Lecture 5: Structural Defects and Color in Minerals

STRUCTURE DEFECTS

X-ray diffraction studies, TEM work and HRTEM techniques provide direct evidence that natural minerals contain imperfections at many scales. These structural errors may be at the unit cell scale or macroscopically visible. Such structural imperfections affect basic properties of crystalline materials such as strength, conductivity, mechanical deformation, and color.

Qualitatively, Fig. 3.34 shows point and line defects and a mosaic of domains separated by defective boundaries. In 3-dimensions, such boundaries would almost certainly be more accurately represented as plane defects. (O.H. 1)

Point Defects

• Schottky defect- cation or anion absent from its site in a structure. This needs to be compensated for by either other defects of the opposite charge or the addition of charged species (like an electron). (O.H. 2)
• Frenkel defect- absence of an ion from its proper site but its location nearby in an interstitial site. More common for a cation because of its usual smaller size.
• Impurity defect- addition of an extra ion into the structure

Line Defects

• concentrations of defects along linear features - dislocation
• Edge dislocation- a plane of atoms that terminates along a line. This kind of error provides a point of weakness for deformation and the defect can migrate through the structure as a slip plane.
• Screw dislocation- errors along a screw axis that normally is not present in the structure but may be a manifestation of pseudo symmetry or near symmetry inherently present in the space group that the mineral has crystallized within. Such spiral steps are important sites of crystal growth because they provide a good site for the addition of atoms.

Plane Defects

• 2-D zones along which slightly misoriented blocks are joined. The individual blocks may have near perfect short-range order but the crystal as a whole does not have perfect long-range order.
• Stacking fault- HCP interrupted by CCP for example- more when we talk about polymorphs and polytypes in the next few weeks.

COLOR

Geol 306 - Gems Lecture on Color

Our perception of color in minerals depends on the type of illumination, the mineral itself, and the human eye. Incandescent, fluorescent and sun light are the most common sources and all have different spectral signatures and thus minerals may look different.

Light: electromagnetic radiation that we can see; very small portion of the total range of possible wavelengths (energy). The shorter the wavelength, the higher the energy. Visible runs from about 375 to 740 nm. We perceive different wavelengths as different colors. (O.H. 1)

The human eye: light sensing portion composed of rods and cones. At low light levels we see in shades of gray as detected by the rods which only contain one pigment. At higher levels of illumination, the cones kick in. Each cone contains one of 3 fundamental pigments with maximum absorption in the red, blue or green. Our brain integrates these signals and arrives at an average color. Our eye is not equally sensitive to each of these colors- it is most sensitive to green wavelengths. (This coincides with the peak solar radiation and is therefore an evolutionary development.)

Light and matter: reflection, refraction, scattering, diffraction, absorbed or transmitted. Part of the energy of the absorbed light can then be emitted as fluorescence. We will look at all of these mechanisms during the next few weeks.

Main categories of interactions leading to color

• dispersed metal ions - perhaps easiest to understand: single atom
• charge-transfer phenomena: small groups of atoms
• color centers: small groups of atoms
• band theory: large clusters of atoms
• physical optics - scattering, diffraction: large structures

Dispersed metal ions

• an ion absorbs light of a particular energy when that energy matches the amount needed to kick an outer shell electron to a higher energy level. This wavelength is thus preferentially removed from the spectrum and the residual is what we see. The electron normally returns immediately to its ground state but usually does so by giving off some of its energy as heat and the rest as electromagnetic radiation which may or may not lie within our ability to see. If red light was absorbed (blue mineral), then any energy loss will shift the emitted light into the infrared and it will be invisible. In a red mineral (blue absorption), red fluorescence is common and this processes enhances the perceived red color of the mineral.
• the identity of the ion, the valence state, the nature of the neighboring ions and the coordination of the site all effect the absorption capacity of a dispersed metal ion in a mineral. Pleochroism in minerals is caused by different absorption characteristics in different crystallographic directions.

  Ex.: rubies (red corundum) and emeralds (green beryl) both owe their colors to Cr+3 in octahedral coordination. The color difference is in the exact position of the absorption band and this hinges on the details of the coordination. (shorter mean Cr-O bond in corundum)

  (Summary O.H. 3)

Charge-Transfer

• when an electron jumps from one atom to another (O.H. 4)
  • - oxygen to metal ion
  • - cation-cation intervalence charge transfer (Fe+2 - Fe+3)
• Aquamarine
• Deep blue of sapphire

Color Centers

• commonly a result of irradiation by natural or synthetic means
  • - radiation can change the oxidation state of metal ions
  • - interact with defects in the crystal i.e. missing atoms or additional interstitial atoms
  • - the removed electron can find a home in one of the defects
  Ex: smoky quartz and amethyst
• smoky quartz is formed by removing an electron from an Al ion that had substituted for a Si. Heating allows the electrons to come home and the smoky color will disappear. (O.H. 1)
• amethyst is formed when Fe+3 substituting for Si is ionized to Fe+4 by radiation. The deep purple color is due to O-2 -> Fe+4 charge transfer which is centered in the yellow-green portion of the spectrum.
• sodalite (hackmanite): electron in Cl- hole in a Na tetrahedron (Demo) (Summary O.H. 4)

Band Theory

• If the band gap is greater than the maximum energy of visible light then no transitions occur, no visible absorbance occurs and the mineral is transparent. Such minerals are inherently electrical insulators. (Slide)
• If the gap is less than the energy of violet light, the high energy end of the spectrum tends to be absorbed leaving the red- this is the cause of the red color of cinnabar.
• If the gap is less than all the energies represented by visible light then the whole spectrum is absorbed. The mineral commonly appears black and opaque. All metals have this property (or no gap at all). Metals appear shiny (metallic luster) because the electrons quickly return to their original energy level, emitting the same energy they absorbed. In cases where some wavelengths are absorbed emitted more efficiently than others, a color is produced (gold vs platinum).
• Nitrogen in diamonds -> yellow
• Boron in diamonds -> blue

Physical Phenomena

• interference caused by thin films - pearls
• diffraction: caused by regular 'layers' on the scale of the wavelength of light - opal
  • 250nm spheres diffract red light
  • spheres down to 140nm diffract the other colors
• scattering: (Rayleigh) from particles smaller than the wavelength of light: reason the sky is blue during the day and red at sunrise and sunset (blue is scattered more efficiently than red)
• opalescence caused by spheres too small to diffract

(Summary O.H. 4)