Petrofabrics of laminated gabbros from the Centre 3 igneous complex, Ardnamurchan, Scotland

SummaryPetrofabric analysis of five specimens of laminated gabbro from the Centre 3 igneous complex, Ardnamurchan, was carried out by measuring orientations of grains of plagioclase, olivine, and augite with a Universal-stage. Linear alignments of primary precipitate mineral grains within the laminar fabrics were revealed by plots of data for coexisting plagioclase and olivine or augite. The plagioclase grains involved have tabular forms elongated parallel to their a-axes (rather than their c-axes). The olivine grains have flattened ovoid forms and the augite grains have sub-ophitic forms developed about prismatic primocrysts. In all specimens plagioclase displayed alignment of long grain-axes directed approximately down the dip of the lamination, save for one case in which a weak oblique alignment was observed. These lineations were reproduced by accompanying olivine and augite. The alignments are tentatively ascribed to flowage of a viscous boundary zone to the magma body in which crystal growth was taking place.

Die Struktur des Rubidiumtrimolybdathydrats Rb2Mo3O10 · H2O / The Structure of the Rubidiumtrimolybdate Hydrate Rb2Mo3O10 · H2O

For the first time single crystals of trimolybdate were obtained through a slow trans-formation of the primary precipitate in a slightly acidified solution of rubidium molybdate at a temperature of 70 °C. A structural analysis of Rb2Mo3O10 · H2O revealed that the anion forms a chain with a hexameric identity period built up from MoO6 octahedra. Each Mo atom is bonded with two terminal oxygen atoms. The anion doesn't contain OH groups.

Origin of the White Allivalite, Western Cuillin, Isle of Skye

SUMMARYTholeiitic contact rocks and a wide zone (White Allivalite) rich in primary precipitate calcic plagioclase are marginal to a layered allivalite series in a funnel-shaped intrusion. The high structural level apparently precludes the possibility that the ultrabasic layered rocks are the basal part of an accumulate from a tholeiitic magma. It is suggested, therefore, that a suspension of calcic plagioclase in a liquid more basic than basalt gave rise to an approximately equal volume of layered ultrabasic rocks by fractional crystallization. Emplacement was probably initiated as a tholeiitic cone-sheet complex, followed by the injection of the ultrabasic suspension in a volume sufficiently large to form a funnel-shaped body within the cone-sheet complex.

Differing Powers of Crystal Nucleation as a Factor Producing Diversity in Layered Igneous Intrusions

To account for a type of rhythmic unit found in the Bushveld complex of South Africa and probably in other layered intrusions, a hypothesis is put forward dependent on the different powers of crystal nucleation from supersaturated magma. It is suggested, for example, that the three common primary precipitate minerals, chromite, bronzite, and bytownite nucleated in this order which is probably dependent on the differing complexity of their crystal structures and that the common rhythmic unit, beginning with chromitite, passing upwards into bronzitite and ending with a bytownite-rich rock is the result of these crystals accumulating successively at the bottom'of the liquid.

Zoned Plagioclases in Layered Gabbros of the Skaergaard Intrusion, East Greenland

A limited amount of careful optical work has been carried out on X three analysed felspars of gabbros belonging to the main layered series of the Skaergaard intrusion, east Greenland (Wager and Deer, 1939). The work was begun with the intention of providing precise optical data to be used in further defining the relationship between composition and optics. The primary precipitate felspar crystals, hitherto thought to be devoid of zoning, were found, however, to possess a zoning which prevents the data being used in this way.