Mineralogical Characteristic of Nanhong Agate with Black Seaweed Pattern Mineral Inclusion

Abstract

Nanhong agate from Liangshan Yi Autonomous Prefecture of Sichuan Province, China has gained great popularity in the gem market in China for its vibrant colour. Its attractive orange-red colour is generated by fine hematite concentrations. Recently, a new kind of agate named "black seaweed pattern agate", belonging to the family of the Nanhong agates appeared in the local market of Liangshan Prefecture. Microscopic examination showed that the fine red and black grain minerals scattered in various forms such as silkiness and disseminated forms in the sample. Infrared spectrometer, Raman spectrometer and X-ray powder diffraction were used to study the gemmological and mineralogical characteristics of the agate. The results proved that the main mineral component of the agate was quartz with a low relative content of moganite and high degree of order. The crystallinity of the agate is about 6.76. FTIR spectra showed a weak peak at 3 674 cm-1 related to OH-. Combined with the XRD testing result (d=8.418 A), it supported the existence of phyllosilicates in the agate. Raman spectrum of red grain mineal showed peaks at 226, 245, 292 cm-1, which matched well with hematite. Meanwhile the spectrum of black grain was consistent with sulfide minerals (peaks at 342, 377 cm-1). Further testing results of scanning electron microscopy and back scattering diffraction confirmed that most of the black grain mineral is pyrite(FeAs(0.015-0.018 5)S(1.826-1.886)), which belongs to isometric system. According to Raman database, the last peak at 289 cm-1 was identical with chalcopyrite. All these evidence showed pyrite had a paragenetic relationship with chalcopyrite in some area of "black seaweed pattern agate". Another important features were that mineral components and morphological distribution of these black grains were obviously different from features such as Fe and Mn oxides and organic grains that appear in common seaweed pattern agate from other places, which may indicate that the origin of "black seaweed pattern agate" is from Liangshan.