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Define Isostructural
Two or more minerals whose atoms are arranged in the same type of crystal system
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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
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List 5 isostructural minerals
- Calcite- CaCO3
- Magnesite- MgCO3
- Siderite - FeCO3
- Rhodocrosite - MnCO3
- Smithsonite - ZnCO3
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Define Polymorphism
The ability of a chemical compound to crystallize with more than one structure
Basically single chemical compound but different structures
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Examples of polymorphic crystal groups
- SiO2alpha- quartz
- beta -quartz
- FeS2
- Pyrite
- marcasite
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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
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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
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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
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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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Magnetite= Fe3O4
= 3Fe2+ + 4O2-
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Rhodocrosite - MnCO3
= Mn2+ + (CO3)2-
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Anhydrite = CaSO4
= Ca2+ + (SO4)4-
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Apatite = Ca5(PO4)3(OH,F,CL)
= 5Ca2+ + (PO4)3- +(OH,F,CL)-
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What is Fe's crystal structure in earths surface
Body - centered cubic packing (8)
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What is Fe's crystal structure in the earths inner core
Hexagonal closest packing (12) due to high pressures
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CN of hexagonal
CN of cubic
CN of octahedral
CN of tetrahedral
CN of triangular
CN of linear
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The higher the radius ratio and atomic radius of the ion results in a _____coordination
higher coordination
K+, Ca2+, Na+ have 12 CN
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What kind of polymorphism needs to occur to go from trigonal alpha quartz to monoclinic coesite?
Recontructive
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Stishovite and coestite require high pressures to form - identify one kind of geologic environment in which these minerals might be found?
Meteorite impact
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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)
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What kind of group does ionic bonding dominate?
What rules do they follow?
Silicate group
Paulings rules
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