Quarterly Crystal: Geocronite in Fluorite
A tight crystal cluster of bright apple-green fluorite and opaque silvery gray geocronite, Pb14(Sb,As)6S23, was recently examined for this edition of Quarterly Crystal. The specimen shown in figure 1 measured 29.60 × 23.84 × 18.35 mm, and the fluorite portion played host to several small, well-formed monoclinic crystals, all situated near the surface. The crystals appeared to be geocronite, a very rare sulfide containing lead, antimony, and arsenic.
The specimen was obtained from Luciana Barbosa of Barbosa Minerals (Asheville, North Carolina). It came from a recent discovery at the Milpo mine in the Atacocha mining district of Pasco Province, Peru. The Milpo mine is known to produce fluorite crystals with inclusions of various sulfides. Since the fluorite and geocronite are intimately intergrown, the opaque silvery gray bodycolor of the inclusions in the fluorite, together with their monoclinic appearance shown in figure 2, strongly suggested geocronite.
The next logical step was laser Raman microspectrometry. Using this technique, we conclusively identified the inclusions as geocronite, thereby confirming our suspicions. It should be noted that, to the authors’ knowledge, this is the first time geocronite has been identified as an inclusion in any mineral.
