The petrography of the Mount Padbury meteorite, previously briefly recorded, is described in some detail. Both the metalliferous host material of the mesosiderite and the varied range of silicate-rich, virtually metal-free enclaves (including both familiar achondrite material and unfamiliar achondrite material) are described. Eucrite, brecciated eucrite, and a peculiar ‘shocked’ form of eucrite (resembling some terrestrial flaser-gabbros) are the calcium-rich achondrite types represented; hypersthene achondrite (including typical diogenite material and unfamiliar material) and olivine achondrite (granular aggregates of olivine not entirely similar to the unique chassignite and single crystals up to 4 in. in length) are the calcium-poor achondrite types represented. The eucrite displays more or less uniform mineralogy, but the mineral constituents are present in varying proportions, and there is a wide range of textural variations recognized. The silicate grain fragments enclosed in the metallic reticulation to form the mesosiderite host material are, significantly, entirely of minerals seen within the achondrite enclaves—plagioclase, hypersthene, pigeonite, olivine, and tridymite.

These results include microscopic analysis of thin sections and polished sections, X-ray diffraction studies, optical determination of refractive indices using mineral grain mounts, and chemical analyses.

The wider implications of this new and unique meteorite find are briefly considered.

Despite a wide range in parent rock compositions, from analcime-olivine theralite to analcime-tinguaite, clinopyroxene phenocrysts in the Square Top intrusion show limited compositional variation. The crystallization trend of the pyroxenes (principally sahlites) lies approximately parallel to the diopside-hedenbergite join of the pyroxene quadrilateral, and in addition reveals slight enrichment in acmite. The limited iron enrichment in the pyroxenes (and in olivine phenocrysts in the theralites and tinguaites) stemmed from progressively more hydrous liquid fractions whose ferric/ferrous iron ratios increased during differentiation.

Properties and possible structures of hydronium trihydrate H9O+4 and hexahydrate H15O+7 are considered. From the 2330 minerals listed in Strunz's mineralogical tables nineteen were selected that appear to contain H+.4H2O or H+.7H2O, and the possibility of discrete H9O+4 or H15O+7 groupings existing in their crystal lattice is discussed. From a crystallochemical point of view the most probable examples of hydronium hydrate compounds are layer lattice minerals like H-montmorillonite, H-vermiculite, troegerite, H-meta-autunite, sabugalite, and hewettite.

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.

A study of inclusions within garnet porphyroblasts demonstrates that planar inclusion bands and trails lie sub-parallel to one of the {110} planes of the host. The inclusions provide a relic of a rock fabric that existed before the growth of garnet. It is suggested that this fabric has determined the crystallographic orientation of garnet and may have arisen by the nucleation and epitaxial growth of garnet within or upon the mica lattice.

Solid-liquid-vapour phase equilibria for the join Ca(OH)2-Ca3(PO4)2-H2O at 1000 bars are determined in the temperature range 700 to 950°C. The isobaric invariant equilibrium portlandite + hydroxyapatite → liquid on the join Ca(OH)2-Ca3(PO4)2 involves a liquid with the composition 96 wt. % Ca(OH)2, 4 wt. % Ca2(PO4)2 and occurs at 765°C. The isobaric invariant equilibrium portlandite + hydroxyapatite + vapour → liquid on the join Ca(OH)2-Ca3(PO4)2-H2O is encountered at 735°C, and involves a liquid containing 92 wt. % Ca(OH)2, 4 wt. % Ca3(PO4)2, and 4 wt. % H2O. The apatites are shown by X-ray diffraction techniques to vary in composition with changes in the bulk composition from which crystallization occurred.

The apatite liquidus is steep; apatite is the first mineral to crystallize from calcium-hydroxide-rich liquids containing as little as 4 wt. % Ca2(PO4)2 (1·8 % P2O5) at temperatures as low as 735°C in the presence of vapour at 1000 bars pressure; and rapid crystal settling of apatite was experimentally noted. These observations suggest that concentrations of apatite in carbonatites are the result of crystal accumulation from liquids relatively poor in P2O5.

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.