1 Color in Minerals GLY 4200 Fall, 2012

2 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

3 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

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

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

6 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”

7 Mineral Spectrum Spectrum of elbaite, a tourmaline group mineral Note that absorbance is different in different directions What color is this mineral?

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

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

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

11 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 12

13 Absorption Spectra of Fe Minerals

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

15 Emerald and Ruby Photos

16 Charge Transfer Delocalized electrons hop between adjacent cations Transition shown produces blue color in minerals such as kyanite, glaucophane, crocidolite, and sapphire

17 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

18 Sapphire

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

20 Fluorite Color Center An electron replaces an F - ion

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

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

23 Smoky Quartz and Amythyst

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

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

26 Rose Quartz Color often due to microscopic rutile needles

27 Blue Quartz

28 Rutilated Quartz

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