Framework Silicates: Silica

• Roughly 2/3 of the crust of the Earth is composed of Framework silicates
• We will examine:
  • SiO₂ group
  • Feldspars
  • Feldspathoids
  • Scapolites
  • Zeolites

Start with Si and O

Si:
• 4 coordination:
• 14th element on periodic table: [Ne] 2s² 3p²
  -> 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 temperature and pressure are varied. A particular 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
      • where G = Gibbs free energy
  • as we change the P and T conditions, we may discover that the structure can more effectively minimize its energy by changing fundamentally than it can 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 more stable 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, but occurs in one phase - stishovite - formed in meteorite impacts (and deep within the mantle).
• Displacive vs Reconstructive Transformations

low pressure, temperature		LOW QUARTZ (alpha) framework distortion	Hex-Trig	2.65 g/cc
higher temperature (moderate P)		HIGH QUARTZ (beta) framework ideal	Hex	2.53
high T (lower P)		LOW TRIDYMITE (HCP)	Mono/Ortho	2.26
high T (lower P)		HIGH TRIDYMITE (HCP)	Hex	2.22
even higher T		LOW CRISTOBALITE (CCP)	Tetra	2.32
even higher T		HIGH CRISTOBALITE (CCP)	Iso	2.20
even higher T		??
high P		COESITE	Mono	3.01
very high P		STISHOVITE	Tetra	4.35

• USING Si, O, and one of the following: Fe²⁺, Fe³⁺, Mg
  1. Write two charge balanced formulas.
  2. 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⁺
  1. Write three charge balanced formulas
  2. Discussion

• Where might Al be located and what have we made?

Silicate Type	Tetrahedral cations: O	Fraction of O shared
Framework	1:2	4
Sheet	2:5	3
Double Chain	4:11	2.5
Simple Chain	1:3	2
Rings	1:3	2
Bow Ties	2:7	1
Island	1:4	0

Framework silicates - simplest examples are the SiO₂ polymorphs.

Here are some structure simulations you can download:

• Alpha quartz (low-temperature quartz) (234K)
• Beta quartz (241K)
• Tridymite (based on HCP) (322K)
• Cristobalite (based on CCP) (194K)
• Coesite (note similarity to feldspars!) (326K)
• Stishovite (rutile structure, note octahedrally coordinated Si) (291K)