Reaction induced fracturing during replacement processes |
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Authors: | Bjørn Jamtveit Christine V Putnis Anders Malthe-Sørenssen |
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Institution: | (1) Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, 0316 Oslo, Norway;(2) Institut für Mineralogie, University of Münster, 48149 Münster, Germany |
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Abstract: | Replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales. The
underlying mechanisms are, in general, poorly understood, but both mechanical and chemical processes are thought to be important.
Replacement of leucite (KAlSi2O6) by analcime (NaAlSi2O6
.H2O) is common in silica-poor igneous rocks. A 10% increase in volume is associated with the replacement process, and this generates
stresses that eventually cause fracturing of the reacting leucite. Experimentally reacted leucite samples display characteristic
fracturing patterns that include both spalling of concentric ‘onion-skin’-like layers near the reacting interface and the
formation of cross-cutting, often hierarchically arranged, sets of fractures that divide the remaining leucite into progressively
smaller domains. These structures may explain the ‘patchy’ alteration patterns observed in natural leucite samples and similar,
so-called, mesh-textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks.
They are also strikingly similar to larger scale patterns formed during spheroidal weathering processes. A simple discrete
element model illustrates the mechanics that control the formation of such systems, and shows how these replacement processes
may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes. |
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