Study On Gemological And Mineralogical Characteristics And Genesis Of "Ganlan Jade" In Ji'an,Jilin

A rare massive yellowish-green“Ganlan jade”,which is essentially the serpentinized dunite,covering a minimum area up to 50m²,has been found in Ji'an County,Jilin Province.The research is based on laboratory analysis via field exploration,conventional gemological testing,polarizing microscope and SEM observation,XRD,Raman spectroscopy,EPMA,LA-ICP-MS and micro-infrared spectroscopy.The gemological and mineralogical,as well as the genesis of the jade are all determined in this article.The jade show uniform color,granular and metasomatic residual structure,dense block,grease-glass luster,translucent-slight transparency and uneven fracture.The sample has a refractive index of 1.56-1.65,a specific density of 2.65-2.82.The hardness is greatly affected by composition,which means the micro hardness is848.24-968.41?due to olivine?,140.19-172.60?due to serpentine and brucite?,and the Mohs ranges from 3 to 6.5.The jade contains primary olivine and secondary serpentine?antigorite?and brucite.Other primary minerals,frequently appearing in ultramafic rocks,have not been identified.The olivine is essentially pure forsterite,with an Mg#?100×Mg/?Mg+Fe??of 99.63-99.70,and display no intragranular and intergranular composition variations.They are extensively ruptured and hydrated?i.e.,serpentinized?,with the fractures?and the grain boundaries as well?filled by fine-grained antigorite(ideally Mg₆(Si₄O₁₀)?OH?₈)and brucite?ideally Mg?OH?₂?.The hydration zones along the grain boundaries of the olivine are wider than those in the fractures within the olivine grains,indicating a hierarchic fluid-transport system operating in the hydration process.For both hydration zones,the serpentine aggregates,appearing as layers,often locate themselves next to the olivine and sandwich the layers of the brucite aggregates.These secondary phases are also extremely poor in Fe,indicating a good chemical equilibrium with the forsterite.The serpentinization reaction in the jade may have proceeded as4Mg₂SiO₄+6H₂O?fluid?=Mg₃Si₂O₅?OH?₄+2Mg?OH?₂ at temperatures of 425±25?and at relatively low but undetermined perssures?0-10Kbar?.The water content in olivine is 1450±300wt ppm.The fluid was likely a B-rich,but Si-poor dilute aqueous fluid,as implied by the trace element characteristics and water-related infrared features of the forsterites in equilibrium.Due to these distinct features,the Ganlan jade reported here might stand for the ancient and refractory lithospheric mantle of the North China Craton,which was somehow transported into the Liao-Ji Proterozoic rift zone.Another protential process that could have led to the formation of the Ganlan jade is that the forsterite might have quickly crystallized at relatively low temperatures from a special hydrous magma with its anhydrous components close to the forsterite composition.