VARIATIONS IN VIOLET: CORRELATING TRACE ELEMENTS IN AMETHYST WITH COLOR DENSITY

Amethyst is a variety of quartz recognized by its purple hue. Often, the purple color intensity is highly variable across an amethyst crystal. This study probes this variability, looking to determine why it occurs. Quantifying the color change in these gems is important for understanding the evolution in their environment during growth. The purple color is caused by the presence of Fe impurities within the crystal that have undergone irradiation. Fe incorporates into the quartz crystal lattice during periods of growth, as silica transport occurs via local hydrothermal processes. The amethyst crystal studied here likely formed within a vug, an internal cavity enclosed by rock filled with mineral materials such as silica. Proximity to a natural source of radiation causes the Fe to expel an electron from its orbital, specifically Fe³⁺ becoming Fe⁴⁺. Charge transfer between Fe⁴⁺ and O^2-, also present in the quartz crystal lattice, then causes the purple hue. Laser ablation ICP-MS measurements were taken along two directions of an amethyst where the crystal changes from colorless to deep purple, giving us in situ concentration values for several elements along with Fe. Digital imaging techniques were used on a high-quality picture of the sample to create a graphical representation of the change in color density over the crystal's length along the data lines created by laser ablation. We find that the concentration of Fe does not directly correlate with the purple color. No element appears to correlate best, with many exhibiting complex profiles. This may indicate a change in the amount of radiation in the environment as the crystal was growing or in the available Fe. Other elements, such as Ga and Al, may help elucidate how the growth environment was evolving.