Micro-World Gems & Gemology, Spring 2019, Vol. 55, No. 1

Quarterly Crystal: Orpiment in Barite

Bright yellow inclusions inside barite crystals.
Figure 1. Numerous bright yellow crystals dominate the interior of this 74.31 ct barite cluster from Nevada. Photo by Nathan Renfro.

The 74.31 ct crystal cluster of transparent to translucent colorless barite columns shown in figure 1 measures 37.89 × 20.83 × 15.27 mm. It plays host to numerous small yellow crystals and radial sprays, which together give the cluster a light yellow color. The barite specimen is from the Regent mine in the Rawhide District of Mineral County, Nevada. The Regent mine was known to produce barite crystals with yellow orpiment inclusions in the mid-1990s. The pure yellow bodycolor of these numerous inclusions suggested they might be orpiment. Laser Raman microspectrometry was able to confirm our suspicions. Focused energy-dispersive X-ray fluorescence (EDXRF) analysis was used in an attempt to examine the chemical makeup of the inclusions, since orpiment contains arsenic as a main constituent. The EDXRF analysis showed the presence of arsenic. Based on chemistry, we determined that the yellow inclusions shown in figure 2 are indeed the arsenic sulfide mineral orpiment.

Inclusions identified as orpiment, a yellow arsenic sulfide.
Figure 2. A combination of Raman and EDXRF analysis identified the inclusions as orpiment, a yellow arsenic sulfide. Photomicrograph by Nathan Renfro; field of view 3.83 mm.

John I. Koivula is analytical microscopist at GIA in Carlsbad, California.