Introduction to MineralogyDr. Tark Hamilton

Chapter 2: Lecture 4

Camosun College GEOS 250

Lectures: 9:30-10:20 M T Th F300

Lab: 9:30-12:20 W F300

Physical Properties of Minerals(Interplay with light)

• Asterism• Crystal form• Crystal Habit• Chatoyancy• Cleavage• Colour• Density (S.G.)• Fracture• Fluorescence• Hardness

• Luminescence• Lustre• Magnetism• Parting• Phosphorescence• Piezo-, Pyroelectricity• Play of colours• Radioactivity• Tenacity• Streak

Crystallography (faces)

Perfect & Poor Dodecahedron 12

Pyritohedron 12

Rhombohedron 6

Perfect & Poor Cube 6 Perfect & Poor Octahedron 8

Crystallography

• External & internal crystal form

• Methods: Visual, microscopy, refraction, XRD, ED, SEM, TEM

• Forms: Pedion, Pinacoid, Dome; (hkl)

• Dihedral angles: (<180°, internal)

• Symmetry elements: 2- 3- 4- 6-rotation, screw axes, mirror planes, glide planes

6 Crystal Systems (Ch.6: p.128-129)

32 Crystal Classes grouped by center of symmetry or none

• Triclinic: all different edges and angles, a ≠ b ≠c , no 90° angles, 1= no sym, Ī = centre

• Monoclinic: all different edges, a ≠ b ≠c , β > 90° , α=γ=90°, symmetry: 2,m, 2/m

• Orthorhombic: all different edges but all 90° angles, a≠b≠c , α=β=γ=90°: 222, mm2, 2/m 2/m 2/m

• Tetragonal: 2 different edges, all 90° angles, a=b≠c , α=β=γ=90°: 4, 4bar, 422, 4mm, 4bar2m, 4/m, 4/m 2/m 2/m

6 Crystal Systems (Ch.6: p.128-129)

32 Crystal Classes grouped by center of symmetry or none

• Tetragonal: 2 different edges, all 90° angles, a=b≠c , α=β=γ=90°: 4, 4bar, 422, 4mm, 4bar2m, 4/m, 4/m 2/m 2/m

• Hexagonal (hexagonal): 2 different edges, 120° & 90° angles, a1 = a2 = a3 ≠ c , α= α= α = β = 90° γ = 120°: 6, 6bar, 622, 6mm, 6bar m2, 6/m, 6/m 2/m 2/m

• Hexagonal (trigonal): 3 equal edges, 120° & 90° angles, a1 = a2 = a3 = c , α = β = 90° γ = 120°: 3, 32, 3m, 3bar, 3bar2/m

6 Crystal Systems (Ch.6: p.128-129)

32 Crystal Classes grouped by center of symmetry or none

• Tetragonal: 2 different edges, all 90° angles, a=b≠c , α=β=γ=90°: 4, 4bar, 422, 4mm, 4bar2m, 4/m, 4/m 2/m 2/m

• Hexagonal (trigonal): 3 equal edges, 120° & 90° angles, a1 = a2 = a3 = c , α = β = 90° γ = 120°: 3, 32, 3m, 3bar, 3bar2/m

• Isometric (cubic): 3 equal edges, 3 90° angles, a1 = a2 = a3 , α = β = γ = 90°: 23, 432, 4bar3m, 2/m3bar, 4/m3bar2/m

Habit – Face Development• Euhedral: good, well formed faces, taking

its characteristic crystal form. E.g. hexagonal quartz prisms or cubic pyrite

• Subhedral: Some good faces - some curved

• Anhedral: Mineral lacking crystal faces, curved, rounded, embayed, irregular

Crystal Habit (Growth Shape)(Environmentally controlled)

• Granular Micaceous Bladed

• Fibrous Acicular Radiating

• Dendritic Mammilary Colliform

• Vuggy Concentric• Oolitic/Pisolitic

Crystal Habit (Growth Shape)Other terms:

• Compact: too fine grained for naked eye e.g. Kaolinite

• Massive: Lacking crystal faces, multiple fine grains e.g. olivine in dunite, goethite

• Sugary: mass of fine crystals, e.g. gypsum, anhydrite

• Earthy: massive, compact, dull, e.g limonite

Crystal Habit (Growth Shape)

Other terms: (and interpretation)• Banded: layers with colours or compositional differences, planar

version of concentric, e.g. agate, fluorite (changing fluid composition, trace elements, oxidation)

• Blocky: euhedral but similar in all dimensions e.g. analcite, feldspar (grew in free space or unencumbered, e.g. magmatic)

• Botryoidal: mammilary, reniform a curved bumpy grape-like mass with involute intersections often radial or concentric in cross section e.g. chalcedony, prehnite (growth during fluid concentration, dessication, at water table)

• Fibrous: thin filaments often curved e.g. asbestos (Grew in free space, across a vein)

Crystal Habit (Growth Shape)

Other terms: (and interpretation)• Geode: hollow rock cavity or concretionary mass with concentric

layering or drusy crystals, e.g. barite, calcite, amythest often with shelves or half filled geopedal structures showing way up (shallow void or solutional space often above water table)

• Triple point: granular often monomineralic with 120° grain boundaries, common in quartzite & marble (equal pressure in all directions)

• Lamellar, foliated: layered e.g. graphite, molybdenite (pseudo-hexagonal crystal form)

• Needles, whiskers: Longer than wide or thick = acicular e.g. apatite, millerite (preferential growth in one direction or form, quench, bacterial)

Crystal Habit (Growth Shape)

Other terms: (and interpretation)

• Needles, whiskers: Longer than wide or thick = acicular e.g. apatite, millerite (preferential growth in one direction or form, quench, bacterial)

• Prismatic: elongate euhedral forms e.g. pyroxene, amphibole, quartz (Commonly monoclinic or hexagonal)

• Pisolitic: Large concentric concretionary or residual masses, e.g. bauxite, cave pearls (formed at or below water table by solution precipitation processes)

• Tabular: similar length & width but lesser thickness e.g. feldspars (Commonly monoclinic, orthorhombic, tetragonal, grew in free space, e.g. magmatic)

Cleavage directions & formsOctahedral 4 @ 109.5°Cubic 3 @ 90° Dodecahedral 6 @ 116.6°

Prismatic 3 = (2 + pinacoid) Pinacoidal, Basal 1Rhombohedral 3No 90° or 60°/120°

Introduction Minerals & Light

Reflectance, scattering, transmittance, refraction,

absorption, energy effects…

Diaphaneity: ability to transmit light

• Transparent: Transmitting some light; quartz, calcite, halite, ulexite, gems

• Translucent: Diffuse transmittance of light, cloudy bright, bathroom glass, most silicates, sulphates, carbonates, salts; moonstone, gypsum, anhydrite, aragonite

• Opaque: Blocks transmittance of light even on thinnest edges, metal sulphides & oxides; Magnetite, Pyrite, Galena, Copper

Lustre: appearance in scattered + reflected light (interaction between photons of visible light and

bonding electrons in mineral)

• Metallic: highly reflective, shiny

• Sub-Metallic: darkly reflective

• Non-metallic: various, glassy ceramic-like

Colour

• It is a spectral thing ROYGBIV long short

• Depends on energy, E = h ν = h c/λ

• It depends on our eyes: Gold absorbs blue so it looks yellow!

Streak

• True colour of powdered mineral (depends on compound not structure)

Luminescence

• Mineral absorbs usually higher energy and emits cold light (not incandescence)

• Triboluminescence: shock emits light, quartz; hammers, explosions, quakes

• Thermoluminescence: heat emits light, caused by cosmic ray damage, dating use

• Phosphorescence: stores & emits light

• Fluorescence: uV emits visible light on-off