New version page

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

Upgrade to remove ads
Upgrade to remove ads
Unformatted text preview:

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,

View Full Document
Download Lecture Overview for Part 3 Growth and Form of Minerals
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...

Join to view Lecture Overview for Part 3 Growth and Form of Minerals and access 3M+ class-specific study document.

We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Lecture Overview for Part 3 Growth and Form of Minerals 2 2 and access 3M+ class-specific study document.


By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?