Silica minerals

Start with Si and O

Si:
• 4 coordination:
• 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 energy
  • 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, but occurs in one phase - stishovite - formed in meteorite impacts (and the mantle).

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
  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 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.

Here are some structure simulations you can download:

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