Mineral growth - CH 5.

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kevindevriendt
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175447
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Mineral growth - CH 5.
Updated:
2012-10-08 14:05:19
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Mineral growth
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Mineral growth
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  1. Forsterite(olivine) - Mg2SiO4
    what are the ions in this mineral?
    what ion sits in the octahedra?
    How do the M1 and M2 sites differ?
    • --> 2Mg2+, Si4+, O2-
    • -Magnesium is in the octahedra
    • - M1 has lots of edge sharing, less stable structure, and as a result distorted
  2. What is a mineral? newely defined
    • Naturally ocurring,
    • Homogeneous solid
    • Definite but not generally fixed chemiscal composition
    •  - "Solid solution"
    • ordered atomic arrangement
  3. Substitution basic rules?
    Simple substitution? (substitution solid solution)
    coupled substitution?
    • -ion sizes must be similar
    • -charge neutrality must be maintained.
    • -if difference in size is less than 15%, extensive substitution is possible,if not, this is limited

    Substituion cations have the same charge and about the same size. (olivine - fayalite (fe) and forsterite (mg))

    charge balance is maintained by coupling one substiution that increases the charge with another that reduces the charge - PLAGIOCLASE
  4. What it Omission solid solution?
    E.g. Pyrrhotite

    Fe1-XS (X= 0 -0.02)

    3Fe2+  <--> 2Fe3+ + ____

    Charge balance is maintained when ions of different charge substitute for each other by leaving structuralsites vacant or unfilled
  5. What is interstitial substitution?
    E.g. Beryl

    A variation of couples substitution in which charge balance in maintained by placing ions in sites that are normally vacant.

    ___ +Si4+ <--> Al3++(K+,Rb+,Cs+)
  6. Crytal growth process
    which crystals are the most prominant on a crystal and why?
    • - Adding a new materical to a face leaves many bonds unsatisfied
    • - Having an EDGE availabile allows new material to be added without as many unsatisfied bonds.
    • - Growth on a SCREW DISLOCATION (a crystal defect)allows the face to grow in a continuous spiral

    The slowest growing crystal faces are the most likely to be prominent on a crystal. The fastest growing faces {111} have a net charge and high surface energy as opposed to the (100) faces which are uncharged and grow slow. The fastest growing however grow at a speed which results in their dissapearence.
  7. Why are their only 8 abundant minerals that make up the earths crust?
    • - primordial metarial accumulated to form the earth - prepresented by chondritic meteorites whose major minerology includes olivine and pyroxine.
    • - less abundant minerals are only known from meteorites because their chemiscal and physical conditions needed for formation are not found on earth
  8. what are the three types of defects that influence properties of a crystal
    Point,line,planar defects

    Vacancy defers, mislocation, foreign atoms in unused sites, foreign ion substitutes.
  9. Mineral formulars,
    How should a mineral be named, by what order?
    • 1. cations are first followed by anions
    • 2. charges must balance
    • 3. cations litsted in order of their coordination number (high to low)
    • 4. halogens and hydroxy ions at the end of the formula

  10. What is nucleation?

    Homogeneous Vs Heterogeneous?
    Homo :clusters of atoms or ions find each other spontaneously and randomly to form the nucleus of a new crystal

    • Hetero: New minearls nucleate by taking advantage of the structure of an existing mineral
  11. Nucleation + crystal growth:
    • -Need to start nucleus
    • -need to add material to the surface
    • -need to move unwanted materical away and move required -material to the crystal surface
  12. Define : twinning

    What is twin law?
    • A symmetrical integrowth of two or more crystals of the same mineral.
    • - it is a planar structural defect

    • related by symmetry = twin operation
    • -reflection, rotation, inversion

    Twin law : decreibes the twin operation and identifies the crystal planes or axis associated with twinning {hkl}
  13. Contact twin?
    • joiined on a rational composition plane (not intergrown) only two parts
  14. Penetration twin?
    twin segments are Intergrown, related by rotation

  15. Significance of a defect
    Influences diffusion of atoms through a crystal lattice

    and colour
  16. what are polysynthetic twins
    twin segments jokined on successive parallel composition planes repeasted by the same twin law - triclinc plagioclase.
  17. Rate of crystal grow

    What faces are most likely to be prominent on a crystal?
    Slowest growing crystal faces are most likely to be prominent on a crystal.

    fast  growing on the (111) plane where there is a very strong attraction for sucessive laters (have a net charge and high surface energy

  18. Polysynthetic twinning
    • Twin segments joined on succesive parallel composition planes repeated by the same twin law (plagiclase repeated on {010})

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