Elsevier

Earth-Science Reviews

Volume 20, Issue 4, August 1984, Pages 265-344
Earth-Science Reviews

Orbicules: An indication of the crystallisation of hydrosilicates, I

https://doi.org/10.1016/0012-8252(84)90021-7Get rights and content

Abstract

Observations relative to orbicular granites support later rimming layers built up on a previously formed nucleus; plasticity and a soft stage during the development of the orbicule; initial fluidity and a repeated fluidity of the inter-orbicular matrix after or during orbicule formation; diffusion of material to add onto and build up individually in each orbicule the successive rhythmic “shells”; continuous growth of the orbicules even after some disruption; and crystallisation of the orbicular material later than the formation of the orbicular structure.

To be consistent with these observations, the “magma” in which orbicules develop must have the diffusive and rheological properties of a concentrated macromolecular paste or gel of mixed hydrosilicates. The way in which all available water “dissolves” in such rock-forming magmas at elevated temperature is by reaction with the silicates to form disordered solvated precrystalline hydrolysates such as silica gel, clay and hydrous ferromagnesian minerals. Such an intrusive mass has the appropriate diffusive and thixotropic properties for: (1) the formation of accretions (aggregated nuclei); (2) the overgrowth of concretions (rhythmic rimming structures); and (3) the repeated isothermal reliquifaction of the interorbicular matrix.

The orbicules and matrix have subsequently crystallised and further heated by exothermic dehydration reactions where the rate of reaction is governed by the rate at which water escapes from the system. Crystallisation is not by cooling through a melting point.

Twenty-eight characteristic features of orbicules are described and illustrated which fully accord with twelve properties and phenomena attributable only to particle interactions in an alternately dynamic and static colloidal system. Such an extraordinarily high correlation between the unique properties of the envisaged system and the resultant distinctive complex orbicular structures leaves little room for any alternative interpretation. Therefore orbicules can be confidently assumed to be an indication of the crystallisation of precursor hydrosilicates.

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