A Comparative Study on Black Feicui (Omphacite Jade) from Myanmar and Black Feicui (Jadeite Jade) from Guatemala

Black Feicui is a special variety of Feicui that normally shows black under reflective light and may appear vivid green to dark green under transmitted light. Black Feicui samples from Myanmar and Guatemala were studied through observations of appearance characteristics and analyses of structure and composition, in attempt to provide a theoretical basis for the effective identification of the two. The results showed that the black Feicui samples from Myanmar are fine-grained, with more uniform particles and finer texture, and appear dark green under transmitted light. The black Feicui samples from Guatemala sometimes exhibit visible grainess to the naked eye with uneven particle sizes, grayish-white flocs and dots can be observed in the interior, and the bulk form samples are generally opaque under transmitted light, the relative density of the Guatemala samples are slightly higher than that of black Feicui samples from Myanmar. Some samples from both origins have an outer skin (weathering crusts), which is yellowish-brown partly. The testing results of Fourier-transform infrared spectroscopy (FTIR) showed that the main mineral component of the black Feicui samples from Myanmar is omphacite, which is relatively pure. While, the main mineral components of the black Feicui samples from Guatemala exist both jadeite and omphacite. Electron probe microanalysis (EPMA) results showed that the black Feicui samples from Myanmar mainly contain omphacite, and the samples from Guatemala mainly contain jadeite with minor aegirine or augite. The contents of MgO and CaO in black Feicui samples from Myanmar were higher than that in Guatemalan ones, while the content of FeO was lower. Analyzing the appearance characteristics of the testing points and content calculations of Fe²⁺ and Fe³⁺, it can be inferred that the colouration of black Feicui is closely related to Fe content and may be dominated by Fe³⁺, and the high content of Fe can also influence the transparency of the samples. With scanning electron microscopy (SEM), it can be found that the black Feicui samples from Myanmar are simpler in mineral components, the opaque skin part(weathering crust) has abundant and dense fractures and the part far away from the skin has less fractures, which may represent that the transparency can be influenced by the amount of fractures. The black Feicui samples from Guatemala are more complex in mineral components, the impurity minerals include omphacite, aegirine, titanite, allanite, etc. The white-grayish flocs or dots in the black Feicui samples from Guatemala are found to be titanite or allanite.