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