. A procedure is outlined by which any chemical analysis for which a density is available can be utilized, provided X-ray data are available for a reasonable range of analysed specimens. The effects of possible errors in the determination of water, and in the assessment of essential and non-exsential water, are discussed both qualitatively and quantitatively.

A survey of all available data for anthophyllite has not disclosed any specimens in which the number of oxygen atoms per unit cell is significantly in excess of 96 with the possible exception of the Glen Urquhart gedrite and the Edwards (New York) material. But it is certain that the number of cations per unit cell is normally well in excess of 60 (partial occupation of the A lattice positions), and that, although ‘excess’ water in sonic fibrous anthophyllites may be adsorbed impurity, the number of hydroxyl groups in others is well in excess of 8 per unit cell; it is also fairly clear that the number of hydroxyl groups may fall below 8 per unit cell.

Sepiolite has been identified in the Keuper Marl at a Midlands locality by X-ray diffraction data, electron microscopy, differential thermal analysis, and chemical analysis. It has been calculated that a sample representing 30 ft. thickness of clay may contain as much as 39 % sepiolite. Apart from sepiolite, the Keuper Marl in the area examined consists mainly of mica, quartz, calcite, and ferric oxide.

Electron micrographs of Na-saturated Unter-Rupsroth montmorillonite show that many of the individual flakes, which are probably 10 Å. thick, have straight edges meeting in 120° angles. This is evidence in support of the pseudohexagonal structure of montmorillonite. Similar pictures were obtained with Lisaturated clay and with ‘Calgon’-dispersed material. The usefulness of electron microscopy for the study of clay mineral morphology depends upon the efficiency of dispersion and the details of the mounting techniques rather than the nature of the exchange cation or the particular dispersing agent employed.

Quartz from a granite pegmatite near Kalkovo, Bulgaria, has undergone partial replacement by biotite along cracks parallel to (01;0); the biotite is in regular orientation relative to the quartz, and appears to constitute an instance of epitaxic replacement, defined as a replacement in which the replacing mineral develops in a regular, structurally controlled orientation relative to the mineral replaced.

Thin-section analysis is essentially an areal measurement, the measurement area usually being the upper surface of the section. If transmitted light is used for the measurement, the apparent areas of opaque grains will in general be somewhat larger than their true areas on the measurement surface. For strictly spherical opaque particles in a transparent matrix the expected excess of apparent over true area is shown to be (πr2k)/(2r + k) where r is the spherical radius and κ is the thickness of the thin section. A table shows the relation between true and apparent area as a function of r/k.