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 Mineralogy Dr. Tark Hamilton Chapter 3: Lecture 9 The 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 q 1 q 2 )/r Bond Energy is proportional to product of charges & inverse to bond length Ionic Bond

Electronegativity: atom’s attraction for electrons in a bond (empirical) H – H = 0, pure covalent O – H = 1.4, polar covalent F – Na = 3.1, pure ionic Electron Donors Electron Acceptors

ΔElectronegativity vs % Ionic Character, Pauling (1960) NaCl HO CO SiO AlO, BeO TiO KO, BaO CsF CaF AuTe BeO ZnS AsS IC = 1 – e -0.25(a-b)exp2

Coesite: SiO 2, 2/m, 3Gpa, 700°C Electron Density Distribution: Contour: 0.1 e - /Ang 3 + ______, - ……… Si --- O 1.61 Angstroms after Geisinger & Gibbs (1983) O Si

Polarization = Van der Waal’s Bonds Nucleus of Neighbour atom attracts electrons Induced dipole London Dispersion Forces (weak dipoles) Hydrogen Bonding is strongest

Cleavage follows Van der Waal’s Bonds 1.42 Angstroms C-C bonds in sheets delocalized & same as Benzene or Diamond Perfect Cleavage Moh’s Hardness = 1

Orthorhombic Sulfur (2/m 2/m 2/m) Sulvere – Sanscrit Sulphurium - Latin S 8 rings Covalent Van der Waal’s Bonds Hardness ~ 2.0 Melts 112.9°C Volcanic exhalites & Bacterial reduction of (SO 4 ) -2 in sediments S-S, 2.06 angstroms in rings SSS, 108° bond angle 98° dihedral angle Colour depends on Traces of other polymorphs  95.3°C β-Sulfur Monoclinic Rosickyite

Packing Styles of Polar Molecules Dipoles alternate H - bonding Dipole H 2 O molecule Cubic Ice 1c 4c3

Common Coordination Polyhedra

Radius RatioCoordination no. Binary (AB) Structure- type r + /r - = 1 12none known 1 > r + /r - > CsCl > r + /r - > NaCl > r + /r - > ZnS Limiting Radius Ratios – Coordinating anions contact each other & cation (Pauling’s 1 st rule)

What is the Numerical Value of 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 r o = r+ + r- Ionic radii change with coordination number –r 8 > r 6 > r 4 {use the appropriate one!} Use the same scale for cation and anion