packing of atoms in minerals
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49 Terms
how do ions arrange themselves?
surround themselves with ions of opposite charge to form regular repeating 3D structures
what is the coordination number?
number of nearest neighbour ions
what is a coordination polyhedron?
the shape obtained by joining centres of these nearest numbers
what determines the way ions pack togeher?
the size of the ion
what are larger anions or cations?
anions
how does ionic radius change down a group
increases
if there are two ions of the same element will the more highly charged one be smaller or larger?
smaller
what is the formula for radius ration?
r+/r-
what is the coordination number for a radius ratio of <0.155?
2
what is the coordination number for a radius ratio of 0.155-0.255?
3
what is the coordination number for between 0.255 to 0.414?
4
what is the coordination number for a radius ratio of between 0.414 to 0.732?
6
what is the coordination number for a radius ratio of between 0.732 and 1?
8
what shape does a coordination of 2 make?
linear
what shape does a coordination of 3 make?
equilateral triangle
what shaped does a coordination number of 4 make?
tetrahedron
what shape does a coordination of 6 make?
octahedron
what shape does a coordination number of 8 make?
cube
what are complex ions?
many minerals possess covalently bonded complex ions which usually remain as such even in solution
what is ionic substitution?
one ion can replace another of a similar ionic radius (± 15%) e.g. in olivine Fe2+ and + can replace each other
what is incomplete substitution?
in some minerals not all the atoms can be substituted .e.g. in Sphalerite only 50% by weight of the zinc atoms can be replaced by iron atoms
what is coupled ionic substitution?
it is not necessary that substituting ions should have the same charge however electrical neutrality must be maintained by simultaneous substitution elsewhere in the structure
what is the general formula for Plagioclase?
Na1xCaxAl1+xSi3-xO8
what is exsolution?
in the Alkali Feldspar’s ionic Subsitution of Na and K is much more extensive at higher temperatures than at low temperatures
what is camouflage?
Although Rubidium is a relatively abundant element there are no rubidium minerals but its ionic radius is very close to that of potassium so all available Rubidium.
what unit is chemical analysis given in earth science?
weight percent
what unit is chemical analysis of silicates given in?
weight % oxides
how is glass formed?
from rapidly cooled liquid creating a disordered solid
what is a phase diagram used to show?
represent the stability fields of the phases ice, water and steam
what are phase boundaries?
lines separating the stability fields
what is the triple point?
the point where solid, liquid and gas live in equilibrium
what are polymorphs?
minerals with the same chemical composition but different crystal structures
why are different individual polymorphs formed?
they are stable under different pressure and temperature conditions
what does the geothermal gradient represent?
represents the estimated change of P and T with depth
what depth is the diamond stability field?
over 100km depth
what are displacive transformations?
transformations involving the bending of bonds
what are reconstructive transformations?
transformations involving the breaking of bonds
over what timescale do displacive transformations take place?
usually instantaneous
over what timescale do reconstructive transformations usually take place?
usually slow leading to meta-stable persistance of a mineral
what is a pseudomorph?
can be seen where the shape of one thing has been replaced by something else
how can polymorphs exist outside their stability field?
an energy barrier has to be overcome e.g. for diamond to be converted graphite energy has to be provided to break the bonds of diamonds
what is order-disorder polymorphism?
in certain minerals the distribution of some atoms may be random whereas in others the distribution may be ordered
what are the three main types of defects?
point defects, line defects and domain defects
what main effect do point defects have on minerals?
their colour
what are point Schottky defects?
missing atom
what are point Frenkel defects?
atoms in an interstitial position
what are point impurity defects?
impurity atom on an interstitial position
when do screw dislocations form?
during crystal growth
what do edge defect control?
the mechanical behaviour of crystals and rocks