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Mineral Structures

Mineral Structures. From definition of a mineral: “…an ordered atomic arrangement…” How do Pauling’s rules control “ordered atomic arrangement?” How can crystal structure make one mineral different from another? Can mineral structures be used to group minerals (e.g. classify them)?.

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Mineral Structures

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  1. Mineral Structures • From definition of a mineral: • “…an ordered atomic arrangement…” • How do Pauling’s rules control “ordered atomic arrangement?” • How can crystal structure make one mineral different from another? • Can mineral structures be used to group minerals (e.g. classify them)?

  2. Illustrations of mineral structures • 2-D representation of 3-D materials • Ions represented as spheres – drawn to scale • Stick and ball method • Polyhedron method • Hybrid: Stick and Ball, plus polyhedron • Map view – unit cell dimensions

  3. Fig. 4-10 Unit cell outline Olivine – view down a crystallographic axis

  4. Structures • Isostructural minerals • Same structure, different composition • Polymorphism – polymorphic minerals • Same composition, different structures

  5. Isostructural Minerals • Many minerals have identical structures, different compositions • Example: halite (NaCl) and Galena (PbS) • Differ in many physical properties - composition • Identical symmetry, cleavage, and habit – elemental arrangement

  6. Isostructural group • Several isostructural minerals • Have common anion group • Much substitution between cations • Example: calcite group

  7. Polymorphism • The ability for compounds with identical compositions to crystallize with more than one structure • Polymorphs • Polymorphic groups

  8. Caused by balance of conflicting requirements and environmental factors: • Attraction and repulsion of cations and anions (charge) • Fit of cations in coordination site (size) • Geometry of covalent bonds • P & T primary environmental variables

  9. P and T controls: • High P favors tightly packed lattice, high density • High T favors open lattice, low density, wide substitution • Composition of environment unimportant • All same elements in polymorphs • Presence or absence of polymorphs provide information on P and T conditions

  10. Four types of mechanisms to create polymorphs: • Reconstructive – break bonds • Order-disorder – cation placement • Displacive – kink bonds • Polytypism – stacking arrangement

  11. 1. Reconstructive polymorphism • Requires breaking bonds – major reorganization • Symmetry and/or structural elements may differ between polymorphs • Symmetry and/or structural elements may be similar because identical composition • Example: C

  12. C = Diamond and Graphite • Diamond – all 100% covalent bonds • Graphite – covalent bonds within sheets, van der Waal bonds between sheets • What conditions cause one mineral or the other to form?

  13. Graphite – stable at earth surface T and P • Diamond stable only at high P and T – but found on earth surface • Won’t spontaneously convert to graphite • Minerals that exists outside of their stability fields are metastable

  14. ~200 km depth ~100 km depth Where on (in) the earth would diamond form/be stable? What are temperatures at these depths? Fig. 4-11 Found on a Phase Diagram – e.g. for single component Increasing Depth (linear) Single component = C Increasing Depth (non-linear)

  15. Diamond stability versus geothermal gradient Kimberlite Red line is geothermal gradient Diamond window Stability Boundary of Diamond and Graphite Lithosphere Asthenosphere Phase diagram Conceptual model of earth

  16. Metastable minerals occur because of energy required for conversion • Bonds must be broken to switch between polymorphs • Cooling removes energy required to break bonds • Rate of cooling often important for lack of conversion – e.g. fast cooling removes energy before reactions occur • Quenching – “frozen”: e.g. K-feldspars • Example of Order-disorder polymorphism

  17. 2. Order-disorder polymorphism • The mineral structure remains same between polymorphs • Difference is in the location of cations in structure • Good examples are the K-feldspars • One end-member of the alkali feldspars

  18. Idealized feldspar structure Fig. 12-6 Si or Al K (or Na, Ca) • K-feldspar has 4 tetrahedral sites called T1 and T2 (two each) Si or Al

  19. “K-spars” • KAlSi3O8 – one Al3+ substitutes for one Si4+ • High Sanidine (high T) – Al can substitute for any Si – completely disordered • Low Microcline (low T) – Al restricted to one site – completely ordered • Orthoclase (Intermediate T) – Intermediate number of sites with Al

  20. Fig. 4-13 Order-disorder in the K-feldspars High Sanidine – Al3+ equally likely to be in any one of the four T sites Microcline – Al3+ is restricted to one T1 site. Si4+ fills other three sites

  21. Degree of order depends on T • High T favors disorder • Low T favors order • Sanidine formed in magmas found in volcanic rocks – quenched at disordered state: metastable • Microcline found in plutonic rocks – slow cooling allows for ordering to take place • Over time, sanidine will convert to microcline

  22. 3. Displacive Polymorphism • No bonds broken • a and b quartz are good examples • b quartz (AKA high quartz) • 1 atm P and > 573º C, SiO2 has 6-fold rotation axis. • a quartz (AKA low quartz) • 1 atm P and < 573º C, SiO2 distorted to 3-fold axis

  23. a quartz b quartz View down c-axis Fig. 4-12 6-fold rotation axis 3-fold rotation axis • Conversion can not be quenched, always happens • Never find metastable b quartz

  24. External crystal shape may be retained from conversion to low form • Causes strain on internal lattice • Strain may cause twinning or undulatory extinction • Must have sufficient space for mineral to form Undulatory extinction

  25. (4) Polytypism • Stacking diffrences • Common examples are micas and clays

  26. Fig. 4-14 Common Sheet silicates – like clay minerals Orthorhombic, two stacking vectors, not 90º Monoclinic, single stacking vector, not 90º Orthorhombic, single stacking vector, 90º

  27. Eventually will get to controls on compositional variations • First some “housekeeping” – necessary skills: • Scheme for mineral classification • Rules for chemical formulas • A graphing technique – ternary diagrams

  28. Mineral Classification • Based on major anion or anionic group • Consistent with chemical organization of inorganic compounds • Families of minerals with common anions have similar structure and properties • Cation contents commonly quite variable

  29. Follows from Pauling’s rules • 1, 3, and 4 (coordination polyhedron & sharing of polyhedral elements) - anions define basic structure • 2: (electrostatic valency principle) anionic group separate minerals

  30. Mineral Formulas • Rules • Cations first, then anions or anionic group • Charges must balance • Cations of same sites grouped into parentheses • Cations listed in decreasing coordination number • Thus also decreasing ionic radius • Also increasing valence state

  31. Examples • Diopside – a pyroxene: CaMgSi2O6 • Charges balance • Ca - 8 fold coordination: +2 valence • Mg - 6 fold coordination: +2 valence • Si – 4 fold coordination: +4 valence • Anionic group is Si2O6

  32. Substitution within sites indicated by parentheses: • Ca(Fe,Mg)Si2O6 • Intermediate of two end-members: Diopside CaMgSi2O6 – Hedenbergite CaFeSi2O6complete solid solution series (more on “solid solution” in a moment)

  33. Can explicitly describe substitution • E.g. Olivine: (Mg2-x,Fex)SiO4 0 ≤ x ≤ 2 • Alternatively: Can describe composition by relative amounts of end members: • Forsterite = Fo • Fayalite = Fa

  34. General composition of olivine is (Mg,Fe)2SiO4 • All of the following are the same exact composition: • (Mg0.78Fe0.22)2SiO4 • Mg1.56Fe0.44SiO4 • Fo78Fa22 (here numbers are percentages of amount of each mineral) • Fo78 (here implied that the remainder is Fa22) • Fa22

  35. How to calculate chemical formulas for solid solutions • Eg. Plagioclase feldspars: • Albite, Ab – NaAlSi3O8 • Anorthite, An – CaAl2Si2O8 • What is chemical composition of say Ab25An75?

  36. Graphic representation • Common to have three “end members” • Ca2+, Mg2+ and Fe2+ common substitutions between silicate minerals • Also K, Na, Ca – e.g. the Feldspars • Ternary diagrams • Used to describe distribution of each end member • Total amount is 100%

  37. See page 84

  38. 8% Fs 50% Wo 42% En Fig. 4-17 Ca2Si2O6 Pyroxenes: (Mg,Fe,Ca)2Si2O6 Composition is: En42Fs8Wo50 (Mg0.42Fe0.08Ca0.5)2Si2O6 Mg2Si2O6 Fe2Si2O6

  39. Compositional Variation • Think of minerals as framework of anions • Form various sites where cations reside • Principle of parsimony • Not all sites need to be filled • Some sites can accommodate more than one type of ion (e.g. polymorphism in feldspar, solid solution in olivine)

  40. Solid solution • Occurs when different cations can occur in a particular site • Three types: Substitution, omission, and interstitial • Anions can substitute for each other, but this is rare

  41. Tourmaline – an example of extreme amount of substitution Na(Mg,Fe,Li,Al)3Al6[Si6O18] (BO3)3(O,OH,F)4 W = 8-fold coordination, not cubic; usually Na, sometimes Ca or K X = Regular octahedral; usually Mg and Fe, sometimes Mn, Li, and Al Y = 6-fold coordination; usually Al, Less commonly Fe3+ or Mg, links columns B = trigonal; Borate ions, B is small, Fig. 15.9

  42. Terms • Substitution series or solid solution series: the complete range of composition of a mineral • End members: the extremes in the range of compositions • E.g. olivine: Forsterite and Fayelite

  43. Terms • Continuous or complete solid solution series: all intermediate compositions are possible • E.g. Olivine • Incomplete or discontinuous solid solution series: a restricted range of compositions • E.g Calcite - magnesite

  44. Substitutional Solid Solution • Two requirements for substitution • Size – substituting ions must be close in size • Charge – electrical neutrality must be maintained

  45. Size • Comes from Pauling rule 1: coordination • In general size of ions must be < 15% different for substitution • Tetrahedral sites: Si4+ and Al3+ • Octahedral sites: Mg2+, Fe2+, Fe3+, Al3+ • Larger sites: Na+ and Ca2+

  46. Temperature is important • Example is K and Na substitution in alkali feldspar (Sanidine and Albite) • Size difference is about 25% • Complete solid solution at high T • Limited solid solution at low T • Results in exsolution

  47. Types of substitution • Substitutional solid solution • Simple substitution • Coupled substitution • Omission substitution • Interstitial substitution • Different types have to do with where the substitution occurs in the crystal lattice

  48. Simple Substitution • Occurs with cations of about same size and same charge • Example: Olivine

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