Unique Metal Sulfide Inclusion in “Bing Piao” Red Agate from Liangshan, China
Over the past decade, with the discovery of “Nanhong” agate ore deposits in Sichuan Province, a significant amount of this material has been mined in Meigu County, located in the northeast of Liangshan Yi Autonomous Prefecture. Commercial gem mines in Meigu County are located in three main areas, including Jiu Kou, Wa Xi, and Lian He. Field observations show that the ore body is found either in the interlayer fissures and cavities of Permian Emeishan basalt crystallized from siliceous hydrothermal fluid or as conglomerates of the Permian Leping Formation.
Previous reports found that “Nanhong” agate is colored by natural hematite inclusions. Moreover, these agates are divided into five major categories, including “Bing Piao” (a Chinese term that refers to their inclusions “floating on ice”). This type of agate (see figure 1, left) is colorless and contains red hematite particles distributed in attractive patterns.
Recently, a special type of “Bing Piao” red agate called “black grass” agate by merchants appeared in the market at Liangshan. A bead of this material (figure 1, right) was obtained for examination at the National Gold-Silver Gem & Jewelry Quality Supervision & Inspection Center (Sichuan).
Gemological properties of the semitransparent bead, including its RI of 1.54 and hydrostatic SG of 2.65, suggested agate; this was confirmed by FTIR spectroscopy. Magnification revealed tiny red inclusions that were identified as hematite by Raman spectra (peaks at 225, 244, 291, 410, 610, and 1320 cm–1) using a 785 nm laser. In addition to these attractive red forms made of hematite, the bead hosted many unusual black mineral particles displaying various shapes such as blades, planar clusters, and the like (figure 2). When viewed with reflected light, microscopic observations of their opaque, anhedral form and metallic luster suggested the particles were likely an iron sulfide. A Raman spectrum obtained from a surface-reaching black particle showed two major peaks at 342 and 377 cm–1, matching well with pyrite, according to the RRUFF database (reference spectrum R100166), as shown in figure 3.
To obtain confirmation, the bead was polished down to a plate to perform chemical analysis and phase identification of the black particles using a scanning electron microscope in combination with an energy-dispersive spectrometer (SEM-EDS) equipped with an electron backscatter diffraction (EBSD) detector. Qualitative analysis revealed the expected major elements of Fe, As, and S, along with minor Si, likely from the agate host. This was consistent with pyrite composition. The EBSD analyses indicated a cubic structure, and a search of the NIST Structural Database using Aztec software revealed a match for pyrite (FeS2).
Except for Liangshan, such distinctive black minerals of various forms have not been found in other “Bing Piao” red agate deposits. The pyrite particles, as inclusions that are unique to “Bing Piao” red agate from Liangshan, may therefore be considered reliable indicators of origin. Further gemological studies on these inclusions are required to construct the database for origin determination.
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