Crystal structure CH-4

  1. Define Isostructural 
    Two or more minerals whose atoms are arranged in the same type of crystal system
  2. Isostructural group
    Group of minerals that are isostructual and also chemically related by having a common anions or anionic group.

    basically : same structure different chemistry
  3. List 5 isostructural minerals
    • Calcite- CaCO3
    • Magnesite- MgCO3
    • Siderite - FeCO3
    • Rhodocrosite - MnCO3
    • Smithsonite - ZnCO3
  4. Define Polymorphism
    The ability of a chemical compound to crystallize with more than one structure 

    Basically single chemical compound but different structures
  5. Examples of polymorphic crystal groups
    • SiO2
    • alpha- quartz
    • beta -quartz
    • FeS2
    • Pyrite
    • marcasite

    • Carbon
    • graphite
    • Diamond
  6. What is reconstructuve polymorphism
    • - Requires major re-organisation of crystal structure (e.g. trigonal to monoclinic)
    • -Chemical bonds must be broken
    • -some structural similarities

    Graphite to Diamond for example. Changing conditions are required such as high pressure and temperatures. Diamond on the earths surface are called metastable/

    • Diamond - covalent bonds
    • Graphite - Metallic + van der waals 
  7. What is Displacive polymorphism
    Involves a distortion or bending of the crystal structure but does not involve and breaking of bonds.

    • example is Quartz (beta and alpha)
    • Beta= high T polymorph, 6 fold screw axes, hexagonal 
    • Alpha = low- T polymorph 3 fold screw axes trigonal
  8. What is order-disorder polymorphism
    The cation distribution within structuralsites changes; degree of order strongly influenced by crystallization T and cooling history

    • K-feldspar example
    • High sanidine (completely disordered structure)
    •  = Al is equally likely to be in and 4 sites on average which contains 25% AL -  
    • Microcline (fully ordered structure) = 
    • Al is preferentially in T1 site; SI occupies the other 3 sites and structure is slightly distorted (monoclinic to triclinic - less stable) --> Two up pointing and two down pointing tetrahedra
  9. What is the percent ionic character of the Si- O bond that dominates the earths crust?
    50% ionic -covalent character

    [ Variation exists between ionic and covalent and separetly covalent and metallic bonds]

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  10. SiO2 =
    Si4+ + 2O2-
  11. Hematite = Fe2O3
    • = 2Fe3+ +3O2-
  12. Magnetite= Fe3O4
    = 3Fe2+ + 4O2-
  13. Rhodocrosite - MnCO3
    = Mn2+  + (CO3)2-
  14. Anhydrite = CaSO4
    = Ca2+ + (SO4)4-
  15. Apatite = Ca5(PO4)3(OH,F,CL)
    = 5Ca2+ + (PO4)3- +(OH,F,CL)-
  16. What is Fe's crystal structure in earths surface
    Body - centered cubic packing (8)
  17. What is Fe's crystal structure in the earths inner core
    Hexagonal closest packing (12) due to high pressures
  18. CN of hexagonal
    CN of cubic
    CN of octahedral
    CN of tetrahedral
    CN of triangular
    CN of linear
    • =12
    • =8
    • =6
    • =4
    • =3
    • =2
  19. The higher the radius ratio and atomic radius of the ion results in a _____coordination
    higher coordination

    K+, Ca2+, Na+ have 12 CN
  20. What kind of polymorphism needs to occur to go from trigonal alpha quartz to monoclinic coesite?
  21. Stishovite and coestite require high pressures to form - identify one kind of geologic environment in which these minerals might be found?
    Meteorite impact
  22. How do metal atoms tend to pack together?
    Pack together in highly ordered arrangements to minimize space (Hexagonal, cubic and body centered cubic closest packing)
  23. What kind of group does ionic bonding dominate?

    What rules do they follow?
    Silicate group

    Paulings rules
Card Set
Crystal structure CH-4
Crystal structure