Mineralogical Study On Nephrite From Somaliland,Africa

Here we report the new found nephrite jade deposit in Somaliland,Africa.Grey nephrite is the main type with rounded river-alluvial nephrite discovered occasionally.Presumably this place contains white jade.The nephrite showing brown weathering rind can also be found.The deposit consists of migmatitic amphibolite,marble(M?)and granite(?)intrusion veins in NE-trending.It looks that the rocks related to the genesis of nephrite jade are marble and epidote-altered rocks.The nephrite and wall rocks of the Godirood Mountain and Laas Ma.Aan Mountain,which is two open tracts,were sampled on the spot and shipped back to China to complete the indoor testing.Geological,mineralogical and spectroscopic properties of nephrite recovered from the deposit are presented.Three jade samples have been tested.The gemological properties of them are determined.Somaliland nephrite is yellow-green and dark-green,with fine texture.RIs range from 1.60 to 1.61 and SGs are 2.81-3.02.Their hardness values vary from709 to 881kg/mm~2.All the nephrite samples are inert to longwave and shortwave UV radiation.The gemological parameters of somoliland jade are similar to nephrite in other places globally.The samples'X-ray powder diffraction,FTIR absorption spectra and Raman spectra imply the truth that somaliland jade samples are composed of amphibole mostly.By the way,they also show the diffraction peaks of main accessory minerals such as diopside and apatite.Somaliland nephrite's structure is divided into metasomatic structure and secondary filling structure.Mostly,Somaliland jade is a crypto-crystalline,which is invisible by naked eyes.Fiber crytalloblastic structure is a typical structure of them.Under scanning electron microscopy,Somaliland jade mainly has parallel or near-parallel fibrous crytalloblastic structure,fibrous-column crytalloblastic structure,lamellar structure,etc.Slender fibers interpenetrate tightly with each other,which is an inherent factor of good toughness.In the test of electronic probes,Somaliland jade consists of tremolite(Tr)and actinolite(Act)as the main minerals,and other minor minerals include edenite(Ed),pargasite(Prg),diopside(Di),apatite(Ap),and epidote(Ep).Pargasite(Prg)is relatively rare in the oher nephrite placecs.In the study,two epidote-amphibole altered rocks are tested.This kind of rocks have poor transparency and are always mistaken for jade because of similar color in some cases.Mineral components of epidote-amphibole altered rocks are composed of actinolite(Act)and epidote(Ep)with tremolite(Tr),titanite(Ttn),rutile(Rt),zircon(Zrn)and apatite(Ap)and etc.Two dolomitic marble samples have a granoblastic texture composed of coarse-grained(2–3 mm)dolomite under the observation.Meanwhile serpentine(Srp)and apatite(Ap)were distributed in the wall rocks.Using BSE images,the order of formation of the constituent minerals,the contact relationship such as account,intersect,harbor and other structures are judged.Field investigations and laboratory experiments all reveal that it is a dolomitic marble-related(D-type)nephrite deposit.It is speculated that the possible ore-forming process of Somaliland nephrite jade is as follows:1)dolomite marble?tremolite/actinolite,2)dolomite marble?diopside?actinolite,3)dolomite marble?diopside?pargasite?actinolite,4)Coarse grain tremolite/actinolite?fine grain tremolite/actinolite.