Pargasite is a calcareous amphibole with the chemical formula NaCa₂(Mg₄ Al)(Si₆ Al₂)O₂₂(OH)₂, which can appear in different shades of green depending on the Fe content. Pargasite usually occurs in corundum as prisms, needles, and platelets.

Recently, the authors examined several Madagascar rubies, and we observed a prismatic, transparent crystal displaying the monoclinic crystal habit and striae on the surface in one sample, which was identified by Raman spectroscopy as pargasite (figure 1). Based on the complete prismatic morphology and surface characteristics of the pargasite, we reckon that this crystal is protogenetic that existed in the growth environment before the formation of the ruby host.

The geological environment of ruby formation in Madagascar is extremely complicated, including both magmatic and metamorphic. Occurrences of Madagascar ruby from the metamorphic origin are found in metamorphosed M-UMR (mafic-ultramafic rocks), which is also called amphibolite-type. The most common assemblage of these deposits is corundum, anorthite, amphibole (gedrite, pargasite), and margarite, which formed under amphibolite and granulite facies conditions (Giuliani et al., 2020).

Pargasite occurs as a common ruby inclusion and has been found in rubies from the Mogok and Hunza deposits in Burma, the Montepuez deposit in Mozambique, and Madagascar (Kane et al., 2005; Groat et al., 2019).

Figure 1. Pargasite inclusions in a Madagascar ruby. Photograph by Huixin Zhao; field of view 1.46 mm. Darkfield illumination.

References

Giuliani G., Groat L. A., Fallick A. E., Pignatelli I., Pardieu V. (2020) Ruby Deposits: A Review and Geological Classification. Minerals, Vol. 7.

Kane R. E., Boehm E. W., Overlin S. D., Dirlam D. M., Koivula J. I., Smith C. P. (2005) A Gemological Pioneer: Dr. Edward j. Gübelin. Gems & Gemology, Vol. 41, No. 4, pp. 298-327.

Groat L. A., Giuliani G., Sundberg J. S., Sun Z. Y., Renfro N. D., Palke A. C. (2019) A Review of Analytical Methods Used in Geographic Origin Determination of Gemstones. Gems & Gemology, Vol. 55, No. 4, pp. 512-535.