Acta Petrologica et Mineralogica

The Alxa area in Inner Mongolia has experienced hundreds of millions years of geological activities, producing Gobi agate with rich colors and compact structure. The gem mineralogical characteristics and coloring causes of green Gobi agate were studied by conventional gemological test, polarizing microscope and scanning electron microscope observation, X-ray diffraction, electron microprobe analysis, infrared spectrometer and ultraviolet-visible-near infrared spectrophotometer. With naked eyes, the agate is dark green to brown green, translucent to opaque. The relative density, refractive index and Mohs hardness are all in line with the characteristics of quartzite jade stone. Through microscopic observation, celadonite is enriched in the surface layer and grows radially inward. Both calcite and quartz have cryptocrystalline texture. Through scanning electron microscope, celadonite is distributed between quartz and calcite in granular form. X-ray diffraction analysis shows that the mineral phase composition of green Gobi agate is quartz, calcite and celadonite. The EPMA results show that the main chemical composition of celadonite are SiO₂, FeO, Al₂O₃, K₂O and MgO. Infrared spectrum analysis also shows that there are characteristic peaks of corresponding groups of celadonite. The test results of surface celadonite under UV-Vis-NIR spectrophotometer show the characteristic spectra of Fe²⁺ and Fe³⁺. The charge transfer between Fe²⁺ and Fe³⁺ is the main reason for its coloration. Celadonite is the main chromic mineral of green Gobi agate.

green Gobi agate; celadonite; quartzite jade; charge transfer of Fe²⁺-Fe³⁺