11 Color in Minerals GLY 4200 Fall, 2014

22 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

33 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

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

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

66 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”

77 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

99 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

10 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

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

12 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

13 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 13

14 Absorption Spectra of Fe Minerals

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

16 Emerald and Ruby Photos

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

18 Tanzanite Tanzanite (a variety of zoisite, Ca 2 Al 3 Si 3 O 12 (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

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

20 Tetrahedral vs. Octahedral Co 2+ in cobaltian calcite from the Kakanda Mine, Zaire, causes a typical reddish color, on an octahedral site In tetrahedral sites, Co 2+ causes blue color such is found in some spinels from Baffin Island.

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

22 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

23 Sapphire

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

25 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.

26 Fluorite Color Center An electron replaces an F - ion

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

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

29 Smoky Quartz and Amythyst

30 Amber Calcite Amber Calcite from the Tri-state district, USA, with amber color from natural irradiation next to a colorless calcite cleavage rhomb.

31 Quartz, variety Chrysoprase Green color usually due to chlorite impurities, sometimes to admixture of nickel minerals

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

33 Rose Quartz Color often due to microscopic rutile needles

34 Blue Quartz

35 Rutilated Quartz

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

37 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 38 Blue Halite from Germany

Purple Halite Carlsbad, New Mexico 39