Introduction to Mineralogy Dr. Tark Hamilton Chapter 3: Lecture 9 The Chemical Basis of Minerals (sizes, shapes & directions) Camosun College GEOS 250.
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Introduction to MineralogyDr. Tark Hamilton
Chapter 3: Lecture 9The Chemical Basis of Minerals
(sizes, shapes & directions)
Camosun College GEOS 250
Lectures: 9:30-10:20 M T Th F300
Lab: 9:30-12:20 W F300
“NaCl Structure”
2.8 Ang
u = (A q1 q2 )/r
Bond Energy is proportionalto product of charges
& inverse to bond length
Ionic Bond
Electronegativity: atom’s attractionfor electrons in a bond (empirical)
H – H = 0, pure covalentO – H = 1.4, polar covalent
F – Na = 3.1, pure ionic
ElectronDonors
ElectronAcceptors
ΔElectronegativity vs % IonicCharacter, Pauling (1960)
NaCl
HO
CO
SiO
AlO, BeOTiO
KO, BaO
CsFCaF
AuTe
BeO
ZnS
AsS
IC = 1 – e -0.25(a-b)exp2
Coesite: SiO2 , 2/m, 3Gpa, 700°C
Electron DensityDistribution:
Contour: 0.1 e- /Ang3
+ ______ , - ………
0
0
0Si --- O 1.61 Angstroms
after Geisinger & Gibbs(1983)
O Si
Si
Polarization = Van der Waal’s Bonds
Nucleus ofNeighbour atomattracts electrons
Induced dipoleLondon Dispersion
Forces (weak dipoles)
HydrogenBonding
is strongest
Cleavage follows Van der Waal’s Bonds
1.42 Angstroms
C-C bonds in sheetsdelocalized &
same as Benzeneor Diamond
Perfect CleavageMoh’s Hardness = 1
Orthorhombic Sulfur (2/m 2/m 2/m)
Sulvere – SanscritSulphurium - Latin S 8 rings
Covalent
Van der Waal’s BondsHardness ~ 2.0Melts 112.9°C
Volcanic exhalites &Bacterial reduction of(SO4)-2 in sediments
S-S, 2.06 angstroms in ringsSSS, 108° bond angle
98° dihedral angleColour depends on
Traces of other polymorphs
95.3°C β-SulfurMonoclinic Rosickyite
Packing Styles of Polar Molecules
Dipolesalternate
H - bonding
Dipole H2Omolecule
Cubic Ice 1c 4c3
Common Coordination Polyhedra
Radius Ratio Coordination no. Binary (AB) Structure-type
r+/r- = 1 12 none known
1 > r+/r- > 0.732 8 CsCl
0.732 > r+/r- > 0.414 6 NaCl
0.414 > r+/r- > 0.225 4 ZnS
Limiting Radius Ratios – Coordinating anions contact each other & cation (Pauling’s 1st rule)
What is theNumerical Valueof ionic radius?
What's the Numerical Value of a specific Ionic Radius?
• Ionic Radii in most scales do not generally meet at experimental electron density minima, because of polarization of the anion by the cation
• The various scales are designed to be self-consistent in reproducing ro = r+ + r-
• Ionic radii change with coordination number – r8 > r6 > r4 {use the appropriate one!}
• Use the same scale for cation and anion
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