Growth heterogeneity and zonation characteristics of emeralds from Davdar, Xinjiang

doi:10.13745/j.esf.sf.2021.11.9

Abstract

Abstract:

The chemical composition, microstructure and element-mapping characteristics of the color-zoned emeralds from Davdar, Xinjiang are investigated using gem microscope, polarized microscope, electron microprobe (EPMA) and X-ray diffraction techniques. The microstructure reveals reliable evidence of periodic growth, which shows the emerald core is fragmented and corroded and the rim is relatively complete. X-ray diffraction data indicate that the Davdar emeralds can be assigned to “octahedral” beryl, with Al substitutions mainly by mafic ions, and alkali ions, mainly Na⁺, occupying the beryl channels for charge compensation. Chemical composition analysis shows the alkali metal contents decrease from the core to the rim in both new and old mine samples. The alkalinity of new mine samples is low (alkali metal <1%), while old mine samples have medium or high alkalinities (alkali metal between 1%-2% or > 2%). The growth zoning types include multiple-color or colorless core-green rim types, and Cr and V are the dominant coloring elements; and chemical heterogeneity of the growth microzones reflects the variable contributions from coloring ions and alkali and mafic ions.

Key words: Emeralds from Davdar, X-ray diffractometer, EPMA, zonation character, heterogeneity

CLC Number:

CUI Di, WU Qiong, LIAO Zongting, QI Lijian, ZHOU Zhengyu, ZHANG Lingmin, ZHONG Qian, LIU Yiceng, LI Ling. Growth heterogeneity and zonation characteristics of emeralds from Davdar, Xinjiang[J]. Earth Science Frontiers, 2023, 30(2): 401-414.

Figures/Tables 16

Fig.1 Simplified geological map of the Davdar emerald mine, Xinjiang. Modified after [21].

Fig.2 Emerald samples from Davdar, Xinjiang

Fig.3 Zonation characteristics of sample XE-1

Fig.4 XRD patterns of samples XE-1, LE-1, LE-2 and LE-3

Table 1 Crystal cell parameters for samples

祖母绿产地	a/ $A ˙$	c/ $A ˙$	c/a	备注
标准绿柱石	9.219 0	9.198 0	0.996 5	卡片编号01-073-1918
新疆达布达	9.229 9	9.197 4	0.996 4	本文实测
新疆达布达	9.230 5	9.197 3	0.994 6
新疆达布达	9.248 3	9.198 4	0.996 2
新疆达布达	9.231 5	9.196 8	0.995 1
新疆达布达	9.233 0	9.206 0	0.995 3	任伟等^[7]
新疆达布达	9.243 8	9.200 3	0.995 0	郭燕^[9]
新疆达布达	9.241 7	9.195 4	0.995 4	郭燕^[9]

Table 1 Crystal cell parameters for samples

祖母绿产地	a/ $A ˙$	c/ $A ˙$	c/a	备注
标准绿柱石	9.219 0	9.198 0	0.996 5	卡片编号01-073-1918
新疆达布达	9.229 9	9.197 4	0.996 4	本文实测
新疆达布达	9.230 5	9.197 3	0.994 6
新疆达布达	9.248 3	9.198 4	0.996 2
新疆达布达	9.231 5	9.196 8	0.995 1
新疆达布达	9.233 0	9.206 0	0.995 3	任伟等^[7]
新疆达布达	9.243 8	9.200 3	0.995 0	郭燕^[9]
新疆达布达	9.241 7	9.195 4	0.995 4	郭燕^[9]

Fig.5 Crystal structure of beryl. Modified after [24].

Table2 Chemical compositions (w. /%) of color zone sections in new mine sample XE-1 from Davdar, Xinjiang

Table 3 Chemical compositions (w. /%) of color zone sections in new mine sample LE-1 from Davdar, Xinjiang

Table 4 Chemical compositions (w. /%) of color zone sections in old mine samples LE-2 and LE-3 from Davdar, Xinjiang

Fig.6 (a) Microstructural and (b) chemical compositional characteristics of color zones in sample XE-1

Fig.7 Microstructural and chemical compositional characteristics of color zones in samples LE-1 and LE-2

Fig.8 Correlation plots of Al vs. Fe + Mg + Cr + V (apfu values) for samples XE-1, LE-1 and LE-2

Fig.9 Correlation plots of Fe + Mg vs. Na + K + Rb + Cs (apfu values) for samples XE-1, LE-1 and LE-2

Fig.10 Microscopic images showing the periodic growth patterns (a-e) and chemical compositional characteristics of the elemental mapping area (f) in sample LE-3

Fig.11 Element-mapping results and SEI and BSE images of sample LE-3

Fig.12 Genetic model of color zoning in Davdar emerald

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