On synneusis |
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| Authors: | Joseph A Vance |
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| Institution: | (1) Department of Geological Sciences, University of Washington, 98105 Seattle, Washington |
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| Abstract: | Synnneusis is the process of drifting-together and mutual attachment of crystals suspended in a melt. This process is episodic, is most characteristic of the earlier stages of consolidation, and appears to be related to magmatic turbulence. Union of crystals in synneusis relation normally occurs on their broader faces in preferred orientations which coincide with positions of low interfacial energy. Quantitative studies of several common igneous minerals indicate that crystals of a single mineral characteristically show a strong affinity for synneusis and typically unite in parallel or twinned orientation. Some pairs of unlike minerals join readily in synneusis relation, but most appear to be antipathetic.Synneusis structures have generally been overlooked or misinterpreted as epitaxial intergrowths, primary twins, irregular growth forms, or the random union of crystals which have grown into contact. These possibilities must be rejected where it can be shown that two or more distinct crystals are involved, that they were relatively large when they came in contact, and that they are oriented with prominent faces in common.Synneusis is responsible for three major features of the magmatic fabric: (1) the small scale segregation of minerals; (2) the systematic mutual orientation of adjacent crystals in synneusis relation; and (3) the morphology of their common boundary. Because synneusis structures are restricted to igneous rocks and are widespread and easily recognized, they provide a definitive and ready criterion of magmatic origin. |
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