Pan-African plates: Additional evidence from igneous events in Saudi Arabia

The chemical and geological nature of thec. 600 m.y. igneous complex of the Al-Hadah escarpment of Saudi Arabia is fitting to that of a modern subduction environment. It appears that plate motions were occurring during this Pan-African tectonic event but the scale of both plates and motions was smaller than that of the present time.     

Dislocations and voids in pyroxene from a low-temperature eclogite: Mechanism of eclogite formation

New occurrences of the phosphates sarcopside/graftonite (Fe, Mn)₃(PO₄)₂, farringtonite (Mg, Fe)₃(PO₄)₂, and brianite Na₂CaMg(PO₄)₂ in iron meteorites are reported. The sarcopside in Duel Hill (1854) (IVA-Anom) contains less than 0.1 mole % Mn₃(PO₄)₂ and more nearly approaches the Fe-rich end member than does any previously reported natural occurrence. This identification of farringtonite in Barranca Bianca (IIE) is the first report of this mineral in a meteorite group other than the pallasites.     

Omphacite in Californian metamorphic rocks

Omphacite is a common mineral in greenstones, metasediments and related Franciscan rocks of the glaucophane schist facies. It also occurs in late veins cutting amphibolites, glaucophane schists, eclogites, greenstones, and occasionally metagraywackes. It is apparent that this mineral is stable under glaucophane schist facies conditions in rocks of a suitable bulk composition, and is not restricted to the eclogite facies. Association with albite, quartz and lawsonite, and late veining of omphacite veins by aragonite indicates that pressures necessary to form omphacite are reasonably close to those calculated from an ideal solution model.     

Weathering of apatite under extreme conditions of leaching

Apatites occurring in lateritic residues derived from high-grade metamorphic rocks from Sri Lanka are hydroxy-apatite as indicated by EPMA and XRD. Weathered phosphates and weathered crystalline products are composed of carbonate fluorapatite, crandallite, wardite, fluellite, kaolinite, goethite and gibbsite. EDAX, DTA and SIMS show the presence of amorphous materials, such as oxides of Fe, Al, Si, Al+Si, and S, on the surface of weathered phosphate. Using SEM, TEM, EDAX and electron diffraction techniques, the presence of these amorphous and crystalline materials with several transitions has been clearly identified in weathered phosphate. Recrystallization of phosphate occurred during weathering by dissolution, nucleation and precipitation. The lattice images of hydroxyapatite are gradually increased from 8.3-A basal spacings by weathering, and form mosaic bound fragments. Chondrite-, shale- and phosphorite concretion-normalized REE patterns of these apatites, known to have sedimentary origins, suggest enrichment and remobilisation during post-depositional reworking.