Crystal Growth Mineral Formation & Stability Gibbs Free Energy ...

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http://www.brocku.ca/earthsciences/people/gfinn/petrology/ab-an2.gif Nucleation EQ xtalization -> final xtal comp = starting L comp E. Goeke, Fall 2006 • For minerals to nucleate, the new cluster of atoms/ions must have a lower ΔG then the other phases in the system (e.g. melt, aqueous solution, other minerals) • Whether or not a mineral will nucleate will depend on P, T, and chemical composition of the system • Size of nucleus will determine whether the new mineral will grow in size or dissolve – Greater surface energy = less stable – Small xtals have a high surface area to volume ratio S.A. = 6 * 2 * side length V = (side length) 3 http://www.tulane.edu/~sanelson/eens212/metamorphreact.htm S.A. = 6 * 2 * 5 V = 5 3 S.A./V = 60/125 S.A. = 6 * 2 * 1 V = 1 3 S.A./V = 12/1 E. Goeke, Fall 2006 E. Goeke, Fall 2006 http://www.brocku.ca/earthsciences/people/gfinn/petrology/ab-an3.gif – T d = melt has an equal ΔG to the xtal, so if nuclei form, the high surface energy causes the xtals to simply dissolve back into the melt – T a = xtals have a slightly lower ΔG than the melt, but nuclei must be above a critical size in order to avoid being resorbed; only a small number of nuclei are present & grow – T b = xtals have a significantly lower ΔG than the melt, so many nuclei form & grow fractional xtalization -> final xtal comp ≠ starting L comp E. Goeke, Fall 2006 Winter, 2001, An Introduction to Igneous & Metamorphic Geology Heterogeneous Nucleation • Heterogeneous nucleation occurs by a new xtal taking advantage of a pre-existing surface or flaw to nucleate – Largely eliminates the need for seed nuclei – Reduces surface energy problems • Can occur on: – Specific crystallographic faces of a pre-existing xtal = epitaxial nucleation – On a structural defect such as as grain boundary or another imperfection, where the surface energy was higher for the original xtal but the new nucleation lowers the surface energy E. Goeke, Fall 2006 E. Goeke, Fall 2006, 12:041 Mineralogy 4 T d E. Goeke, Fall 2006
Crystal Growth • Still have to worry about surface energy – Less when new growth is along a pre-existing row or at flaw in the xtal – Screw dislocations cause growth that continually has an edge available – New growth will try and reduce the surface energy of the pre-existing xtal Zoned Crystals • As a xtal grows, it changes the composition of the melt/rock that its growing from • Some minerals can have a variation in their composition (e.g. garnet (Fe,Mg,Ca,Mn) 3(Al,Fe) 2Si 3O 12, olivine (Fe,Mg) 2SiO 4, plagioclase (Na,Ca)Al(Al,Si)Si 2O 8), which means that the xtal could change compositions as it grows • Changes in P, T, and bulk composition can all cause zoning E. Goeke, Fall 2006 Rate of Growth • Xtals will grow fastest usually along longer dimensions of the unit cell (e.g. halite will grow fastest along the {111} faces & slower along the {100} faces) • Slow growing faces tend to be more prominent – Faces with higher surface energy will grow faster – Xtal growth tries to minimize surface energy, so the slow growing faces will tend to dominate in the end E. Goeke, Fall 2006 http://www.mtholyoke.edu/acad/geo/faculty/sd/zoned.html E. Goeke, Fall 2006 E. Goeke, Fall 2006 http://www.engr.ku.edu/~rhale/ae510/lecture2/sld027.htm http://www.jwave.vt.edu/crcd/farkas/lectures/dislocations/tsld007.htm E. Goeke, Fall 2006 Structural Defects • Point defects = xtal imperfections dealing with single locations in the structure – Vacant sites = Schottky defects – Atoms in the improper position = Frenkel defects – Extra atoms/ions = Interstitial defects – Atoms/ions substituted into the structure = Substitutional defects • Line defects = due to deformation at high temperatures; xtals deform along specific crystallographic planes & in preferred directions (slip system) – Edge dislocation = at right angles to the main stress – Screw dislocation = movement is parallel to the main stress • Planar defects = mismatch of the xtal structure along a surface – Grain boundaries – Stacking faults = layers are either out of position or sequence (e.g. ABABAABABA) – Antiphase boundaries = separate two+ sections of a xtal that are related by a simple translation • Both have the same crystallographic orientation http://wwwuser.gwdg.de/~upmp/neu/Arbeitsgruppen/Jooss/vizinal.html E. Goeke, Fall 2006 E. Goeke, Fall 2006, 12:041 Mineralogy 5
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