Mineral Composition Variability GLY 4200 Fall, 2012

Ionic Substitution - Size Size: Fe2+ ↔ Mg2+ ↔ Ni2+ (0.86Å) (0.80Å) (0.77Å)

Ionic Substitution - Charge Coupled substitution Ca2+ & A13+ ↔ Na+ and Si 4+ Example: Plagioclase feldspar NaAlSi3O8 ↔ CaAl2Si2O8 Void Ca2+ & Void ↔ 2 Na+

Victor M. Goldschmidt Swiss-born Norwegian mineralogist and petrologist who laid the foundation of inorganic crystal chemistry and founded modern geochemistry Born 1888, died 1947 More Information: http://www.todayinsci.com/1/1_27.htm Source: http://www.todayinsci.com/cgi-bin/indexpage.pl?http://www.todayinsci.com/1/1_27.htm

Goldschmidt’s Rules - Size Atomic substitution is controlled by size (i.e., radii) of the ions Free substitution can occur if size difference is less than ~15% Limited substitution can occur if size difference is 15 - 30% Little to no substitution can occur if size difference is greater than 30% If there is a small difference of ionic radius the smaller ion enters the crystal preferentially Source: http://classes.colgate.edu/rapril/geol201/summaries/atsub.html

Goldschmidt’s Rules - Charge Atomic substitution is controlled by charge of the ions --> cannot differ by more than 1 For ions of similar radius but different charges, the ion with the higher charge enters the crystal preferentially

Other Factors Affecting Solid Solution Temperature Minerals expand at higher T Minerals contract at lower T Greater tolerance for ionic substitution at higher T Pressure Increasing pressure causes compression Less tolerance for ionic substitution at higher P Availability of ions – ions must be readily available for substitution to occur

Spin State High-spin versus low-spin Source: http://www.dur.ac.uk/a.l.thompson/MainPage/SpinCrossover.htm High-spin versus low-spin

Solid Solution Source: http://classes.colgate.edu/rapril/geol201/images/olivinesln.gif and http://classes.colgate.edu/rapril/geol201/images/plag1.gif

Types of Crystalline Solution 1. Substitutional - Mg2+ for Fe2+ 2. Omission - Ca2+ & void for 2 Na+

Crystalline Substitution 2 3. Vacancy - normally vacant sites can be filled as part of a coupled substitution. An important example is in the mineral group amphibole. An abundant, end-member component of this group of minerals is tremolite which ideally has the formula: []Ca3Mg5Si8O22(OH)2 where [] represents a vacant crystallographic site. Minerals can utilize this vacant site in coupled substitutions such as: [] + Si4+ = Na+ + Al3+

Crystalline Substitution 3 4. Interstitial - Atom or ion occupies space in between the normal sites Often this is H+, a very small cation In some crystal structures these voids are channel-like cavities.  A good example is the mineral beryl (Be3Al2Si6O18) Image: http://www.tech.farmingdale.edu/depts/met/met205/imperfections.html

Beryl Cavities

Schottky Defect Source:http://www.tech.farmingdale.edu/depts/met/met205/imperfections.html

Frenkel defect Source: http://ttb.eng.wayne.edu/%7Egrimm/BME5370/L2F3.gif

HCP Stacking Defect ABABABCABAB H H C H H

CCP Stacking Defect ABCABCABABCABC C C H C C

Grain Boundary Defect Two lattices grow together, with some displacement of ions (shown in blue) Source: http://www.cmm.wsu.edu/cmm_research/edge_1.jpg

Polymorphous Minerals Source: http://metafysica.nl/holism/implicate_order_2.html All have the formula Al2SiO5

Ditypous Minerals Top – sphalerite (aka zinc blende) CCP Bottom – wurzite HCP Source: http://www.indigo.com/models/gphmodel/zinc-blende-SiC-wurtzite-model-W.html

Pseudomorphism Pseudomorphic goethite after cubic pyrite crystals clustered on a terminated aegerine crystal Group is 4.6cm Eric Farquharson specimen Source: http://www.onlineminerals.com/article17.htm

Mineraloids Upper left –amber Lower left – obsidian Sources: Upper left: http://mineral.galleries.com/minerals/mineralo/amber/amb-37.jpg Lower left: http://mineral.galleries.com/minerals/mineralo/tektites/tek-30.jpg Right: http://mineral.galleries.com/minerals/mineralo/tektites/tek-30.jpg Upper left –amber Lower left – obsidian Right – tektite glass

Exsolution Augite with pigeonite exsolution lamellae Pigeonite is a Ca-poor clinopryoxene Left: http://www.geosci.unc.edu/Petunia/IgMetAtlas/template.html Right: http://jaeger.earthsci.unimelb.edu.au/Images/Mineralogical/html/Im68.html Exsolution in pyroxene