ERTH 2001: Physical Properties

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ERTH 2001 Physical Properties Nesse, Ch.6
Properties related to   1. Mass density specifi
c gravity   2. Mechanical cohesion hardness clea
vage fracture parting   3. Interaction with
light lustre colour  
 4. Magnetism diamagnetism paramagnetism ferrom
agnetism ferrimagnetism   5. Electrical conducti
vity piezoelectricity pyroelectricity   6.
Miscellaneous taste feel reactivity
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ERTH 2001Physical Properties Nesse, Ch.6
Interaction of Minerals with Light   Light
striking a mineral can be reflected transmitted
(and refracted) absorbed generally some
combination of these   opaque minerals - most
light absorbed transparent minerals - most
light transmitted
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ERTH 2001Physical Properties Nesse, Ch.6
Lustre our perception of the effect of light
at/near surface of mineral   metallic lustre -
minerals are opaque, reflective normal
metallic - 20-50 light reflected
submetallic lt 20 reflected   non-metallic lustre
- minerals are transparent or transluscent
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ERTH 2001Physical Properties Nesse, Ch.6
Lustre
metallic and submetallic
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ERTH 2001Physical Properties Nesse, Ch.6
Lustre
various forms of non-metallic
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ERTH 2001Physical Properties Nesse, Ch.6
Colour of Minerals determined by selective
absorption of certain ? of visible spectrum
where some aspect of light energy (? in visible
spectrum, 400-700 nm) matches some aspect of
energy levels in electronic structure of atoms in
crystal lattice a) absorbed energy --gt
excitation of electrons to higher energy
level absorbed ? not transmitted eye recombines
remaining ? as perceived colour   b)
absorbed energy will be re-emitted in some form
when excited electron falls back to its original
energy level (if in visible spectrum --gt
fluorescence)
absorption emission
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ERTH 2001Physical Properties Nesse, Ch.6
Colour of Minerals our perception of wavelengths
of reflected and transmitted light
(colour) depends on wavelengths NOT absorbed by
mineral   
ruby absorbs V, G, Y transmits B, O, R
emerald absorbs I, O, R transmits G, Y
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ERTH 2001Physical Properties Nesse, Ch.6
Colour of Minerals our perception of wavelengths
of reflected and transmitted light
(colour) depends on wavelengths NOT absorbed by
mineral   
alexandrite absorbs certain ? (violet-blue,
green-orange) incident white light transmitted
through alexandrite emerges looking purple-grey
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ERTH 2001Physical Properties Nesse, Ch.6
alexandrite absorbs certain ? (violet-blue,
green-orange) incident reddish light
transmitted through alexandrite emerges looking
orange-red incident bluish light transmitted
through alexandrite emerges looking
greensish-blue
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ERTH 2001Physical Properties Nesse, Ch.6
garnet
dominantly short ?
dominantly long ?
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals   1.
Crystal field transitions cations with
partially filled d orbitals transition metals,
Ti, V, Cr, Mn, Fe, Co, Ni, Cu can be part of
essential mineral composition or substitutional
impurities  interaction with co-ordinating anions
(generally O) develops variable energy levels
in d orbitals absorbtion of light can promote
electrons lower --gt higher levels   if absorbed
light is short ? (blue-violet), we perceive
mainly long ? (red)  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals   1.
Crystal field transitions cations with
partially filled d orbitals transition metals,
Ti, V, Cr, Mn, Fe, Co, Ni, Cu can be part of
essential mineral composition or substitutional
impurities  interaction with co-ordinating anions
(generally O) develops variable energy levels
in d orbitals absorbtion of light can promote
electrons lower --gt higher levels   if absorbed
light is short ? (blue-violet), we perceive
mainly long ? (red)  
example ruby some Cr3 substitutes for Al3 in
corundum (Al2O3)
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 2.
Band gaps affects minerals with a combination
of covalent or ionic banding (most light
transmitted) and metallic bonding (most light
absorbed by conduction band)   the energy gap
between valence band and conduction band may
correspond to some ? in visible spectrum (band
gap) absorption of light --gt promotion of
electrons from valence to conduction band  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 3.
Charge transfer cations with different
charge occupy adjacent lattice sites (essential
or substituted)   absorption of light by
cation with lower charge --gt transfer of an
electron to adjacent cation with higher charge
(e.g. Fe2 --gt Fe3 Fe2 --gt Ti4)   example
- sapphire (substitution of Fe2 or Ti4 for
Al3 in corundum, Al2O3)  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 3.
Charge transfer cations with different
charge occupy adjacent lattice sites (essential
or substituted)   absorption of light by
cation with lower charge --gt transfer of an
electron to adjacent cation with higher charge
(e.g. Fe2 --gt Fe3 Fe2 --gt Ti4)   example
- sapphire (substitution of Fe2 or Ti4 for
Al3 in corundum, Al2O3)  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 4.
Colour centres some defects in crystal structure
--gt changes in local electronic structure to
maintain charge balance (electron traps) can be
growth defects or can result from post-growth
damage (e.g., radiation) light energy may be
absorbed by displaced electrons or may displace
electrons in some cases, colour can be removed or
changed by heating   electron colour centres
Frenkel defects - missing anion  hole colour
centres electron missing from its normal
location  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 4.
Colour centres electron colour centres Frenkel
defects - missing anion  hole colour centres
electron missing from its normal location  
electron removed to balance charge
difference around O2-
colour centres in quartz
ideal quartz
Si4 replaced by Al3 H
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 4.
Colour centres electron colour centres Frenkel
defects - missing anion  hole colour centres
electron missing from its normal
location example - amethyst substitution of
Fe3 for Si4 in SiO2 (quartz) --gt hole colour
centre exposure to sunlight (UV light)
sufficient to cause colour change  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 4.
Colour centres    
electron traps
colour induced by irradiation may fade after a
few minutes
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 4.
Colour centres    
electron traps
in zircon, presence of U (impurity) can cause
permanent colour change may be removed (or
changed further) by heat treatment
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 5.
Mechanical causes (microstructure)   foreign
inclusions (solid or fluid) exsolution
lamellae twinning stacking
arrangement   colour related to diffraction
(scattering) of light by host and/or absorption
of light by inclusions  
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 5.
Mechanical causes (microstructure)   foreign
inclusions (solid or fluid) exsolution
lamellae twinning stacking arrangement  
e.g., labradorite iridescence caused by
submicroscopic exsolution lamellae
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 5.
Mechanical causes (microstructure)   foreign
inclusions (solid or fluid) exsolution
lamellae twinning stacking arrangement  
e.g., opal diffraction of light by spheres of
amorphous silica
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 6.
Luminescence absorbed energy (not necessarily
light) re-emitted as visible light  triboluminesce
nce - mechanical energy absorbed thermoluminescenc
e - absorbed heat energy trapped photoluminescence
- light energy trapped exposure to light
or some other form of radiation electrons
excited and trapped at higher energy levels
energy released upon heating electrons return to
ground state   type and amount of light
released on heating can be used to determine
duration of initial exposure i.e., this is a
dating method!!! 
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ERTH 2001Physical Properties Nesse, Ch.6
Factors contributing to colour in minerals 6.
Luminescence (cont'd) photoluminescence -   ?
absorbed (higher energy, shorter ?) different
from ? emitted (lower energy, longer ?)
(e.g., UV light --gt red/green fluorescence)
fluorescence (immediate) phosphorescence
(delayed)  
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ERTH 2001Physical Properties Nesse, Ch.6
Fluorescence
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ERTH 2001Physical Properties
Causes of colour in some minerals
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ERTH 2001 Strategies for Study Nesse, Ch. 10
1. Mineral Identification  a) separation  b)
hand sample ID  c) thin section ID  d) grain
mount ID  e) polished section ID  f) electron
microprobe  g) X-ray diffraction    2. Mineral
Association  never random! requires knowledge
of rocks and rock-forming processes (ERTH
2002)   3. Problems in Paradise  a)
alteration  b) mixtures
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ERTH 2001 Gemstones Study of Minerals Module,
GIS Lab

• What are gemstones? 
• What distinguishes gemstones from other
  minerals? 
•  
• What determines the value of gemstones? 
•  
• What common minerals also have gem varieties? 
• How are gemstones identified and validated? 
• How are gemstones cut and polished? 
•  
• How are gemstones formed and where are they
  mined?