302-pp6 - GY 302: Crystallography & Mineralogy...

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Unformatted text preview: GY 302: Crystallography & Mineralogy GY 302: Crystallography & Mineralogy UNIVERSITY OF SOUTH ALABAMA Lecture 6: Polymorphism and Crystal Habit Lecture 6: Polymorphism and Crystal Habit Last Time 1. Space Groups (glides and screw axes) 2. Crystal Growth (nucleation, transport of ions) 3. Twinning (Lots of Twin Laws) a Space Groups But there are two other combination symmetry operations that must be considered: Glides : a combination of translation and reflection Pyroxene structure a m The Point Groups But there are two other combination symmetry operations that must be considered: Screw Axes : a combination of translation and rotation Possible combinations: 2-fold, 3-fold, 4-fold, 6-fold) This is an example of a 4-fold screw axis (4 translations of distance a in 360 of rotation) a Space Groups 230 space groups The combination of reflection, rotation, inversion, glide and screw axes with Bravais Lattices leads to an additional 157 space groups (total 230). 1) Volume energy . Consider a crystal of volume v. G v = [ G f(crystal)- G f(fluid) ]v If G v is < 0, the crystal is at a lower energy and is stable. It can precipitate under these conditions. 2) Surface energy . G s = a Where is the surface energy per unit area and a is the unit area of the crystal. Gibbs Free Energy of Formation (for a crystal) Crystal Growth Gibbs Free Energy of Formation G f = G v + G s Consider a spherical crystal of radius r: G f = G v 4r 3 + G s 4r 2 3 Crystal Growth r c In order for crystal nucleation to occur, crystals must overcome the nucleation energy energy Crystal radius nucleation energy Twins and Twinning There are two basic styles of twins 1) Contact Twins : where a twin plane acts as a compositional surface separating 2 individual crystals 2) Penetration Twins : where an irregular surface generated by a twin axis or a twin center separates two individual crystals 1) Orthoclase with {021} as the twin plane 2) Orthoclase with [001] as the twin axis Todays Agenda 1. Polymorphs and Polymorphism 2. Pseudomorphs and other definitions 3. Crystal Habit Polymorphs & Polymorphism Polymorphism : literally translates as many forms Polymorphs & Polymorphism Polymorphism : literally translates as many forms In crystallography/mineralogy, it generally means that one...
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302-pp6 - GY 302: Crystallography & Mineralogy...

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