论文部分内容阅读
Demantoid is a rare gem mineral. The Xinjiang Cr-bearing demantoid reported in this paper has beendiscovered for the first time in China. It contains more than 90% end-member molecule of andradite, closeto the pure end-member composition, as revealed by chemical analysis, X-ray powder diffraction, Mossbauerspectrometry and both infrared and optical absorption spectrometry.The Fe~(2+) content lies beyond thedetection limit of Mossbauer spectrometry. It was found that the green color of demantoid is directly relatedto Fe~(3+) in the octahedral site and that the presence of Cr~(3+) greatly enhances its emerald-green hue. From itsgeological occurrence and mineral association the genesis of demantoid is inferred as follows. In ultramaficrocks olivine and pyroxene were replaced by serpentine under the reaction of residual volatiles, creating anFe-rich and Al-poor environment. In such an environment late-stage hydrothermal metasomatism wasdeveloped along the fissures of serpentine, resulting in the crystallization and precipitation of demantoid inasbestos veins. On the other hand, local tectonic compression and dislocation gave rise to the formation ofdemantoid aggregates cemented by serpentine asbestos. Evidence has shown that the trace Cr was derivedfrom chromite and Cr-spinel in the ultramafic rock bodies.
The Xinjiang Cr-bearing demantoid reported in this paper has beendiscovered for the first time in China. It contains more than 90% end-member molecule of andradite, closeto the pure end-member composition, as revealed by chemical analysis, X-ray powder diffraction, Mossbauerspectrometry and both infrared and optical absorption spectrometry. The Fe ~ (2+) content lies beyond the limit of determination of Mossbauer spectrometry. It was found that the green color of demantoid is directly relatedto Fe ~ (3 +) in the octahedral site and that the presence of Cr ~ (3+) greatly enhances its emerald-green hue. From itsgeological occurrence and mineral association the genesis of demantoid is inferred as follows. In ultramaficrocks olivine and pyroxene were replaced by serpentine under the reaction of residual volatiles, creating anFe-rich and Al-poor environment. In such an environment late-stage hydrothermal metasomatism wasdeveloped along the fissures of serpentine, resultin g in the crystallization and precipitation of demantoid inasbestos 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 derivedfrom chromite and Cr-spinel in the ultramafic rock bodies.