introduction to mineralogy dr. tark hamilton chapter 2: lecture 5 camosun college geos 250 lectures:...
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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?
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