Ionic Coordination and Silicate Structures

Transcription

1 Ionic Coordination and Silicate Structures Pauling s Rules A coordination polyhedron of anions forms around a cation Ionic distance determined by radii Coordination number determined by radius ratio. May result in a complex ion Atoms and Ions Have Different Radii Elemental Abundance in Crust Element Ionic Radius (R) R/R Oxygen O Si Al / /0.42 Mg Fe Fe Ca / /0.86 Na / /0.89 K / /1.24 C

2 Coordination Number Coordination number: total number of neighbors around a central atom Styrofoam Ball Investigation of Coordination Number Four sizes of styrofoam balls Controlled by ratio of ionic radii Arranged for closest packing Exercise 1: CN of Si and O Largest = O, Smallest = Si Exercise 2: CN of Fe and O Largest = O, Medium = Al Exercise 3: CN of Ca and O Largest balls = O, Large = Ca Element Ionic Radius (R) R/R Oxygen O Si Fe Ca CN=4: Tetrahedral CN=6: Octahedral 2

3 CN=8: Cubic CN=12: Hexagonal or Cubic Close Packed Coordination of Common Crustal Ions General Formula for Silicates Element R/R Oxygen CN Coordination with O Si Tetrahedral Al /0.42 4/6 Tetrahedral/Octahedral Mg Octahedral Fe Octahedral Fe Octahedral Ca /0.86 6/8 Octahedral/Cubic Na /0.89 6/8 Octahedral/Cubic K /1.24 8/12 Cubic/Closest Ions in silicates tetrahedral, octahedral, or cubic/closest packed coordination General Formula: X m Y n (Z p O q ) W r X = Cubic/Closest Y = Octahedral Z = Tetrahedral O = Oxygen W = OH, F, Cl Site CN Ions Z 4 Si 4+, Al 3+ Y 6 Al 3+, Fe 3+, Fe 2+, Mg 2+, Mn 2+, Ti 2+ X 8 Na +, Ca K +, Ba 2+, Rb + 3

4 Mineral Formula Examples General Formula X m Y n (Z p O q ) W r Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al) 2 O 6 Muscovite KAl 2 (Si 3 Al)O 10 (OH,F) 2 Plagioclase (Na,Ca)(Si,Al) 4 O 8 Requirements of Pauling s Rules Stable coordination numbers for Si produces complex anion (SiO 4 ) 4- (SiO 4 ) 4- must bond to balance charge Insufficient cations Tetrahedra commonly bond with other complex anions Pauling s Rules Shared edges or faces of polyhedra decrease stability. The closer the cations the greater the repulsion The higher the cation charge the greater the repulsion Requirements of Pauling s Rules Si 4+ has high charge and low coordination number Silica tetrahedra will not share sides or faces Arrangements of silica tetrahedra based on sharing of apices 4

5 Metals: Form Positive Ions (Cations) Non-Metals: Form Negative Ions (Anions) Mineral Classes Elemental Abundance in Crust Minerals involve bonds between cations and anions (metals and non-metals) Only a few elements form common anions (C, N, O, P, S, Cl) Minerals classified based on anions (non-metals) Element % by wt mol wt % by mol O Si Al Fe Ca Na K Mg

6 Common Crustal Elements Common Crustal Elements Metals Most Minerals Are Silicates Silica Tetrahedron Crust is mostly Si & O Crust is made up mostly of minerals that include Si and O Silicates The fundamental architectural unit of silicate minerals is the silica tetrahedron One Si ion bonded with four O ions 6

7 Isolated Tetraheda Silicates (Nesosilicates) Tetrahedra do not share any oxygens with neighboring silicon ions Charge balance achieved by bonding with cations e.g., Olivine, Garnet, Kyanite Isolated Tetrahedra Silicates Sheet Silicates Olivine Garnet Biotite (Crystal) Biotite (Broken) 7

8 Paired Silicates (Sorosilicates) Pairs of tetrahedra share one oxygen Remaining charge e.g., Epidote Ring Silicates (Cyclosilicates) Sets of tetrahedra share two oxygens to form a ring Remaining charge e.g., tourmaline, beryl Ring Silicates Single-Chain Silicates (Inosilicates) Sets of tetrahedra share two oxygens to form a chain Remaining charge e.g., pyroxenes Tourmaline Beryl 8

9 Double-Chain Silicates (Inosilicates) Sets of tetrahedra share oxygens (2 and 3 alternation) to form a chain Remaining charge e.g., amphiboles Sheet Silicates (Phyllosilicates) Sets of tetrahedra share three oxygens to form a sheet Remaining charge e.g., micas Chain Silicates: Habit Chain Silicates: Cleavage Pyroxene Amphibole Amphibole Cleavage (120 / 60) Pyroxene Cleavage (90) 9

10 Framework Silicates (Tectosilicates) Framework Silicates Tetrahedra share all 4 oxygens to form a 3-D network If all tetrahedra cored by silicon then no charge imbalance e.g., quartz (SiO 2 ) If some tetrahedra cored by Al, remaining charge e.g., feldspar (NaAlSi 3 O 8 ) Feldspar (Orthoclase) Nepheline Silicon Content of Silicates STRUCTURE EXAMPLE FORMULA Si:O Ratio Nesosilicates Mg 2 SiO 4 1:4 Sorosilicates Zn 4 (OH) 2 Si 2 O 7.H 2 O 1:3.5 Cyclosilicates Al 2 Be 3 Si 6 O 18 1:3 Inosilicates (Single Chain) Inosilicates (Double Chain) CaMgSi 2 O 6 1:3 Ca 2 Mg 2 (Si 4 O 11 )OH 2 1:2.75 Phyllosilicates Al 2 Si 4 O 10 (OH) 2 1:2.5 Tectosilicates SiO2 1:2 10