Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2014 GEOLOGY 284 MINERALOGY Ionic Radius and Cation Substitution in Minerals see Chapter 13 Perkins Effective Ionic Radius Measured bond lengths of oxides Assume average ionic radius for O2 of 1 32 1 ngstrom 10 8 cm It is possible to tabulate ionic radii of common cations Ionic radii depend on Coordination Number C N number of nearest neighbor anions of a cation Coordination Number with Oxygen Tetrahedral Coordination C N 4 Smaller cations Si4 Al3 fit Octahedral Coordination C N 6 Medium sized cations Mg2 Fe2 Ca2 Na1 fit Cubic Coordination or higher C N 8 Large cations K fit Periodic Table with Effective Ionic Radii inside front cover Perkins Ion Effective Ionic Radii of Common Cations and Anions C N w Ox Ionic Radius O2 1 32 F 1 33 Cl 1 72 K 8 1 59 K 6 1 46 Na 6 1 08 Ca2 6 1 08 Mg2 6 0 80 Mn2 6 0 83 Fe2 6 0 77 Fe3 6 0 68 Al3 6 0 61 Al3 4 0 47 Si4 4 0 34 C N Coordination Number number of nearest anion neighbors Two Cations substitute readily for each other if their ionic radii differ by less than about 15 they have the same charge or if their charges differ the charge difference must be compensated by another substitution elsewhere in the mineral they form bonds of similar character i e they have similar electronegativity Substitution is easier and more common at high temperature than at low temperature Ions that readily substitute for each other Ion C N w Ox Ionic Radius O2 1 32 F 1 33 Cl 1 72 K 8 1 59 K 6 1 46 Na 6 1 08 Ca2 6 1 08 Mg2 6 0 80 Mn2 6 0 83 Fe2 6 0 77 Fe3 6 0 68 Al3 6 0 61 Al3 4 0 47 Si4 4 0 34 C N Coordination Number number of nearest anion neighbors Ions that readily substitute for each other Mg2 Fe2 Mn2 very common substitutions in mafic silicates Mg2 Fe2 Mn2 Ca2 Ca2 substitutes for Mg2 Fe2 in some silicates Ca2 Na1 in plagioclase charges different must be coupled to Al3 Si4 to balance charge written Ca2 Al3 Na1 Si4 Al3 Si4 in some of the tetrahedra C N 4 in sheet silicates requires presence of K or Na between t o t sandwiches to balance the charge Ions that readily substitute for each other continued Na1 1 08 K1 1 46 in alkali feldspar violates the 15 size difference rule 30 difference That s the driving force for exsolution perthites Fe3 and Al3 can substitute for Mg2 Fe2 in octahedral positions but they re a little small and the charge difference must be compensated Anions also substitute for eachother F and Cl commonly substitute for OH Br for Cl in halides Se2 for S2 in sulfides Periodic Table with Effective Ionic Radii inside front cover Perkins Ion Ions that readily substitute for each other C N w Ox Ionic Radius O2 1 32 F 1 33 Cl 1 72 K 8 1 59 K 6 1 46 Na 6 1 08 Ca2 6 1 08 Mg2 6 0 80 Mn2 6 0 83 Fe2 6 0 77 Fe3 6 0 68 Al3 6 0 61 Al3 4 0 47 Si4 4 0 34 C N Coordination Number number of nearest anion neighbors