Investigation of recently discovered common green opals from Anosy (Madagascar)

Abstract

The physical, chemical and gemological properties of recently discovered common green opals from Anosy (South-Eastern Madagascar) have been investigated using different analytical methods such as standard optical and gemological analyzes, X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), Micro-Raman Spectroscopy and Laser Ablation Microprobe (LA-ICP-MS). Anosy opals have a yellowish green color with a light yellowish rim, translucent / opaque diaphaneity with greasy luster, red oxide spots and are inert to long and short wavelength UV radiation (366–254 nm).

The values for refractive index and specific gravity range between 1.435–1.460 and 2.03-2.07, respectively. According to XRPD and Raman analyses, the opals are CT type with tridymite more abundant than cristobalite and also contain clay minerals (saponite). Iron and, subordinately, V and Cu appear the main chromophores that determine the green-yellowish color, while Ni and Cr are very subordinate. Other detected trace elements are Mg, Al, Ca, K, and Na. The high contents of Ba are noteworthy since this element is rarely reported in opals worldwide. SEM observations show three types of structures already observed in the CT opals: homogeneous and very fine microspheres, lepispheric cauliflower structures and globular aggregates of different sizes formed by laminae.