SUMMARY OF CRUSTAL ELEMENTAL ABUNDANCE AND CHEMICAL DETAILS
Extracted from p 222 Klein and Hurlbut and radii from Shannon and Prewitt (1969) from Bloss, p. 209
Extract: wt % cations charge ionic radii common site
27.7 % Si 4+ 0.26 tetrahedral
8.13% Al 3+ 0.39 tetrahedral/octahedral 5.00% Fe 2+, 3+ 0.61, 0.55 octahedral 3.63% Ca 2+ 1.00+ octahedral or larger 2.83% Na 1+ 1.02+ octahedral or larger 2.59% K 1+ ~ 1.59 cubic
2.09% Mg 2+ 0.72 octahedral
0.40% Ti 4+ 0.61 octahedral
0.14% H 1+ water or OH
0.09% Mn 2+, 3+ ~ 0.67 octahedral
46.60% O 2- ~ 1.40 (varies with CN #)
RADIUS RATIO RULE (Pauling) and consequences for coordination by oxygen, radius ~ 1.4 )
< 0.155 CN # = 2 < 0.22
0.155 - .255 CN # = 3 0.22 - 0.36
0.255 - .414 CN # = 4 0.36 - 0.58
0.414 - .732 CN # = 6 0.58 - 1.02
0.732 - 1.0 CN # = 8 1.02 - 1.44
> 1.0 CN # = 12 1.44 +
Lets make some minerals!
Start with Si and O:
Si - 4 coordination:
Si: 14th element on periodic table: [Ne]2s23p2 -> Si4+ with tetrahedral sp3 hybrid orbitals.
Si bonds with O: 50% ionic, 50% covalent.
Simplest compounds based on Si and O : formula = ?
Structural arrangement ? Pauling bond strengths in tetrahedra, unsatisfied charge on O ?
Polymerization network: polymorphs
Effects of pressure and temperature: Thermodynamics
When energy in the form of heat is added to a mineral, part of the ENERGY added is used to do WORK.
A crystal structure changes in subtle ways as varying temperature and pressure. The compound will adopt a structure that is optimized for a RANGE of P and T conditions.
-low pressure, temperature : LOW QUARTZ (alpha) framework distortion -higher temperature (P) : HIGH QUARTZ (beta) framework ideal
-high T (lower P) : TRIDYMITE (HCP) -even higher T : CRISTOBALITE (CCP) -even higher T ?? -high P :COESITE
-very high P ...:STISHOVITE
USING Si, O, and one of the following: Fe2+, Fe3+, Mg
Write two charge balanced formulas.
Look at the Si:O ratio. What kind of silicate might you have made....(discussion)
Write three charge balanced formulas.
Where might Al be and what have we made?
Silicate Type Tetrahedral cations: O Fraction of O shared
Framework 1:2 all Sheet 2:5 three Simple Chain 1:3 two Rings 1:3 two Bow Ties 2:7 one Island 1:4 none
Framework silicates - simplest examples are the SiO2 polymorphs.
Framework silicate structures derived from SiO2 frameworks (above).
Most important group: FELDSPAR minerals!
phase relationships amongst feldspar minerals
Structural details about feldspars: structural variations based on cation ordering and distortion of the feldspar framework.