In Lecture 3.3, we took a look at some popular silicate gemstones. Here, we’ll look at how the colorless “parent” compounds turn into colored gemstones with the help of point defects. For each of the colored gemstones below (images from the Smithsonian National Museum of Natural History), write the point defect(s) responsible for creating the color in Kröger-Vink notation.
Parent compound: Corundum (Al2O3)
(a) Ruby https://naturalhistory.si.edu/explore/collections/geogallery/10002910: Red color originates from Cr3+ substitution on the Al3+ site
(b) Sapphire https://naturalhistory.si.edu/explore/collections/geogallery/10002691: Blue color originates from Fe2+ and Ti4+ impurities that both substitute on the Al3+ site (write each substitution on the Al3+ site separately in Kröger-Vink notation)
Parent compound: 𝛼-Quartz (SiO2)
(c) Amethyst https://naturalhistory.si.edu/explore/collections/geogallery/10002933: Purple color originates from Fe3+ substituting Si4+, with formation of an alkali metal interstitial for charge balance (write both defects separately in Kröger-Vink notation)
(d) Pink quartz https://naturalhistory.si.edu/explore/collections/geogallery/10025996: Pink color from Al3+ and P5+ impurities that both substitute on the Si4+ site. (write each substitution on the Si4+ site separately in Kröger-Vink notation) Parent compound: Beryl = Be3Al2(Si6O18)
(d) Aquamarine https://naturalhistory.si.edu/explore/collections/geogallery/10002729: Light blue color from isovalent substitution of Fe2+ and Fe3+ on the appropriate metal cation site (write each substitution separately in Kröger-Vink notation)
(e) Emerald https://naturalhistory.si.edu/explore/collections/geogallery/10026501: Green color originates from substitution of Cr3+ onto Al3+ sites, and yellowish-green is from V3+ substitution onto Al3+ sites (write each substitution separately in Kröger-Vink notation)
