Color in Minerals

1
Color in Minerals

• GLY 4200
• Fall, 2009

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
• 1s2 2s2 2p6 3s2 3p6 3d10-n 4s1-2, where n1-9

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

12
Absorption Spectra of Fe Minerals
13
Emerald and Ruby Spectra

• The field around Cr3 in ruby is stronger than in
  beryl
• Peaks in emerald are at lower energy

14
Emerald and Ruby Photos
15
Charge Transfer

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

16
Sapphire Charge Transfer

• Sapphire is Al2O3, but often contains iron and
  titanium impurities
• The transition shown produces the deep blue color
  of gem sapphire

17
Sapphire
18
Sapphire Spectrum

• Sapphires transmit in the blue part of the
  spectrum

19
Fluorite Color Center

• An electron replaces an F- ion

20
Fluorite

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

21
Smoky Quartz

• Replacement of Si4 with Al3 and H produces a
  smoky color

22
Smoky Quartz and Amythyst
23
Quartz, variety Chrysoprase

• Green color usually due to chlorite impurities,
  sometimes to admixture of nickel minerals

24
Milky Quartz

• Milky quartz has inclusions of small amounts of
  water

25
Rose Quartz

• Color often due to microscopic rutile needles

26
Blue Quartz
27
Rutilated Quartz
28
Quartz, variety Jasper

• Color due to admixture of hematite in quartz