Transcript Introduction to Environmental Geochemistry

Color in Minerals
GLY 4200
Fall, 2012
1
Color Sources
• Minerals may be naturally colored for a
variety of reasons - among these are:
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Selective absorption
Crystal Field Transitions
Charge Transfer (Molecular Orbital) Transitions
Color Center Transitions
Dispersion
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Characteristic Color
• Color is characteristic for some minerals, in
which case it is idiochromatic and thus may
serve as an aid to identification
• Color is often quite variable, which is called
allochromatic, and thus may contribute to
misidentification
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Visible Light
• Visible light, as perceived by the human
eye, lies between approximately 400 to 700
nanometers
4
Interaction of Light with a Surface
• Light striking the surface of a mineral may
be:
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Transmitted
Refracted
Absorbed
Reflected
Scattered
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Absorption
• Color results from the absorption of some
wavelengths of light, with the remainder
being transmitted
• Our eye blends the transmitted colors into a
single “color”
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Mineral Spectrum
• Spectrum of elbaite, a tourmaline group mineral
• Note that absorbance is different in different
directions
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• What color is this mineral?
Crystal Field Splitting
• Partially filled 3d (or, much less common,
4d or 5d) allow transitions between the split
d orbitals found in crystals
• The electronic configuration for the 3d
orbitals is:
 1s2 2s2 2p6 3s2 3p6 3d10-n 4s1-2, where n=1-9
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Octahedral Splitting
• Splitting of the five d
orbitals in an octahedral
environment
• Three orbitals are
lowered in energy, two
are raised
• Note that the “center
position” of the orbitals
is unchanged
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Tetrahedral Splitting
• Tetrahedral splitting
has two orbitals
lowered in energy,
while three are raised
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Square Planar Splitting
• a) octahedral splitting
• b) tetragonal
elongation splits the
degenerate orbitals
• c) total removal of
ions along z axis
produces a square
planar environment
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Factors Influencing Crystal Field Splitting
• Crystal Field Splitting (Δ) is influenced by:
 Oxidation state of metal cation – Δ increases
about 50% when oxidation state increases one
unit
 Nature of the metal ion – Δ3d < Δ4d < Δ5d
 About 50% from Co to Rh, and 25% from Rh to Ir
 Number and geometry of ligands
 Δo is about 50% larger than Δt
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Absorption Spectra of Fe Minerals
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Emerald and
Ruby Spectra
• The field around Cr3+ in ruby is stronger than in beryl
• Peaks in emerald are at lower energy
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Emerald and Ruby Photos
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Charge Transfer
• Delocalized electrons hop between adjacent
cations
• Transition shown produces blue color in minerals
such as kyanite, glaucophane, crocidolite, and
sapphire
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Sapphire Charge Transfer
• Sapphire is Al2O3, but often contains iron
and titanium impurities
• The transition shown produces the deep
blue color of gem sapphire
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Sapphire
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Sapphire
Spectrum
• Sapphires transmit in the blue part of the spectrum
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Fluorite Color Center
• An electron
replaces an Fion
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Fluorite
• Grape purple
fluorite, Queen
Ann Claim,
Bingham, NM.
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Smoky Quartz
• Replacement of Si4+
with Al3+ and H+
produces a smoky
color
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Smoky Quartz and Amythyst
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Quartz, variety Chrysoprase
• Green color usually
due to chlorite
impurities, sometimes
to admixture of nickel
minerals
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Milky Quartz
• Milky quartz
has inclusions
of small
amounts of
water
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Rose Quartz
• Color often due
to microscopic
rutile needles
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Blue Quartz
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Rutilated Quartz
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Quartz, variety Jasper
• Color due to
admixture of hematite
in quartz
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