Mineral/Crystal Chemistry and Classification of Minerals Revisited

1. Mineral/Crystal Chemistry and Classification of Minerals Revisited Atomic Substitution and Solid Solution, Polymorphism and Isomorphism; Mineral Classes, Subclasses, Groups, Series, Species, and Varieties

2. Pyrite, FeS2 (Fe+2S2), Cubic Marcasite, FeS2 (Fe+2S2), Orthorhombic Polymorphism • The same chemical formula applies to two (or more) distinct mineral species. • Chemical composition may not be sufficient to designate a specific mineral species (physically homogeneous and separable portion of a material system). • Different polymorphs occur as a result of differing environmental conditions principally temperature and pressure.

3. Polymorphs • Diamond and Graphite (C); Geobarometer: (determines pressure of formation)

4. Polymorphs • Quartz, Tridymite, and Cristobolite (SiO2); Silica mineral phase diagram

5. Polymorphs • Calcite and Aragonite (CaCO3);

6. Isomorphism • Substances with analogous formulas where the relative sizes of cations and anions are similar and crystal structure is closely related • Typically (but not always) the basis for grouping and classification, e.g. • Garnet group, Amphibole group, Mica group, Pyroxene group Galena, PbS Halite, NaCl

7. Isomorphism • Anions and cations of isomorphous minerals have • the same relative size • the same coordination and • crystallize in the same crystal structure • Share similarity of crystal structure but not (necessarily) chemical behavior Galena, PbS Halite, NaCl

8. Isomorphism • Aragonite (orthorhombic) and Calcite Groups (trigonal)

9. Atomic Substitution/Solid Solution • Homogeneous crystalline solids of variable chemical composition • Many minerals vary in their composition • Elements are readily substituted (atomic substitution) for one another in many crystal structures (when ionic radius and charge permits)

10. Atomic Substitution/Solid Solution • Requires valences of substituting ions are no more different than 1 • Na+1 for Ca+2 * • At room temperature difference in the size of substituting ions must be <15% *Na+1 IR = .97

11. Some isomorphous minerals occur in complete solid solution i.e. any composition (mixture) may occur between end member compositions Plagioclase feldspar series: Albite (Na) to Anorthite (Ca) series; NaAlSi308 to CaAl2Si2O8 Atomic Substitution/Solid Solution

12. Plagioclase Feldspar Continuous Solid Solution Series (Ca, Na)(Si,AL)AlSi2O8 • Composition of plagioclase feldspar suggests temperature of formation

13. Incomplete Solid Solution • Some solid solutions do not have isomorphous (that is they have dimorphic) end members • Sphalerite (Zn, Fe)S (cubic) and Pyrrhotite Fe1-xS (hexagonal)

14. Atomic substitution is greater at higher temperature (crystal lattices are more open) and can accommodate greater ionic radius deviation (than 15%) Na+1 IR = 0.97 K+1 IR = 1.33 Ca+2 IR = 0.99 Incomplete Solid Solution (atomic substitution) & Temperature Controls

15. Isomorphism and solid solution are distinct concepts The carbonate minerals Galena and Halite Isomorphism Without/with Minimal Solid Solution

16. Classification of Minerals • Mineral classes • Mineralogists use crystal chemistry to organize and classify minerals • Based on the anion or anionic complexes in the crystal structure

17. Classification of Minerals:Mineral Classes • Chemical composition (anion/anionic complex) determines class • Types of bonding and structures are the same (or similar) • Physical properties can be very similar within classes

18. Carbonates Sulfates Phosphates Silicates (Hydroxides) Cyclo- Neso- Soro- Mineral Classes (for our purposes) • Mineral Classes (8) • Native Elements • Sulfides • Oxides • Halides • Silicates are complex and subdivided into subclasses: • Tecto- • Phylo- • Ino- Silicates

19. Subclasses (mostly otbo structure) e.g. Six silicate subclasses. Nesosilicates: SiO4, independent silica tetrahedra. Sorosilicates: Si2O7, double silica tetrahedra. Cyclosilicates: SiO3, ring of silica tetrahedra. Inosilicates: Si4O11, chains of silica tetrahedra. Phyllosilicates: Si2O5, sheets of silica tetrahedra. Tektosilicates: Si02, frameworks of silica tetrahedra. Hierarchy of Mineral Classification:

20. Hierarchy of Mineral Classification: • Mineral Groups; mineral species with close chemical and structural relationship e.g. (typically due to atomic substitution/solid solution) • Amphibole, Feldspar, Mica, Pyroxene, Garnet, Olivine.

21. Hierarchy of Mineral Classification: • Mineral Species within a Group or Solid Solution Series: • “naturally occurring homogeneous crystalline substance of inorganic origin, possessing characteristic physical properties, with either definite chemical composition or range in composition between certain limits”e.g. • Albite: NaAlSi308; • Anorthite: CaAl2Si2O8. • Plagioclase (Series): • (Na,Ca) (Al,Si)AlSi2O8.

22. Hierarchy of Mineral Classification: • Mineral Variety: • slight variation in trace (non-structural) element content and resultant distinctive physical properties (typically color) e.g. • Corundum —> Ruby (red), Sapphire (blue) [AL2O3] • Classification tending to move away from species and variety names to names using a modifier of main species e.g.: • Fe in magnesite (MgCO3) —> ferroan magnesite Ruby Sapphire