The Study On Gemological Characteristics Of Nigerian Sapphire

At present,a large number of sapphires of basaltic origin on the market are produced in Nigeria,but there are few literatures on mineralogy and gemology about these sapphire.The sapphire samples in this paper were produced in basaltic?secondary?deposits on the Mambilla Plateau,Nigeria.This study use microscope,FTIR spectrometer,ultraviolet-visible spectrometer,Laser Raman spectrometer,EDXRF and LA-ICP-MS to make systematical study on the gemological characteristics of the sapphires from this area.In addition,due to the similarity in appearance with the Shandong Changle sapphire,which is also of basalt origin,this paper will compare the chemical composition and spectroscopic characteristics of these two different sapphire using instruments which is commonly used in the laboratory.Nigeria Sapphire is usually dark blue,occasionally come with light blue,yellow,yellow/blue bicolor,green and greenish blue.The rough crystals are generally small,mostly fragmented,with a melt-like appearance on the surface,and in high roundness.Open fissure often filled with brownish secondary ferriferous minerals.The Sapphire contain numerous tiny titanium-contianing particle inclusions,which resulting in poor transparency and a milky appearance.The fluid and melt inclusions vary in shape and are distributed along the healing fracture or along a certain crystallographic direction.The trace element of Nigerian sapphire,in general,show the characteristics of magmatic sapphire,containing high Fe content,generally above 7000ppm,Ga between156 and 244ppm,Mg less than 40ppm,with Fe/Mg ratios>100,Ga/Mg ratios>6.Due to presence of minute particle inclusions,the Ti content varies greatly.Compared with the sapphire from shandong changle,the content of Fe,Ti,Ga of Nigerian sapphire is lower,with Fe content generally below 1%.LA-ICP-MS also detected trace elements such as Be,Zr,Nb,Ta and Hf,these element were related to the Ti-containing particle inclusions in the sapphires.Infrared spectroscopy shows that the sapphires generally have 3310 cm^-1-series peaks generated by the absorption of hydroxyl-OH in the structure,which is a typical feature of basalt sapphire.UV-Vis spectroscopy indicates the main chromophores of the Nigeria sapphires are the single Fe³⁺,Fe³⁺-Fe³⁺ion pairs,Fe²⁺-Ti⁴⁺ion pairs and Fe²⁺-Fe³⁺ion pairs,the absorption produced by the intervalence charge transfer of Fe²⁺-Fe³⁺ion pairs has a greater impact on the color,which resulting in low saturation of the blue sapphire.