Micro-World Gems & Gemology, Summer 2020, Vol. 56, No. 2

Fluorophlogopite in Burmese Spinel

Richard W. Hughes

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Surface-breaking crystal seen in red spinel.
Figure 1. This crystal broke the surface of a dark red spinel believed to be from Mogok. It had a lower luster than the spinel, and undercutting indicated a lower hardness. Photomicrograph by Richard W. Hughes, diffuse overhead illumination; field of view 2.5 mm.

Lotus Gemology in Bangkok received a large parcel of dark red spinels for identification. Their colors, inclusion features, and trace element chemistry suggested that they originated from Myanmar’s Mogok Stone Tract.

Fluorophlogopite crystal identified by Micro-Raman analysis.
Figure 2. In darkfield illumination, cleavage was visible inside the crystal. Micro-Raman analysis showed it to be fluorophlogopite. Photomicrograph by Richard W. Hughes, darkfield illumination; field of view 2.5 mm.

In one faceted specimen, a crystal was found breaking the surface (figure 1). The crystal’s surface luster suggested a refractive index below that of the host spinel, and undercutting suggested it was also significantly lower in hardness. The inclusion’s interior also displayed signs of cleavage (figure 2). Thinking it was possibly a carbonate, we proceeded to do micro-Raman analysis on the crystal. To our surprise, the result was fluorophlogopite.

Fluorophlogopite, KMg3(AlSi3O10)(F,OH)2, is a species in the mica group and a member of the trioctahedral mica subgroup. To the best of our knowledge, this is the first reported instance of this species in spinel. Nor are we aware of it having been found included in any gem mineral to date.

Richard W. Hughes is an award-winning gemologist and the co-founder of Lotus Gemology in Bangkok.