The Rock Forming Silicate Minerals: Basic Principles. Importance of the Silicates. Abundance ~25% of all known minerals Make up ~90% of earth’s crust Composed of dominant elements in earth’s crust (O, Si, Al, Fe, Mg, Ca, Na, K) Rock-forming minerals Economic uses
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Results in a structurally stable, anion complex
Suitable for the formation of polymers (two or more shared complex ions of the same type)
Charge reaching the cation is exactly 1/2 the charge on the anion
Limited ways in which polymers can form (Pauling’s Rules)
Charge balance in minerals attained by:
Sharing oxygen between adjoining tetrahedra
Linking tetrahedra with other cations
Neso: SiO4-4, separate tetrahedra (1:4; Si:0)
Soro: Si2O7-6, double tetrahedra (1:3.5; Si:0)
Cyclo: Si3O9-6 (3 tetrahedra ring), Si4O12--8 (4 tetrahedra ring), Si6O18- 12 (6 tetrahedra ring) (1:3; Si:0)
Ino: SiO3-2 (single chain of tetrahedra) (1:3; Si:0), Si4O11-6 (double chain of tetrahedra) (1:2.75; Si:0)
Phyllo: Si4O10-4 (OH)2, sheets of tetrahedra (1:2.5; Si:0)
Tecto: SiO2, three dimensional frameworks (1:2; Si:0)
SiO4More Silicates Structure
* temperature dependence of substitution is a result of high temperature expansion of mineral crystal lattice and the ability to accommodate greater variation in ionic radii of ions involved in the substitution.
Results in growth of a single mineral
Single mineral formed at high temp will exsolve (unmix) at lower temp
If slowly cooled
Forms two intergrown mineralsT-X Phase Diagrams and Exsolution
Two chemical analyses for a silicate mineral are provided.
a. Calculate the gram molecular proportions (gmp) for each sample (in the table).
b. Determine the silicate mineral group and subclass to which these samples belong.
c. Write a general formula for this mineral group
d. Determine the molecular proportions of Mg and Fe and write a chemical formula for each analysis.
Using the plagioclase feldspar T-X phase diagram:
a. On the diagram, label the liquidus and solidus
b. A liquid of composition An60 begins to crystallize. Draw the crystallization path on the diagram and determine (a) the temperature at which the first crystals form and (b) the composition of the first crystals?
c. Determine the temperature at which a plagioclase crystal of composition An40 first began to crystallize from the liquid and the composition of the liquid from which this crystal grew:Collaborative Activity