The Study On Gemological And Mineralogical Characteristics And Genesis Of Moss Agate From Fuxin, Liaoning Province

Moss agate is a kind of agate decorated with different color dendritic inclusions. In this paper, testing the oridinary gemological characteristics and using methods of microscope observation, SEM, C L, IR, X-ray diffraction, Raman spectroscopy, EPMA and TGA etc, we study on the gemology, mineralogy and genesis of moss agate in Fuxin, Liaoning Provience. Compared with the moss agate from India, we summarize the similarities and differences. Moreover, the appraisal standard of moss agate is also discussed as well.The green inclusions in moss agate from Fuxin district turn out to be celadonite as a kind of dioctahedral phyllosilicates, while the red-yellow inclusions are goethite or hematite or the both. Moss agate is consist of granular and fibrous α-quartz together with few moganite and opal-CT. Micro-Raman spectroscopy yielded by moganite is characterized by 502cm-1. Paracrystalline opal-CT is a kind of hydrous silicas, consisting of disordered α-cristobalite and α-tridymite. XRD pattern yielded by opal-CT is characterized by three broad reflections, the most intense of which occurs at about 4.12 to 4.05 ?, with a subsidiary peak or shoulder at about 4.32 ?. There is also a weaker reflection at about 2.50 ?. The red-yellow color of moss agate is caused by goethite or hematite or the both, while the hematite dehydrated of goethite caused the color darker.The intergrowth of celadonite and silica phases always show different zones. By observing the sample sheet under polarizing microscope, celadonite shows microstructure as needle, scaly, spherulitic structure, while the silica phases are always fibrous and microgranular and macrogranular in shape. The silica phases shows fibrous, spherulite, as well as rohombohedral surface, and the first two alternate growing is the reason why the bands are observed by naked eyes. The fibrous can be nearly parallel, as well as showing concentric structures stacked by many layers. The spherulite silica consists of many tiny spherulite silica which are stacked together, coating with needle goethite and hematite.Deuterium and oxygen isotope analyses of moss agates reveal o ne component of the agate forming fluids is of meteoric origin. Using oxygen isotope thermometer to calculate the formation temperature of moss agate(with different water sources), it turns out that the formation temperatures are <250 ℃.The formation of moss agates is a complex process, associated with late- or post-volcanic alteration or weathering of volcanic host rocks(especially of volcanic glass). Mineralization consists of the following stages:(1) under oxidation conditions, the hydrothermal alter the andesite with high magnesium, potassium to be celadonite and brown silica.(2) The deposition of latter silica proceeds very microcrystalline and stressed chalcedony or opal-CT, found in the rim at the walls of the vesicles and the rim of celadonite, or even traverse the celadonite and the silica phrase.(3) Then cooling and a further decrease of pressure taking place, the quartz crystallites are still fibrous but less stressed and slightly coarser than in the initial stage.(4) The siliceous liquid or gel forms fine-quartz deposit.(5) At the final stages, coarse quar tz crystals grow in the residual cavities and pores of the vesicle as well as in pores in the fine-quartz layers slowly at very low supersaturations...