On the genesis of demantoid from Xinjiang, China

Demantoid is a rare gem mineral. The Xinjiang Cr-bearing demantoid reported in this paper has been discovered for the first time in China. It contains more than 90% end-member molecule of andradite, close to the pure end-member composition, as revealed by chemical analysis, X-ray powder diffraction, Mossbauer spectrometry and both infrared and optical absorption spectrometry. The Fe2+ content lies beyond the detection limit of Mossbauer spectrometry. It was found that the green color of demantoid is directly related geological occurrence and mineral association the genesis of demantoid is inferred as follows. In ultramafic to Fe3+ in the octahedral site and that the presence of Cr3+ greatly enhances its inferred as follows. In ultramafic rocks olivine and pyroxene were replaced by serpentine under the reaction of residual volatiles, creating an Fe-rich and Al-poor environment. In such an environment late-stage hydrothermal metasomatism was developed along the fissures of serpentine, resulting in the crystallization and precipitation of demantoid in asbestos veins. On the other hand, local tectonic compression and dislocation gave rise to the formation of demantoid aggregates cemented by serpentine asbestos. Evidence has shown that the trace Cr was derived from chromite and Cr-spinel in the ultramafic rock bodies.