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The Rock Forming Silicate Minerals: Basic Principles

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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The Rock Forming Silicate Minerals: Basic Principles

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  1. The Rock Forming Silicate Minerals: Basic Principles

  2. 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 • Building construction (brick, stone, morter, glass) • Technology (silicon chips)

  3. SiO4 Si2O7 SiO3 Si4O11 Si2O6 Si2O5 SiO2 What Are Silicates? • Frameworks based on the linkage of cation centered coordination polyhedron • The silica tetrahedra: the building block of the silicate minerals • SiO4-4 is the basic unit

  4. Frameworks based on the linkage of SiO4-4 tetrahedra 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 SiO4 Si2O7 SiO3 Si4O11 Si2O5 SiO2 The Silicates

  5. Ratio of Si to O determines structural configuration and is the criteria for silicate classification (subclasses) 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) Si2O6 Si4O11 Si2O5 Si2O7 SiO2 SiO3 SiO4 Silicates Structure

  6. Ratio of Si to O determines structural configuration and is the criteria for silicate classification (subclasses) 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) Si2O6 Si4O11 Si2O5 Si2O7 SiO2 SiO3 SiO4 More Silicates Structure

  7. Silicate Subclasses • Neso (a) • Soro (b) • Cyclo (c, d, e)

  8. Silicate Subclasses • Ino • Phylo

  9. Silicate Subclasses • Tecto [SiO2]

  10. The Silicates

  11. Structural Formulas and Silicate Group Minerals • Where do the other cations fit? • Coordination polyhedron with oxygen depends on ionic radius

  12. Structural Formulas and Silicates • Describes the structural and chemical relationship between related minerals (mineral groups) through a generalized description of the chemical formula • Symbology • X = large, weakly charged cations, C.N. >6 (with oxygen) • Na, K, & Ca (sort of) • Y = medium-sized, 2+ to 4+ cations, C.N. = 6 • Mg, Fe+2 , Fe+3, Al, Ti, & Ca (sort of) • Z = small cations, C.N. = 4 • Mainly Si+4, but also Al+3 • W = additional anionic groups • OH-, Cl-, F- • Results in generic formula for all silicates: XmYn(ZpOq)Wr

  13. T-X Phase Diagrams • Diagrams showing the relationship between temperature, composition, and proportion of crystals and liquid in a melt • Liquidus: t-x at which first crystals form from a melt • Solidus: t-x at which the last melt crystallizes

  14. Miscibility gap (opposite of solid solution) Defined by the solvus Outline of the miscibility gap Represents the area of a phase diagram where naturally occurring minerals do not occur The Solvus and Miscibility Gaps

  15. Temperature and Miscibility Gaps • Temperature dependant solid solution/ion substitution • The extent of ion substitution is dependant on • Ionic radius • Valency • Temperature* * 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.

  16. At high T, lattices are more open and can accommodate cations of dissimilar size 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 minerals T-X Phase Diagrams and Exsolution

  17. Answer the following in groups of 2-3: 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

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