Unveiling the Colors of the Mineral Rainbow
Embark on a journey through the intersection of chemistry, physics, and geology as we explore the captivating world of mineralogy and the science behind its vibrant hues.
By Mineralogical and Geological Museum, Harvard University
Created by Isabel Reyes and Kelly Thomas
Rhodochrosite (2023-12-19) by Kelly Thomas photoMineralogical and Geological Museum, Harvard University
Color minerals
The wide variety of colors found in minerals are produced in four different ways: crystal field transitions, color centers, molecular orbital/charge transfer-transitions, and physical impurities.
Wulfenite (2023-12-19) by Kelly Thomas photoMineralogical and Geological Museum, Harvard University
The following information is about the first three color-producing mechanisms, all of which involve electron action!
Crystal field transitions:
Transition metals (the middle elements of the periodic table) are common causes of color in minerals. One way that transition metals can cause color in minerals is through crystal field transitions.
Valence electrons of transition metals are excited by certain wavelengths of light. These wavelengths are absorbed in response to this excitation. Color is perceived as the unabsorbed (transmitted) wavelengths in the visible spectrum.
"Emerald green"
The color of green beryl, commonly emerald (but not always) is caused by small amounts of the transition metal chromium which is not intrinsic to the beryl’s chemical formula (Be3Al2(Si6O18)).
"Azure blue"
In contrast, azurite’s blue is caused by the transition metal copper, which is intrinsic to its formula (Cu3(CO3)2(OH)2).
Charge-transfer transitions:
Charge transfer-transitions happen when valence electrons no longer belong to one ion’s orbital, but transition between the orbitals of neighboring ions in a mineral. When one electron is shared between two ions this requires the ion to accommodate different valence states, a common example being Fe2+ - Fe3+.
This phenomenon is what causes the brilliant blues in sapphire, aquamarine, and kyanite (pictured).
Color Centers:
Color centers are the result of defects in the crystal structure which are often caused by radiation. There are two types of color centers. In one case, the color center may be the result of a missing ion. An excess electron will end up filling iSmithsoniten the hole left by the missing ion which results in an electron color center. This abnormality commonly causes color in mineral fluorite (CaF2) when there is a missing fluorine ion.
This abnormality commonly causes color in mineral fluorite (CaF2) when there is a missing fluorine ion.
Hole Color Center: Color due to the absence of an electron
This results from a cation filling in the space of another ion of higher positive charge (e.g., Al3+ fills in for Si4+ in smoky quartz (SiO2)). This charge imbalance coupled with radiation can then eject an electron from a pair in a nearby anion such as oxygen.
The remaining electron can now enter several excited states, which absorbs light energy and produces color accordingly. This process also results in the purple color of amethyst (also quartz (SiO2)) except for Fe3+ fills in for Si4+.
Sugilite (2023-12-19) by Isabel Reyes photoMineralogical and Geological Museum, Harvard University
...and the that is the story of the Mineral Rainbow.
Sources:
Color in Minerals (berkeley.edu)
Color in Minerals (fau.edu)
Sugilite Value, Price, and Jewelry Information - Gem Society
GMS - Color in Minerals (gamineral.org)
Colors in Minerals
Color | Definition, Perception, Types, & Facts | Britannica
Physics Tutorial: Light Absorption, Reflection, and Transmission
Colours of light — Science Learning Hub
Color in minerals
Color in minerals (minsocam.org)
Story and all pictures by By Isabel Reyes and Kelly Thomas.
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