EASA Mineralogy Homepage

Week 8: March 6 and 8

Each wavelength of light (l) corresponds to some energy (E). The relationship is given by: E = hc/l, where h is Planck's constant and c is the speed of light. (Shorter wavelengths correspond to higher energies.)
Electrons can have different energy levels.
The key: If the energy difference between two energy levels that an electron may occupy corresponds to the energy of a certain wavelength of light, then that wavelength will be absorbed. (However, note that if the electron drops down to the original energy level, the same wavelength will be re-emitted and no net absorption occurs. We need at least three energy levels for the electron to occupy so that the wavelength absorbed during the jump from level 1 to 3 is not equal to the wavelength emitted during the drop from 3 to 2 and 2 to 1. In this case, a net absorption of a particular wavlength occurs. If the emitted wavelengths are in the visible light range, then fluoresence of that color occurs.)

Crystal Field Transitions - electron transitions between partly filled 3d orbitals of transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni), called the "chromophores".

in an isolated state, the five 3d orbitals have the same energy levels; no absorption occurs
But in a crystal lattice, the coordination of the transition element causes the "splitting" of the energy levels leading to different energy levels between the various 3d orbitals. (The uneven distributiojn of the coordinating anion's electron clouds and the 3d orbitals causes some of the 3d orbitals to have higher energies.)
example 1: In olivine, (Mg, Fe)₂[SiO₄], the Fe⁺² is in octahedral coordination with O, which leads to diffrences in energy levels of the 3d orbitals of Fe that absorb red, and net yellow green color is observed.
example 2: In almandine garnet, Fe₃Al₂[Si₃O₁₂], the Fe⁺² is in cubic coordination with O. This leads to energy level differences that lead to a net emission of a red color.
Other examples: Cr⁺³ in ruby causes red color; Cr⁺³ in emerald causes green color

Molecular orbital transitions ("Charge transfer transitions") - electrons "jumping" from one ion to another absorb certain wavelengths.

example 1: electrons jumping between Fe⁺² and Fe⁺³ cause the blue color in the mineral glaucophane.
example 2: electron transfer between (Fe⁺², Fe⁺³) and (Ti⁺³, Ti⁺⁴) causes the blue color of sapphire.

example 1: In fluorite, CaF₂, there may be a shottky defect where a F ion is missing. In place of the F there may be an electron to maintain charge balance. This electron can have several energy levels and the transitions of the electron between the levels causes the purple color of fluorite.
example 2: In amethyst, a variety of quartz (SiO₂), an Fe⁺³ may be present. Irradiation may knock off one of the electrons from one of the oxygen ions coordinated with the Fe ion. The ramaining valence electron again may have several E states, and transitions of the electron between the E levels causes a net absorption of certain wavelength and the purple color of amethyst.

Note: In all the above causes, there is some condition that leads to multiple vacant energy levels that an electron may occupy. Transitions of the electron from lower to higher energy levels requires absorption of certain energies (wavelengths) of light. Then when the electron(s) drops back to a lower energy level, different wavelengths are emmitted (that may or may not be visible). The result is a net absorption of a certain color (wavelength of light). What we see is white light minus that absorbed color (with the addition of possible fluoresence of other colors).

Color may also be caused by interference of light - as light is reflected off of a surface. With layers that have a thickness on the order of the wavelength off light one can get constructive and destructive interference of certain wavelengths of light as it is reflected off the upper and lower interface of the layer (depending on the thickness of the layers and the angle that the light strikes the surface). This is the cause of color in:

some plagioclases - some plagioclases that have fine exsolution lamellae (only those formed in really slowly cooled plutons) exhibit a play of colors known as "labradorescence". The thickness of the exsolution lamellae is on the order of the wavelength of light.
Opal - ("opalesence") opal consists of layers of spheres of silica. The layers have the appropriate thickness for interference of light.
(Oil slicks - the layer of oil on the water is the right thickness.)
(Color in soap bubbles - the thickness of the soap bubble wall is the right thickness.)

metalic luster - most light is reflected; hence mineral is opaque.
non-metallic luste - vitreous, resinous, pearly, greasy, silky, adamantine, earthy, dull

STREAK - the color of the powdered mineral. The streak is not necessarily the same as the unpowdered mineral. This is because the streak tends to emphasize the reflected light color, rather than the transmitted light color.
FLUORESCENCE - emission of visible light during exposure to eletromagnetic radiation such as ultraviolet light or x-rays. Electrons absorb the incident radiation by jumping to higher energy levels then emit visible radiation as they drop down (in steps) to the ground state again.
PIEZOELECTRCITY - the property of some minerals to generate a voltage when they are deformed (and alternatively, to deform if a voltage is applied to them). Exhibited only by minerals that lack a center of symmetry. An example is quartz.
MAGNETIC PROPERTIES - Caused by the presence of elements with orbitals with unpaired electrons; the magnetic field produced by the spin of the lone electron is not cancelled by the opposite spin of another electron.

diamagnetic - essentially non-magnetic; all orbitals have paired electrons. Examples - quartz feldspars
paramagnetic - can be temporarily magnetized in the presence of an external magnetic field. The magnetic dipoles are normally randomly oriented. (Examples - olivine, pyroxenes, micas)
ferromagnetic - all dipoles are parallel; magnetism is retained (but lost above the Curie temperature). (Example - metallic iron)
ferrimagnetic - dipoles are antiparallel but unequal giving a net magnetism that is permanent below the Curie point. (Example - magnetite)

(END OF PHYSICAL PROPERTIES)