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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: 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: 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• What color is this mineral?

Elbaite

• From Paraiba, Brazil8

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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

• 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)

polarized vertically unpolarized polarized horizontally

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Rhodonite

• Mn2+ usually results in a pink color in octahedral sites.

• Rhodonite from Minas Gerais, Brazil Rhodocrosite from Colorado

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Tetrahedral vs. Octahedral

• Co2+ in cobaltian calcite from the Kakanda Mine, Zaire, causes a typical reddish color, on an octahedral site

• In tetrahedral sites, Co2+ causes blue color such is found in some spinels from Baffin Island.

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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 F- ion

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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

Blue Halite

• 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 color

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• Blue Halite from Germany

Purple Halite

• Carlsbad, New Mexico

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