Klein 38-68
Nesse 39-56
Crystal Chemistry
Average chemistry of the crust (in other words... elements we are most
likely to encounter, thus the most likely to end up as mineral constituents).
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Atom structure and mineral properties are intimately related. Mineral
properties are function of (1) the atom types present, (2) the geometrical
arrangement of the atoms and, (3) the bonding forces that exist between the
atom pairs.
(This review is only to highlight concepts that you should already be familiar with and should have been covered in 100-level chemistry)
Quantum chemistry - theory that views the atoms are composed of
particles, with each part containing discrete amounts of energy.
The electrons, represent the outer "reactive" portion of an atom and are
viewed probabilistically, existing with specific "orbitals".
Probability vs. distance
Electrons can be viewed as clouds with a probability of occupying a region
within the cloud. Major energy gaps occur between them.
* see figure 3.12 in Klein - electron orbitals.
Each electron orbital in an atom is assigned a set of principle quantum
numbers. (n)
1. This number describes the volume of the orbital (recall the numbers must be integer values).
2. Orbital shape. s (spherical or sharp), p (dumbbell shaped or principal), d (diffuse or 4-fold symmetry) and f (fundamental), respectively.
Non-spherically shaped orbital have strong directionality's, which is important for bonding.
Arranged in increasing atomic weight. All weights are given relative
to 12C which is taken to be exactly 12.000
Characteristics are dependent upon the electronic structure of the atoms. Therefore, atoms are further arranged (into Groups) by chemical properties.
Group number relates the number of electrons in the outermost orbital.
Think of the periodic table as an ordering of elements based upon their chemical properties, that in part, depend upon the outermost (valence) electrons. These electrons become available for chemical bonding.
Because certain atoms have similar electron configurations, they will
occupy similar crystallographic sites.
Between any pair of oppositely charged ions there is an attractive force
proportional to the products of their charges and inversely proportional
to the square of the distance between their centers. (i.e., Coulomb's
Law)
Where:
Example of determinig Cl- ionic radius in LiCl
Using LiCl, Assume anion-to-anion (Cl-Cl) contact. This is reasonable because Li+ is small (Z=3) and Cl- is large (Z= 17).
Given LiCl unit cell edge as a = 5.14Å
Therefore, the distance between Cl- centers is 3.63Å and the radius = 1.81Å.
Example for the case of structures containing bigger cations (Halite)
Given unit cell dimension of halite (a = 5.627Å) the distance between Na+ and Cl- can be estimated by simply subtracting the Cl- radius from the interatomic distance along the unit cell edge (i.e., a/2 = 2.813Å)
Actual radius of Na+ is 0.95Å. The sodium atom "rattles" around in the lattice.
Using the same logic for Sylvite (KCl) where a = 6.28Å
For K+ : r K+ + r Cl- = 1/2 (6.28Å) , r K+ = 1.33Å (fits just right).
Atomic radii are not necessarily constant from one crystal structure to another. The radius will vary with 1) type of bond, 2) nearing neighbors and the shape of the atoms (recall that atoms and ions are not rigid shapes).
The tendency for an ion to reshape (or redistribution of charge) due to
external electric fields is termed polarization.
The more the electron density is localized between two ions the more "covalent" the bonding.
The forces that bind together atoms in a crystalline solid are electrical in nature.
Grouping of types of electrical forces or chemical bond in five principle
types
Bonding type is determined by crystal structure (packing) and atom types and ionization state.
Physical properties are intimately related to these factors as can be seen from the table below.
Melting temperatures and hardness of various compounds
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Specific bonds can share the character of more than one bond type.
More than one bond type ususally occurs within a crystal structure.
