Introduction to MineralogyDr. Tark Hamilton

Chapter 2: Lecture 5

Camosun College GEOS 250

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

Lab: 9:30-12:20 W F300

Introduction Minerals & Light

Reflectance, scattering, transmittance, refraction,

absorption, energy effects…

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

Crystals affect light

• Slows velocity

• Has density

• Absorbs

• Diffracts

• Refracts

• Excites and emits

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

Light interactions with Minerals p.289

Steep angleHigh R.I.

Short wave

Flaws & inclusions

Excited electronsReturn lower energy

Longer waves

I can’tbelieve

I missed allthe electrons!

Internalreflections

Look out mineralhere I come!

Refractionbent, shorter

& slower

DispersionKinky colour

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

Lustre: appearance in scattered + reflected

light (interaction between photons of visible light and bonding electrons in mineral)

• Metallic: Highly lustrous & reflective, polished silver, native metals, metallic minerals especially S-2, Se-2, Te-2, As-2 or -3 ; (Ag, Au, Cu, Graphite, Arsenopyrite Bornite Galena Molybdenite Pyrite)

• Reason: Abundant d- or f-block metallic conduction band electrons of nearly equal energy in heavy or metallic elements. Also conductive of electricity & heat.

Lustre: appearance in scattered + reflected

light (interaction between photons of visible light and bonding electrons in mineral)

• Sub-metallic: Somewhat or darkly lustrous & reflective, some sulphides & oxides often with internal reflections & reddish glints; cassiterite, hematite, ilmenite, sphalerite, spinel

• Reason: Abundant metals in structure yet large space for transparent O-2 or S-2

Lustre: appearance in scattered + reflected

light (interaction between photons of visible light and bonding electrons in mineral)

• Non-metallic: various, glassy ceramic-like, transparent or translucent silicates & p-block oxy-acid salts (sulphate, carbonate, phosphate etc.) Quartz, feldspar, zeolite, clays, calcite, dolomite, gypsum, apatite

• Reason: Abundant oxide & light elements often ionically or covalently bonded, non-conductive.

Non-metallic Lustre: various

• Adamantine: Brilliant, gem-like, due to high refractive index & internal reflections; diamond, garnet, cerussite, synthetic gems YAG, spinels

• Vitreous: Glassy transparent or translucent with high polish in silicate minerals of light elements; quartz crystal, emerald, beryl, topaz, K-feldspars

• Pearly: Iridescent sheen like Mother of Pearl, Abalone, internal reflection planes off of cleavages, twins, or compositional changes; apophyllite, talc

Non-metallic Lustre: various

• Resinous: translucent with internal reflections like amber, plastics or epoxies; sphalerite

• Greasy: Oil like surface sheen due to pitting or microscopic surface roughness in weathered silicates & salts; milky quartz, nepheline, halite, sylvite

• Silky: Internal reflections from individual mineral fibers in fibrous aggregates resembling silk threads or satin weaves; satin spar gypsum, serpentine, crocidolite tiger’s eye

• Earthy: Dull translucent aggregates of fine particles with variable air & water content like soils; limonite, kaolinite, goethite

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!

Colour has many causes• Depends on Chemical composition either

essential or accidental elements (Fe is dark, Ti tints Amethyst)

• Depends on structure (e- in void in fluorite, graphite vs diamond)

• For essential elements, colour is diagnostic as for reflectance in metallics (S in sulphur, Cu ions blue in azurite, Au2 is gold, molybdenite is silvery blue, galena is lead blue-grey)

Streak

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

Minerals with characteristic Colour

• Varietal gems:

• Beryl: aquamarine Blue, emerald Green

• Corundum: sapphire Blue, ruby Red

• Jadeite: light Green

• Quartz: adventurine Green, citrine Yellow

• Topaz: Yellow

Colours vary in non-metallics

• Colour zonation: fluorite, tourmaline

• K-Feldspar: white, yellow, tan, green, pink

• Plagioclase: white, grey, blue-green, black

• Garnet: red, brown, black, green

• Pyroxene: white, lt-green, dk-green, black

• Quartz: white, yellow, green, purple, black

• Calcite: white, tan, grey, blue, black

Play of Light & Colours

• Asterism, Chatoyancy: bright bands across fibers or inclusions, star sapphire, cats eye (alexandrite), tigers eye

• Iridescence: diffraction sheen like oil film

• Tarnish: thin oxide coating on metallics

• Labradorescence: twin planes in Ca Plag

• Opalescence: diffraction from grid of amorphous spheres of SiO2 & H2O

Diffraction of white light by Precious OpalPlay of colours depends on d and Θ theta

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

Hi E Low E Fluorescence

or X-rays,Cosmic rays

What happens to the rest of the energy?