Gem News International Gems & Gemology, Winter 2020, Vol. 56, No. 4

Update on Inclusions in Emeralds from Davdar, China

Di Cui, Zongting Liao, Lijian Qi, and Zhengyu Zhou

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Black inclusions in Davdar emerald identified as magnetite.
 
 
Figure 1. Left: A Davdar emerald hosted numerous black mineral inclusions identified by laser Raman microspectrometry as magnetite. Right: The Raman spectrum of the inclusions is shown along with the Raman reference spectrum for magnetite in the RRUFF database. Spectra are offset for clarity. Inset photomicrograph by Di Cui.

We recently had the opportunity to examine emeralds containing unknown solid inclusions and numerous multiphase inclusions reportedly from Davdar, China. Three green transparent rough samples were acquired from the former mine owner Guimin Wong. They weighed 0.55, 0.45, and 0.50 ct, respectively, with a diameter from 4.0 to 5.0 mm, a refractive index of 1.578 ± 0.002–1.584 ± 0.001, and SG ranging from 2.64 to 2.76. They were inert to both long- and short-wave UV radiation. Inclusion features of Davdar emeralds have been reported previously in the literature.

Very few mineral species have been identified as inclusions in this emerald—these include feldspar (plagioclase) (S. Saeseaw et al., “Three-phase inclusions in emerald and their impact on origin determination,” Summer 2014 G&G, pp. 114–132; S. Saeseaw et al., “Geographic origin determination of emerald,” Winter 2019 G&G, pp. 614–646) and to a lesser extent tourmaline, mica, scheelite, and fluorite (D. Marshall, “Geological work,” InColor, Spring 2009, p. 29). Black rounded crystals were observed in some of the Davdar emeralds we studied. Previous studies noted the presence of black minerals as well but did not identify them (S. Saeseaw et al., 2014, 2019). We revealed the black mineral to be magnetite via Raman microspectrometry (figure 1). This is potentially the first time magnetite has been identified in Davdar emeralds, though it has been identified in emeralds from other regions (referenced above). Magnetite as a black iron oxide inclusion is commonly seen in emeralds from Zambian and Brazilian localities and other schist-hosted origins (again, see Saeseaw et al., 2019).

Multiphase inclusions occur commonly in Davdar emeralds with solid phases (mainly halides such as halite assumed by D. Marshall et al., “Conditions for emerald formation at Davdar, China: Fluid inclusion, trace element and stable isotope studies,” Mineralogical Magazine, Vol. 76, No. 1, 2012, pp. 213–226). Since previous studies reported that some birefringent phases did not dissolve during heating (again, see Marshall et al., 2012), we hoped to learn the solid phase identity in fluid inclusions. Comprehensive microscopic and Raman spectroscopic analyses indicated the presence of dolomite and likely calcite daughter inclusions (figures 2 and 3). As the solubility of a carbonate such as calcite seems to show an inverse solubility with respect to temperature (see B. Coto et al., “Effects in the solubility of CaCO3: Experimental study and model description,” Fluid Phase Equilibria, Vol. 324, 2012, pp. 1–7), the first identified dolomite daughter crystals could be attributable to the phases that were insoluble with increasing temperature and added more constraints to fluid inclusions in Davdar emeralds.

One solid phase inclusion was identified as dolomite.
 
 
Figure 2. Left: Solid-rich fluid inclusions in Davdar emerald. One of the solid phases was determined to be dolomite. Right: The corresponding spectrum (red) showed peaks from both the host beryl (black) and the reference spectrum for dolomite in the RRUFF database (blue). Photomicrograph by Di Cui.
Raman identified calcite and dolomite in the host beryl.
 
 
Figure 3. Two daughter crystals in separate inclusions were identified as likely calcite based on comparison with Raman reference spectra from the RRUFF database. The top and bottom left images show the spectra of the two daughter inclusions (red), the host beryl (black), and the RRUFF calcite reference spectrum (green). The dominant calcite peak at 1087 cm–1 matches with weak peaks in both daughter crystals. The top and bottom right images show the weak 1087 cm–1 peak corresponding to calcite under magnification. Photomicrographs by Di Cui.

As far as we are aware, the carbonate phase could simply refer to the local geologic background and the host rock lithology, which mainly consisted of sandstone and dolomitic limestone. Study of the inclusions showed interesting results due to the genetic environment of the Davdar emerald mine. In addition to the daughter chloride such as halite, the identified dolomite mineral defined the complex multiphase assemblage for the Davdar fluid inclusion population.

Di Cui is a PhD student of mineralogy, Zongting Liao and Lijian Qi are professors of geology and gemology, and Zhengyu Zhou is an associate professor of gemology, at the School of Ocean and Earth Sciences and the Laboratory of Gem and Technological Materials at Tongji University in Shanghai.