Color in Minerals GLY 4200 Fall, 2014

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

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

Visible Light • Visible light, as perceived by the human eye, lies between approximately 400 to 700 nanometers

Interaction of Light with a Surface • Light striking the surface of a mineral may be:   Transmitted Refracted   Absorbed Reflected  Scattered

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”

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, Brazil 8

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:  1s 2 2s 2 2p 6 3s 2 3p 6 3d 10-n 4s 1-2 , where n=1-9

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

Tetrahedral Splitting • Tetrahedral splitting has two orbitals lowered in energy, while three are raised

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

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 – Δ 3d  < Δ 4d < Δ 5d About 50% from Co to Rh, and 25% from Rh to Ir  Number and geometry of ligands  Δ o is about 50% larger than Δ t

Absorption Spectra of Fe Minerals

Emerald and Ruby Spectra • The field around Cr 3+ in ruby is stronger than in emerald • Peaks in emerald are at lower energy

Emerald and Ruby Photos

Grossular Garnet • V 3+ in grossular garnet (tsavorite variety from Kenya) 17

Tanzanite polarized vertically unpolarized polarized horizontally • Tanzanite (a variety of zoisite, Ca 2 Al 3 Si 3 O viewing direction and polarization of light) 12 (OH), that contains vanadium in multiple oxidation states) shows remarkable pleochroism (color change with 18

Rhodonite • Rhodonite from Minas Gerais, Brazil Rhodocrosite from Colorado • Mn 2+ usually results in a pink color in octahedral sites. 19

Tetrahedral vs. Octahedral • In tetrahedral sites, Co 2+ blue color such is found in some spinels from Baffin Island.

causes • Co 2+ in cobaltian calcite from the Kakanda Mine, Zaire, causes a typical reddish color, on an octahedral site 20

Intervalence Charge Transfer (IVCT) • Delocalized electrons hop between adjacent cations • Transition shown produces blue color in minerals such as kyanite, glaucophane, crocidolite, and sapphire

Sapphire Charge Transfer • Sapphire is Al 2 O 3 , but often contains iron and titanium impurities • The transition shown produces the deep blue color of gem sapphire

Sapphire

Sapphire Spectrum • Sapphires transmit in the blue part of the spectrum

Rockbridgeite (Fe Phosphate) • The iron phosphate, rockbridgeite, is an example of a mineral which, by stoichiometry, contains both Fe 2+ and Fe 3+ • 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

Fluorite • Grape purple fluorite, Queen Ann Claim, Bingham, NM.

Smoky Quartz • Replacement of Si 4+ with Al 3+ and H + produces a smoky color 28

Smoky Quartz and Amythyst

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

Milky Quartz • Milky quartz has inclusions of small amounts of water

Rose Quartz • Color often due to microscopic rutile needles

Blue Quartz

Rutilated Quartz

Quartz, variety Jasper • Color due to admixture of hematite in quartz

Pink Halite • Pink Halite, Searles Lake, CA • Color possibly due to impurity silt 37

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 color 38

Purple Halite • Carlsbad, New Mexico 39