The Study On Gemological And Mineralogical Characteristics Of Celestine In Madagascar

The samples for this thesis were from the Sakoany region on the northwest coast of Madagascar.The celestine crystal is abundant in reserves and good in quality.It is currently one of the major production areas of celestine.Madagascar celestine is a marine sedimentary sulphate,and its output form is dominated by crystal caves.The experimental specimens collected in this study consist of seven single crystals and two crystal cavities.The gemological characteristics are tested by conventional gemological methods.On the basis of these experiment,combined with modern large-scale test instruments such as Fourier transform infrared spectroscopy,Raman spectroscopy,UV-Visible spectroscopic spectroscopy,XRF,XRD,electron probe,scanning electron microscopy and other methods were used to study the spectral characteristics,chemical composition and structural characteristics of celestine crystals.Furthermore,we compared these characteristics with celestite from other producing areas.The results of conventional gemological tests show that the crystals of Madagascar celestine are highly self-formed,often appear dark blue,blue to colorless,Pleochroism is weak,and transparency is good,with glass luster.The internal gas-liquid inclusions can be seen.The crystal habits are plate-shaped,column-shaped and tapered,and are often produced in crystal cavities.The refractive index is 1.619¹.637,the birefringence rate is 0.018,and the relative density is 3.90³.94.The hardness of the microhardness test is lower,and the Mohs hardness is 3³.5.Infrared spectroscopy tests show that the celestite of Madagascar has characteristic absorption at 1208¹061 cm^-1,1245 and 990 cm^-1,642⁶11 cm^-1,465 cm^-1,corresponding to[SO₄]^2-antisymmetric stretching vibration,symmetric stretching vibration,asymmetric angular vibration and symmetric angular vibration.Raman spectroscopy is similar to infrared spectroscopy,with characteristic peak positions of1189 cm^-1,1156 cm^-1,1110 cm^-1,and 1093 cm^-1 antisymmetric stretching vibration,1000 cm^-1 symmetric stretching vibration,656 cm^-1,637 cm^-1,623 cm^-11 antisymmetric bending vibration and 460 cm^-1 symmetric bending vibration.The two characteristics of IR and Raman spectra both have[SO₄]^2-symmetric stretching vibration at 1000 cm^-1.Ultraviolet-visible spectroscopic spectroscopy tests have significant absorption in the infrared region,less absorption in the visible range,and only a small amount of absorption in the red region.The celestite crystals are relatively pure.The main chemical elements of the celestite are Sr,S,O,and a very small amount of elements such as Ca,Al,Ba,Fe,etc.,and the SiO₂ content in the soil-like debris on the back of the crystal cavity is high.The calculating result show that the crystal form of celestite crystal is SrSO₄.The results of X-ray powder diffraction show that the crystals are well crystallized,and the unit cell parameters are calculated as a₀=8.359?,b₀=5.350?,c₀=6.8680?,?=90°,?=90°,?=90°.Scanning electron microscopy showed three groups of cleavage.The crystals showed a typical layered structure and stepped fractures.The crystal planes showed the formation of mounds and pores.Based on the study of the geological background of the northwestern coast of Madagascar,the ore-forming conditions are inferred to be low-temperature water circulation?possibly seawater,providing Sr?and decomposition of organic matter?formation of sulfate?.