Chapter 23 Ans

Chapter 23 Ans - Answers for Chapter 23: Structures and...

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1. What is the difference between rock texture and rock structure ? As used in this text, texture refers to small-scale features in a rock that are penetrative , mean- ing that the texture occurs in virtually all of the rock body at the microscopic scale. Structures are larger-scale features that occur on the hand sample, outcrop, or regional scale. 2. Imagine the mineral albite experiencing sufficiently high temperature that it breaks down to jadeite (new) + quartz (already present). Write the reaction and describe what occurs at the atom- ic scale, using the four steps introduced at the beginning of this chapter. The reaction is Ab = Jd + Qtz. According to the steps described: 1. Detachment of ions from the surface of reacting minerals (presumably dissolving into a small amount of intergranular fluid in most situations). So albite breaks down releasing Na + , Al 3+ , Si 4+ , and O 2- ions which become mobilized. 2. Nucleation of the new mineral(s). Small clusters of jadeite nuclei begin to form. 3. Diffusion of material to the sites of new mineral growth. The Na + , Al 3+ , Si 4+ , and O 2- ions released from the surfaces of decomposing albite crystals migrate to the surfaces of new jadeite growth. Excess Si 4+ , and O 2- ions migrate to preexisting quartz surfaces. 4. Growth of the new mineral(s), incorporating components carried to the surface and trans- portation of excess waste constituents away. Newly formed jadeite and preexisting quartz crystals grow as they incorporate the supply of new ions. 3. How does cataclastic flow differ from intracrystalline plastic deformation ? Cataclastic flow is the mechanical fragmentation of a rock and the sliding and rotation of the fragments. These brittle processes include frictional grain-boundary sliding and fracture, which disrupt the crystals (they lose cohesion). Intracrystalline plastic deformation involves no loss of cohesion in the rock. Several processes may be involved, often simultaneously. These include defect migration, dislocation glide and creep, deformation twinning, etc. 4. What causes strain hardening ? Strain hardening results when dislocation glide occurs along several slip systems with different orientations in a crystal. When glide along different active slip systems intersects, migrating dislocations can interfere, forming dislocation “tangles,” inhibiting further motion on all slip systems. A greater degree of differential stress is then required to overcome the tangle and per- mit slip. This effectively increases the strength of the crystal (hardens it) because more stress is required for ensuing deformation. 5. What is recovery , and what processes aid it? Recovery is the relief of accumulated strain in a crystal by migration of defects to a concen-
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Chapter 23 Ans - Answers for Chapter 23: Structures and...

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