Gem News International Gems & Gemology, Winter 2021, Vol. 57, No. 4

Banded Turquoise from Zhushan County, Hubei Province, China

Tianting Lei, Yan Li, Fengshun Xu, and Mingxing Yang

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Six banded turquoise specimens showing variation in color and width of bands.
 
 
 
 
Figure 1. Banded turquoise specimens from Zhushan County, China. Photos of the six specimens show the variation in the color and width of the bands as well as the sharpness of the boundaries between bands. Photos by Tianting Lei.

Zhushan County in China’s Hubei Province is the world’s most abundant source of commercial gem-grade turquoise (F. Xu, “Study on turquoise in Shiyan City, Hubei Province,” Shanghai Art & Crafts, No. 3, 2017, pp. 30–32). It produces turquoise with special patterns, including “raindrop,” “Tang tricolor,” “growth layer,” “spiderweb,” “water grass vein,” and “Ulan flower” (L. Liu et al., “Unique raindrop pattern of turquoise from Hubei, China,” Fall 2020 G&G, pp. 380–400). Turquoise materials with growth layers display a yellow, red, brownish red, brown, blue, or blue-green banded structure, occasionally accompanied by tiny colored spots. This pattern is called “banded turquoise” in the trade. The gemological and mineralogical characteristics, color origin, and origin traceability of common turquoise have been well studied. However, there are few reports on banded turquoise, which is highly valued in the Chinese market.

Division of bands and FTIR spectra of four specimens.
 
 
Figure 2. Division of bands and FTIR spectra of specimens T1, T3, T4, and T6, which were sliced open. A: T1-1 through T1-6, six bands. B: T3-1 through T3-6, six bands. C: T4-1 through T4-7, seven bands. D: T6-1 through T6-5, five bands. Photos by Tianting Lei.

Six specimens of banded turquoise were obtained through long-term cooperation of local miners from the turquoise market in Zhushan. The specimens displayed numerous bands with various colors, including blue, green, yellow, red, and brown (figure 1). The band profiles of samples T1, T3, T4, and T6 were separated according to color for investigation (figure 2). Specimens T3 and T5 were severely damaged during the cutting process and could not meet the requirements of FTIR testing, but they were measured to analyze the composition and structure. The FTIR absorption spectra, which were typical of those for turquoise, were essentially identical in the various bands within a sample (figure 2). The FTIR absorption fingerprint peaks of turquoise, produced by the vibrations of the phosphate group, range from 483 to 653 cm–1 and from 1000 to 1200 cm–1, respectively. The FTIR absorption peaks of the functional group area, characterized by the vibrations of water molecules and hydroxyl ions (Q.L. Chen et al., “Turquoise from Zhushan County, Hubei Province, China,” Fall 2012 G&G, pp. 198–204), are located near 842, 787, 1659, 3087, 3290, 3461, and 3508 cm–1.

SEM microtopography of different bands in four samples.
 
 
 
 
Figure 3. SEM microtopography of different bands in samples T1, T2, T3, and T6. Images by Tianting Lei; field of view 13.5 µm.

The banded turquoise microcrystals were characterized by platy, columnar, and layered structures visible with scanning electron microscopy (SEM) (figure 3). The samples had a disorderly crystal structure, except for the deep green band (T6-2) with specific directionality. The more deeply saturated turquoise had higher density, less porosity, and more compactness than the lighter-colored turquoise. The edges of the blue band turquoise (T1) microcrystals were straight and clear with sharp corners (T1-5), while the red and brown band turquoise microcrystals did not show sharp corners (T1-2, T1-4). The same was true for specimens T2 and T3. The edges of the turquoise microcrystals were always straight, while the corners of blue band turquoise microcrystals (T2-2, T3-1) were sharper than those of the red bands (T1-2) and yellow bands (T3-3, T3-5) in the same samples.

Table 1. Composition of bands in turquoise samples T1, T3, and T5 by EPMA (wt.%).
 

The chemical compositions for turquoise (as a microcrystalline aggregate mineral) obtained from electron probe microanalysis (EPMA) are often not completely consistent with the theoretical value, which is mainly due to the mixed weathering and leaching of minerals. The chemical composition of bands in specimens T1, T3, and T5 is shown in table 1. There was little difference in the chemical composition of each band within the same turquoise, except for the FeOT content in specimen T1. Additionally, there was no obvious linear relationship between the content of other major elements and the color change of turquoise in each band. In specimen T1, the FeOT contents of the red band (T1-2), brownish red band (T1-3), brown band (T1-4), and tan band (T1-6) were higher than that of the blue band (T1-1, T1-5).

Although bands in this turquoise showed obvious color differences, there was no obvious difference in the infrared spectrum and chemical composition of each band. The crystallinity of blue bands was higher than that of the red, brown, and yellow bands. An explanation of the formation mechanism of this material needs further investigation.

Tianting Lei, Yan Li, Fengshun Xu, and Mingxing Yang are affiliated with the Gemmological Institute, China University of Geosciences in Wuhan.