Micro-World Gems & Gemology, Spring 2023, Vol. 59, No. 1

Apatite in Hackmanite

E. Billie Hughes

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Figure 1. A hackmanite with a faded purple color after storage in the dark (left), illuminated with long-wave UV light (center), and after removal from a few seconds of exposure to the long-wave UV light (right). The color has deepened after exposure to UV light. Photos by Ronnakorn Manorotkul.
 
Figure 1. A hackmanite with a faded purple color after storage in the dark (left), illuminated with long-wave UV light (center), and after removal from a few seconds of exposure to the long-wave UV light (right). The color has deepened after exposure to UV light. Photos by Ronnakorn Manorotkul.

Hackmanite, Na8Al6Si6O24(Cl2,S), is a variety of sodalite notable for its tenebrescence. When kept in darkness, samples can fade to pale purple and even gray or translucent to opaque white. When exposed to long-wave ultraviolet illumination, they can display a strong orange fluorescence reaction. After exposure to sunlight or artificial light with a UV component, samples can quickly develop a much stronger color. One hackmanite sample recently examined by the author (figure 1) showed this phenomenon.

Figure 2. Left: In darkfield illumination, a white apatite crystal is visible in the hackmanite. Right: After exposure to long-wave UV light for approximately one second, a dramatic change can be seen. The overall color of the hackmanite is now a more vivid shade of purple. Photomicrographs by E. Billie Hughes; field of view approximately 5 mm.
 
Figure 2. Left: In darkfield illumination, a white apatite crystal is visible in the hackmanite. Right: After exposure to long-wave UV light for approximately one second, a dramatic change can be seen. The overall color of the hackmanite is now a more vivid shade of purple. Photomicrographs by E. Billie Hughes; field of view approximately 5 mm.

Another hackmanite was submitted that displayed an interesting elongated white inclusion (figure 2, left) that was identified by micro-Raman as apatite. As we examined the sample in the microscope, we tried exposing it to a 6-watt long-wave UV light for approximately one second. Almost immediately, a change was observed. The hackmanite developed a more vibrant purple color that is clearly displayed in figure 2 (right).

E. Billie Hughes is a gemologist and cofounder of Lotus Gemology in Bangkok.