Framework Silicates

FRAMEWORK SILICATES

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
0.10% P
0.09% Mn 2+, 3+ ~ 0.67 octahedral

wt% anions

46.60% O 2- ~ 1.40 (varies with CN #)

RECALL:

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 ?

Bridging oxygens

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.

The work is in the form of THERMAL EXPANSION (e.g., lengthening of bonds, etc.): increases unit cell volume
PRESSURE compresses the structure - shortens bond lengths: decreases unit cell volume (V)

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.

phase = crystal structure, e.g., of SiO2
- dense phases are favored at higher pressure [dG/dP)T = V] G = Gibbs free e nergy
as we change the P and T conditions, may discover that the structure can minimize its energy more effectively by changing fundamentally than by stretching or compressing existing bonds: PHASE TRANSFORMATION.
under some range of conditions, two phases may be exactly equally stable ( in equilibrium) and thus may coexist. If P or T are varied, then only one phase will be stable and the second will be converted to this phase.
Si-O bonds are quite hard to compress.
Under extreme pressure conditions, the bonds may become too compressed, and the structure responds by changing the coordination number of the cation! e.g., IV -> VI coordination. VI coordination for Si is very rare, b ut occurs in one phase - stishovite - formed in meteorite impacts (and the mantle).
polymorphs and the SiO2 system: see p. 316 text.

-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

Introduction to the chemistry of other silicates:

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)

USING Si, O, Al, and one of the following: K+. Na+, Ca2+

Write three charge balanced formulas.
Discussion....

Where might Al be and what have we made?

Polymerization Schemes based on bridging O - simple starting point (to be elaborated upon later)

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.

Alpha quartz (low-temperature quartz)
Beta quartz
Cristobalite (based on CCP)
Tridymite (based on HCP)
Stishovite (rutile structure, note octahedrally coordinated Si)
Coesite (note similarity to feldspars!)

SECOND LECTURE ON FRAMEWORK SILICATES

Framework silicate structures derived from SiO2 frameworks (above).

Most important group: FELDSPAR minerals!

Coupled substitutions

phase relationships amongst feldspar minerals

FELDSPAR STRUCTURES:

Structural details about feldspars: structural variations based on cation ordering and distortion of the feldspar framework.