Formation Mechanisms for Mineral Replacement
For hetero-oriented replacement, dissolution–precipitation is metasomatic mechanism. Two indispensable conditions are required: (1) a mineral prone to be replaced on one side; (2) a same or similar mineral as replacive one on the other side, acting as nuc
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Formation Mechanisms for Mineral Replacement
Formation mechanisms for mineral replacement are dissolution–precipitation (crystallization) and ion exchange. Co-oriented replacement of a certain mineral may arise from appropriate hydrothermal fluids entering along grain boundary with tiny cracks. For hetero-oriented replacement, however, two additional prerequisites are required: (1) a mineral prone to be replaced on one side; (2) a same or similar mineral as replacive one on the other side, acting as nucleation center for its growth. Without the second prerequisite, the hetero-oriented replacement would hardly occur. The passage for hydrothermal fluids from outside is discussed first.
2.1
Passage for Gas and Liquid from Outside
For entering and moving of hydrothermal fluids from outside, the largest openings must be fracture fault zones. The smaller openings are joints and fissures in rocks, while the tiniest openings are grain boundaries, cleavages and micropores in minerals. Many researchers consider that the rock should be broken to provide a space for hydrothermal
fluid to enter and then to cause metasomatism. Some researchers (e.g., Collins L.) believe that the rocks had once been broken, but then recovered, i.e., the traces of deformation were fully eliminated after recrystallization. This idea is quite doubtful. The texture of the rock subjected to metasomatism, however, is quite complete rather than cataclastic. Mineral replacement develops widely in entirely solid rocks, not only in fracture zone or along joint and its sides. The granites that have been subjected to multiple metasomatism might not have been broken before. General speaking, under compression stress, quartz is easily affected, resulting in undulatory extinction from weak to strong, and biotite is deformed, while feldspars are more stable. Under stronger stress, smaller crystals may be crushed to aggregates, and cleavage of biotite and twinned lamella of plagioclase may be curved or disrupted, while K-feldspar, especially larger grains, remain nearly unchanged because of its higher compressive strength and tenacity, and only peripheral corners may be abraded and ground. Nevertheless, whole granitic rocks are still characterized by their original magmatic hypidiomorphic granular textures although having been subjected to tectonic and metasomatic processes.
© Science Press and Springer Science+Business Media Singapore 2016 J. Rong and F. Wang, Metasomatic Textures in Granites, Springer Mineralogy, DOI 10.1007/978-981-10-0666-1_2
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Furthermore, no triple junction textures1 typical for recrystallization occur throughout the whole rock. Therefore, the absence of hints of cracks cannot be simply explained by their elimination due to recrystallization. At most there is coalescence phenomenon in weakly broken quartz. Microscopic observation indicates that the real replacement phenomena have surely occurred at one side or both sides of grain boundaries without obvious traces of deformation. So we can not but admit that grain boundaries and
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