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Mineral/Crystal Chemistry and Classification of Minerals Revisited

Mineral/Crystal Chemistry and Classification of Minerals Revisited. Coordination, Solid Solution, Polymorphs and Isomorphs, Mineral Classes . Mineral and Crystal Chemistry. Minerals have:

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Mineral/Crystal Chemistry and Classification of Minerals Revisited

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  1. Mineral/Crystal Chemistry and Classification of Minerals Revisited Coordination, Solid Solution, Polymorphs and Isomorphs, Mineral Classes

  2. Mineral and Crystal Chemistry Minerals have: • Characteristic geometric arrangement or structure of constituent atoms that is regular and repeating in a 3-D arrangement (CRYSTALLINE SOLID) • Arrangement of atoms depends on their ionic radius and valence • Many minerals can be conceptualized as anions (or anion complexes) tightly packed together with cations filling in the intervening spaces (COORDINATION POLYHEDRA) • Fixed or fixed range of chemical composition (FORMULA) • Substitution of one element for another (SOLID SOLUTION) • Depends on their ionic radius and valence • Substitution may be complete or limited, simple or coupled, and varies with P-T conditions

  3. Ionic radius (IR) and Valence Cations (+) —> generally smaller IR Anions (-) —> generally larger IR Variable dependant on atomic number and interaction with other ions Atomic Theory

  4. The Coordination Principle:Geometry of Atomic Building Blocks • In an ionically bonded substance (all minerals for our purposes) cations are surrounded by anions (or anionic complexes) • In stable mineral crystals • The number and arrangement of anions surrounding a cation forms a Coordination polyhedron

  5. The Coordination Principle Coordination polyhedron • If all atoms are the same size, they can be packed together so that each atom touches 12 others (Hexagonal or Cubic closest packing)

  6. The Coordination Principle Coordination polyhedron • BUT atoms vary in size • So the size and shape of the coordination polyhedron is determined by the ionic radii of the cation and anion (anion complex) involved • Radius ratio (cation radius/anion radius) • This shape is described by the number of anions surrounding a cation: • Coordination number (C.N.)

  7. Coordination Polyhedron A. Triangular (CN = 3) B. Tetrahedral (CN 4) C. Octahedral (CN 6) D. Cubic (CN 8) E. Dodecahedral (CN 12)

  8. The Coordination Principle • Oxygen (O-2) is the most common anion in coordination polyhedron • Cations coordinate with oxygen in predicable ways (see Ch. 4, p. 10) • Minerals have specific physical locations (sites) that can hold only a few types of ions • Each site has specific coordination polyhedron type

  9. The Coordination Principle • Coordination explains anion groups • O is more tightly bonded to central, highly charged cation than to other cations • Examples: • C+4 in triangular coordination (CN = 3) produces (CO3)-2 • S+6 in tetrahedral coordination (CN = 4) produces (SO4)-2 • Si+4 in tetrahedral coordination (CN = 4) produces (SiO4)-4

  10. Formula reflects the proportions of elements (cations and anions) present in the mineral Common conventions: Subscripts indicate atomic proportions; superscripts indicate charge Commas for either-or; ex: olivine [(Mg,Fe)2SiO4] Anion complexes typically in parentheses; ex: dolomite [CaMg(CO3)2] Historically determined by wet chemical techniques Modern techniques typically use focused energy beam (electron or ion microprobe) Reported at wt% of elements or oxides Must be converted to atomic proportions Mineral Formulas

  11. Mineral Formulas: Recalculation • Formula = CuFeS2 • Converts wt% of elements or oxides into a mineral formula (pure substance or solid solution) *Calculated by wt%/atomic wt Ex for Cu: 34.30/63.54 = 0.5398 **Normalized to make the smallestnumber equivalent to 1

  12. 100% C 0% B 100% B 100% A 0% A 100% B 100% A Mineral Formulas: Graphical Representation • Can show relative proportions of elements in a mineral • In wt%, oxide wt%, or atomic proportions • Especially useful in showing multi-component systems and solid solution systems Three components Two components

  13. Mineral Formulas: Graphical Representation • Example: • CaO – MgO – SiO2 system

  14. 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 certain conditions are met

  15. Atomic Substitution/Solid Solution Requires: • Valences of substituting ions are no more different than 1 • Na+1 for Ca+2 • Al+3 for Si+4 • Difference in the size of substituting ions must be <15% (at room temperature)

  16. Some substitutions involve complete solid solution Involves ions of (nearly) equal charge and size Any composition (mixture) may occur between end member compositions Examples: Olivine series: Forsterite (Mg2SiO4 ) to Fayalite (Fe2SiO4) Plagioclase feldspar series: Albite (NaAlSi3O8) to Anorthite (CaAl2Si2O8) Atomic Substitution/Solid Solution

  17. Some substitutions involve partial or limited solid solution Involve ions of different sizes or charges Limited compositions (mixtures) may occur between end members Examples: Carbonates: Limited solid solution between Calcite (CaCO3) to Dolomite [CaMg(CO3)2] and Magnesite (MgCO3) to Dolomite Pyroxene group: Limited solution between Hypersthene (MgSiO3) and Diopside (CaMgSi2O6) Atomic Substitution/Solid Solution

  18. Some substitutions are simple 1 for 1 substitution of ions of equal charge Some substitutions are coupled Substitution involves 2 or more ions Necessary to balance different charges Examples: Carbonates: Simple solid solution between Magnesite (MgCO3) and Siderite (FeCO3) Plagioclase feldspar: Coupled solution between Albite (NaAlSi3O8) to Anorthite (CaAl2Si2O8) Atomic Substitution/Solid Solution

  19. 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 Solid Solution & T-P Controls • Atomic substitution is greater at higher pressure because it can change the size of crystallographic sites and ions, thus accommodate greater ionic radius deviation

  20. 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) • Polymorphs have different crystal forms (atomic arrangements) and different physical properties • Different polymorphs occur as a result of differing environmental conditions, principally temperature and pressure

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

  22. Polymorphs • Examples: • Quartz, Tridymite, and Cristobolite (SiO2); Tridymite Quartz Cristobalite

  23. Polymorphs • Example: • Calcite and Aragonite (CaCO3); Calcite Aragonite

  24. Isomorphism (Isostructuralism) • Minerals with analogous formulas where the relative sizes of cations and anions are similar and crystal structure is identical or 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

  25. Isomorphism • Anions and cations of isomorphic minerals have • The same relative size • The same coordination • Crystallize in the same crystal structure • Share similarity of crystal structure but not (necessarily) chemical behavior • Ex: Halite and galena Galena, PbS Halite, NaCl

  26. Isomorphism • Some isomorphic minerals share closely related formulas and identical crystal structures • Ex: Aragonite (orthorhombic) and Calcite (trigonal) Groups

  27. Some isomorphic minerals have such similar compositions and structures that they form solid solution Complete solid solution between Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8 ) Limited solid solution between Calcite (CaCO3) and Magnesite (MgCO3) Isomorphism and solid solution are distinct though related concepts Isomorphism and Solid Solution • Some isomorphic minerals do not have solid solution; Ex: Halite (NaCl) and Galena (PbS) • Some solid solutions do not have isomorphic end members; Ex: Sphalerite (Zn, Fe)S (cubic) and Pyrrhotite Fe1-xS (hexagonal) Sphalerite Pyrrhotite

  28. Class (chemical - anion complex) Subclass (atomic structure) Group (chemical and structural) Species (individual mineral – name) Variety (specific variation) Hierarchy of Mineral Classification • Class: Silicate (SiO44-) • Subclass: Tektosilicate (framework) • Group: Plagioclase Series (An-Ab) • Species: Oligoclase (70-90% Ab) • Variety: Sunstone (red-orange gemstone)

  29. Hierarchy of Mineral Classification: Mineral Classes • Based on the anion or anionic complexes in the crystal structure • Types of bonding and structures are the same (or similar) • Physical properties can be very similar within classes

  30. Mineral Subclasses Classified on basis of structure Example: silicates 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

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

  32. Hierarchy of Mineral Classification • Mineral Species • “naturally occurring homogeneous crystalline substance of inorganic origin, possessing characteristic physical properties, with either definite chemical composition or range in composition between certain limits” • Example: Plagioclase Group (Series) • Albite: NaAlSi308 • Oligoclase • Andesine • Labradorite • Bytownite • Anorthite: CaAl2Si2O8

  33. Hierarchy of Mineral Classification Mineral Variety • Slight variation in trace (non-structural) element content and resultant distinctive physical properties (typically color) • 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

  34. Collaborative Activity In groups, answer the following. Show all calculations. • Predict the coordination number (CN) and name the coordination polyhedron for each of the following cation-anion pairs: A. Zn (0.6Å) – S (1.84Å) B. Rb (1.61Å) – Cl (1.81Å) • The handout has two analyses of the mineral ilmenite. A. For analysis #1, convert the given weight proportions into atomic proportions and calculate the mineral formula B. Analysis #2 shows that a bit of solid solution is possible in the ilmenite structure, with Mg and/or Mn substituting for Fe. Calculate the formula for this analysis • Plot the following compositions on the TiO2-FeO-Fe2O3 diagram: Ilmenite (from analysis #1), Rutile (TiO2), Hematite (Fe2O3), Magnetite (Fe3O4), and Ulvöspinel (Fe2TiO4) • Plot the following compositions on the same diagram: A. 10% TiO2, 10% FeO, 80% Fe2O3 B. 40% TiO2, 25% FeO, 35% Fe2O3

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