Introduction to MineralogyDr. Tark Hamilton

Chapter 3: Lecture 8The 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

Effective Ionic Radii in MineralsI.R. increases with C.N.

C.N. Na+

Mg+2

Al+3

Si+4

P+5

S+6

O-2

S-2

[4] 0.99 0.57 0.39 0.26 0.17 0.12 1.38 1.84

[6] 1.02 0.72 0.48 0.40 0.38 0.29 1.40 XXX

[8] 1.18 0.89 0.54 XXX

XXX

XXX 1.42

XXX

[12] 1.39 XXX

XXX

XXX

XXX

XXX

XXX

XXX

CATIONS ANIONS

Cation Size Increases With C.N.

3 Trigonal Planar

4 Tetrahedral

6 Octahedral

8 Cubic

12 Dodecahedral

C+4, N+5

Polarization Distorts Electrons

Get a load of that tight little cation!

Ionization PotentialEnergy added to an atom to remove an electron

• X atom – e- X+ cation

– Atom gains energy: ΔEnergy > 0

• X atom + e- X- anion

– Atom loses energy: ΔEnergy < 0

• Unpaired electrons are most easily removed

Ionization & Electronegativity

+ ΔE added to lonegaseous atom to

remove 1 electroneV or kJoules/mol

Atom’s tendancy toattract electrons to

itself in bonds(Pauling, qualitative)

X-RAYS

“NaCl Structure”

2.8 Ang

u = (A q1 q2 )/r

Bond Energy is proportionalto product of charges

& inverse to bond length

Ionic Bond

“Wife Swapping – Melting”

Na Halides& Anion size

Alkaline EarthOxides vs

Cation Size

Alkali Fluoridesvs

Cation Size

Linear

Parabolic

0.74F’s repel

1.02

1.38

1.52

“Wife swapping – Hardness”

Alkaline EarthOxides vsHardness

Bond length& Strength

vs Hardness mp 2700°mp 990°

Conduction Band Electrons: Metals

ConductivityPossible electron path

+ nuclei

+Cu Atom

Conduction band electrons

after R.T. DownsUniv. Arizona

In metals, electrons areHigher in energy than the valence bands, e.g.

promoted by adjacent atoms

Mg 1s2 2s2 2p6 3s2

sp-hybrid

Gas Metal

LatticeDopingGeLi - γSiLi - x

IonicSolidConductivity

Good - Poor

Other Elements

Neighbour atomsExcite or promote Electrons

σ - bond

π - bond

Electron Probability Distribution

σ – bond or sp – bondLike Z or Unlike Z

Covalently Bonded Atoms or IonsStable Inert Gas

Configuration

H-H, Cl-Cl, H-ClO=O, (S=S)-2

Hg-Hg, Au-Au

Lewis DotStructure, :N:::N:

VSEPR Theory

C in Diamond, O in Ice, Si4+ in SiO2

Bonding Types for Different Minerals

NiAsMoS2

Sb2S3

PbSZnS

C Cu

Al2O3

NaCl

Mg2SiO4

(Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2

Groups I & VII or II & VI

Electron sharing

Minerals with more than 1 Bond Type

• C in (CO3)-2 is covalent bonded to Oxygen– Δe.n. 3.5 – 2.5 = 1.0– If +4 it would have an Ionic Radius of -0.08Ang– Thus it is held tighter than the ion in [3] Coordination

• Ca+2 in Calcite is bonded to O-2 in (CO3)-2 – Δe.n. 3.5 – 1.1 = 2.4 thus ionic

• Cleavage must cut pairs of ions, not between• This goes for salts of most oxyanions