Test 2 Flashcards
Linus Pauling (1901-1994)
American Chemist, won the noble prize for Chemistry and Peace
Pauling’s Rules - Rule 1 - The radius ratio Rule
The sum of the ionic radii determines the cation-anion distance, while the cation-anion radius ratio determines the coordination number (C.N.) of the cation
Pauling’s Rules - Rule 2 - The electrostatic valence rule
In stable Ionic Struct., the valence of each anion is = to the sum of the strengths of the electrostatic bonds to it from the cations
Pauling’s Rules - Rule 3 - Sharing of polyhedron corners, edges, and faces
The existence of edges and faces, the more sharing, the less stable. this effect is large for cations with high valency and small coordination #, and is especially large when the radius ratio approaches the lower limit of stability of the polyhedron
Pauling’s Rules - Rule 4 - Crystals containing different cations
In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedron elements with each other
Pauling’s Rules - Rule 5 - The rule of parsimony
The # of different kinds of constituents in a crystal tends to be small
Structure Terms
- Corner Sharing - 1 atom shared
- Edge Sharing - 2 atoms shared
- Face Sharing - 3 atoms shared
Isostructural
Having the same, or a corresponding, structure
Crust
Top Component of Lithosphere
Major elements of Earths crust
Earth’s crust is composed predominantly of eight elements. O, Si, Al, Fe, Ca, Na, K, Mg. Measured in weight %
Minor and trace elements
Minor = 0.1 - 1 weight % Trace = <0.1
Given in ppm or ppb
ex// C
Mantle
layer bounded below by a core and above by a crust
Upper Mantle
The upper mantle is dominated by the mineral olivine, Mg2SiO4
Effects of pressure begin to affect atomic structures
Transition Zone
From about 410 to 660 Km below the surface, Olivine transforms into denser structures
Wadsleyite and Ringwoodite
Hydrous, to about 1 weight % water
Lower Mantle
Pressures are so great that Si becomes (CN = VI), and some Mg becomes (CN = VIII) (perovskite structure)
Core
- Core divided into 2 sections, Liquid outer core, Solid inner core
- There is a definite chemical discontinuity between the lower mantle and the outer core
- The main elements in the core are an Fe and Ni alloy
- Increasing temperature first melts the alloy to make the outer core
- Increasing pressure freezes the alloy to produce the inner core
Outer Core
Liquid, 2900 to 5100 Km below the earth Composition is Fe with about 2% Ni Density of 9.9 gm/cm3 is too low to be pure metal Silica makes up 9-12%
Inner Core
Solid, 5100 to 6371 Km below surface
80% Fe, 20% Ni alloy
Pressures reach about 3 megabars
Temperature at the center is about 7600ºC
Ores
Trace elements in the gold group, the platinum group, mercury, lead, and others
Effects of pressure
As pressure increases, minerals transform to
denser structures, with atoms packed more
closely together
Victor Goldschmidt
Swiss-born Norwegian mineralogist and petrologist who laid the foundation of inorganic crystal chemistry and founded modern geochemistry
Goldschmidt’s Rules - Size
Atomic substitution is controlled by size (i.e.,
radii) of the ions (Free substitution can occur if size difference is <15%, Non if >30%)
If there is a small difference of ionic radius the
smaller ion enters the crystal preferentially
Goldschmidt’s Rules - Charge
Atomic substitution is controlled by charge
of the ions –> cannot differ by more than 1
For ions of similar radius but different
charges, the ion with the higher charge
enters the crystal preferentially
Factors affecting solid solution - Temperature
Minerals expand at higher T
Greater tolerance for ionic substitution at higher T
Factors affecting solid solution - Pressure
Increasing pressure causes compression
Less tolerance for ionic substitution at higher P
Availability of ions
Ions must be readily available for substitution to occur
Spin State - High
Mostly unpaired e-
Bigger atomic radii
Spin State - Low
Paired e-
Smaller atomic radii
Types of Crystalline Substitution - Omission and Substitutional
Substitutional - Mg^2+ ~ Fe^2+
Omission - Ca^2+ + Void ~ 2 Na+
Types of Crystalline Substitution - Vacancy
normally vacant sites ([]) can be filled as part
of a coupled substitution
[] + Si4+ = Na+ + Al3+
Types of Crystalline Substitution - Interstitial
Atom or ion occupies space in between the normal sites
Often H+
Ex// Beryl
Schottky Defect
Vacant lattice site
Frenkel defect
Wen an atom or cation leaves its original place in the lattice structure to create a vacancy while occupying another interstitial position
HCP Stacking defect
H H C H H
CCP Stacking defect
C C H C C
Grain Boundary Defect
Two lattices grow together, with some displacement of
ions
Polymorphous minerals
Polymorphism is the ability of a specific chemical composition to crystallize in more than one form.
Result of pressure
ex// Al2SiO5
Ditypous minerals
Same chemical composition, different stacking
Pseudomorphic minerals
mineral formed by chemical or structural change of another substance
Stable vs. metastable
A material being truly unchanging vs. A material where a change cannot be observed because the changing is too slow to be observed.
Mineraloids
A naturally occurring, inorganic solid that does not exhibit crystallinity.
ex// Opal
Ex-solution
Process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials.
Order-Disorder
If one type of ion substituting for another
prefers a certain type of site over another
the structure is ordered.
Metamict
Alpha radiation emitted from the radioactive
elements is responsible for degrading a mineral’s
crystal structure
If structure destroyed, then it is metamict
Atomic Arrangement
Minerals must have a highly ordered atomic arrangement
Unit Cell
Simplest (smallest) parallel piped outlined
by a lattice
Auguste Bravais
French physicist known for his work in crystallography, the conception of Bravais lattices, and the formulation of Bravais law.
Bravais Lattice
a two or three (space lattice) dimensional array of points
Lattice Requirements
Environment about all lattice points
must be identical
Unit cell must fill all space, with no
“holes”
Types of lattice (P,I,C,F,R)
P = Primitive I = Body - Centered C = Centered F = Face - Centered R = ?
Crystal System - Isometric
P, I, F
a=b=c
α = β = γ = 90 ̊
Crystal System - Tetragonal
P, I
a = b ≠c
c > a
α = β = γ = 90 ̊
Crystal System - Orthorhombic
P, I, C, F
a ≠ b ≠c
c > a > b
α = β = γ = 90 ̊
Crystal System - Hexagonal - Hexagonal
a = b ≠ c α = γ = 90 ̊ β = 120 ̊
Crystal System - Hexagonal - Rhombohedral
a = b = c α = β = γ ≠ 90 ̊
Crystal System - Monoclinic
P, C
a ≠ b ≠c
α = γ = 90 ̊ (β ≠ 90 ̊)
Crystal System -Triclinic
P
a ≠ b ≠c
α ≠ β ≠ γ ≠ 90 ̊
