Transcript Introduction to Environmental Geochemistry

Color in Minerals
GLY 4200
Fall, 2014
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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
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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?
Elbaite
• From Paraiba, Brazil
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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 emerald
• Peaks in emerald are at lower energy
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Emerald and Ruby Photos
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Grossular Garnet
• V3+ in grossular garnet (tsavorite
variety from Kenya)
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Tanzanite
polarized vertically
•
unpolarized
polarized horizontally
Tanzanite (a variety of zoisite, Ca2Al3Si3O12(OH), that contains vanadium in
multiple oxidation states) shows remarkable pleochroism (color change with
viewing direction and polarization of light)
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Rhodonite
• Rhodonite from
Minas Gerais, Brazil
Rhodocrosite from Colorado
• Mn2+ usually results in a pink color in octahedral sites.
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Tetrahedral vs. Octahedral
• In tetrahedral sites, Co2+ causes
blue color such is found in some
spinels from Baffin Island.
• Co2+ in cobaltian calcite from
the Kakanda Mine, Zaire, causes
a typical reddish color, on an
octahedral site
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Intervalence Charge Transfer (IVCT)
• 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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Rockbridgeite (Fe Phosphate)
• The iron phosphate, rockbridgeite, is an example of a mineral which,
by stoichiometry, contains both Fe2+ and Fe3+
• In thin section, the dark green color caused by the IVCT interaction is
apparent when the direction of the linerally polarized light is in the
direction of the chains of Fe atoms.
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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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Amber Calcite
• Amber Calcite
from the Tri-state
district, USA, with
amber color from
natural irradiation
next to a colorless
calcite cleavage
rhomb.
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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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Pink Halite
• Pink Halite, Searles Lake, CA
• Color possibly due to impurity silt
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Blue Halite
• Blue Halite from Germany
• Initially, if halite (common salt) is exposed to gamma
radiation, it turns amber because of F-centers
• They are mostly electrons trapped at sites of missing Cl- ions
• In time the electrons migrate to Na+ ions and reduce it to Na
metal
• Atoms of Na metal, in turn, migrate to form colloidal sized
aggregrates of sodium metal, and are the cause of the blue
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color
Purple Halite
• Carlsbad, New Mexico
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