Water characterization and structural attribution of different colored opals

Niu Li; Qingfeng Guo; Qianqian Wang; Libing Liao

doi:10.1039/d2ra04197a

Water characterization and structural attribution of different colored opals

RSC Adv. 2022 Oct 25;12(47):30416-30425. doi: 10.1039/d2ra04197a. eCollection 2022 Oct 24.

Authors

Niu Li^{1

2}, Qingfeng Guo^{1

2}, Qianqian Wang^{1

2}, Libing Liao³

Affiliations

¹ School of Gemmology, China University of Geosciences Beijing 100083 China qfguo@cugb.edu.cn.
² Jewelry and Mineral Materials Laboratory of Experimental Teaching Demonstration Center Beijing China.
³ Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Sciences and Technology, China University of Geosciences Beijing 100083 China clayl@cugb.edu.cn.

Abstract

The structure type and water content of opal will affect its stability and value. When the structure is Opal-A type, its value is higher. When the water content is high, its stability will decrease. The structural attribution and water distribution of pink, yellow, green, blue and purple opal have been investigated by several methodologies like powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier transform near infrared spectroscopy (FT-NIR). The results show that all the opal samples are CT type, among which the green sample has the highest crystallinity. The water contents of the opals range from 3.06 to 8.78%. The distribution and quantity of molecular water (H₂O_mol) and silanols (H₂O_SiOH) are calculated semi-quantitatively according to intensity of 4500 cm^-1 and 5200 cm^-1 peaks in the near infrared region, and it is found that the water in all the samples is basically composed of molecular water. The precipitation mechanism for opal formation results in different structural types, and the difference in structural composition leads to different water contents. This work provides a theoretical basis for the future study of the opal metallogenic environment.