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WUSTL EPSC 352 - Lecture Overview for Part 3 Growth and Form of Minerals

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D:\352\2009\LectureNotes\Part3GrowthAndForm.wpdEPSc 352: Lecture Overview for Part 3, Growth and Form of Minerals(( Read pp. 217-234 in your text. (For those who are interested, chapter 7 in the book byBlackburn and Dennen is very informative.)K Formation and Growth of Crystals (pp. 217-222 in text)Crystal (crystalline solid): homogeneous solid with long-range, 3-D order internally (whether ornot it has well formed external faces).Internally: Long-range, medium-range, short-range order OR amorphous (glass)Externally: Grain size (macro-morphology) is macro-crystalline, micro-crystalline, crypto-crystalline, or amorphous.Perfection of formation of faces: euhedral (all flat, smooth, well formed), subhedral (some goodfaces), anhedral (faces not formed well)“The plan”: discuss a little about crystal growth from chemical, morphological view; then discussfrom viewpoint of internal structure and geometry why crystals are shaped as they are.Crystal formation processes:vapor º solid condensation/sublimationsolution º solid precipitationmelt º solid freezing/crystallizationsolid A º solid B phase transformation (e.g., polymorphic phase transition) º º 4 mechanisms of crystal formation: evaporation, decrease in temperature, change inpressure, reaction (e.g., during metamorphism of pre-existing minerals)Driving force for crystal formation and growth: ENERGY. Products should have lower freeenergy than initial phases.Two stages of growth are nucleation and grain growth.P Saturation is key: solution/melt cannot dissolve any more of some components; ppt. occursDegree of supersaturation: See hand-out 35.Large degree: many nuclei; form a “gel”Moderate degree: many small crystals formSmall degree: few, large crystalsP Nucleation can be homogeneous or heterogeneoussee nuclei form in the solution/meltnucleation on walls of vessel, on “dust”Pathway to nucleation and grain growth is shown on free-energy diagram. See hand-out 36.2Formation of critical-size nucleus depends on surface area:volume ratio.P Growth differential ratesfill in sites as in “parent”changes in shapeeuhedral, subhedral, anhedral shapesrapid growth – supersaturated, hopper/dendritic growth See hand-out 37.crystal growth limited by diffusionHow does growth take place? Physical nature of growth See hand-out 38.stepwise, trying to satisfy bondsgrowth often favored on “accidental sites”defects/dislocations frequently form, affect mineral’s properties“growth faults” can be propagated by continued growth along surface of mismatch(( For these next topics, please read textbook chapter 10 (pp. 222-234).P Defects are of 3 types: point, linear, planarSchottky – missing ionFrenkel – misplacement POINT DEFECTSinterstitialsubstitution SOLID SOLUTIONEdge and screw dislocations – by initial growth or secondary processSlip – usually secondary process (stress)TEM (transmission electron microscope) to image individual atoms, as well as these defectsP Growth forms of same mineral with itselfPolygranular growth phenomenarandom intergrowthsparallel growth – accidental, interruptedtwins – 2 or more parts of a complex object are identical; related by symmetry operationcreated by errors in the first...coordination sphere of an atomContrast with Oriented intergrowths of unlike crystalsepitaxy: B grows on Atopotaxy: overgrowing phase B is a reaction product of underlying phase ATwin operation – adds symmetry not available in the original single crystalPrimary (growth) and secondary (glide, deformation) twins3Phase transformations (polymorphism) can create twinsMultiple (polysynthetic) twinning – striationsContact vs. penetration twins(( For these next topics, please read textbook chapter 12 (pp. 266-281).P Relations among Mineral StructuresIsostructural – same structure, but different composition, e.g., among NaCl, KCl, PbS. Classification, e.g., “calcite group” vs. “aragonite group”Polymorphism – different structure, but same composition (e.g., TiO2, SiO2, C).3 types of solid-solid transformation:displacive: energetically easy, rapid, reversible. Example, "-$ quartz transition.reconstructive: energetically demanding, sluggish. Example, graphite-diamondtransition.order-disorder: more subtle, re-arranging of atoms in a structure (e.g., K-feldspar)1) temp., 2) degree of disorder, 3) symmetrysanidine, orthoclase, microclinePolytypes – compare to stacking choices with the ping-pong-ball layersPseudomorphs – chemical replacement, with retention of original (physical) morphology. Example: goethite pseudo after pyrite.Environments of Mineral Formationtemperature, pressurechemical componentsdegree of saturationeffects on grain size, shape,


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