Study On The Color Formation And Chromaticity Of Agate In The Gobi Desert Of Alxa League, Inner Mongoli

Alxa Gobi agate is a kind of natural quartz jade produced in Alxa Gobi Desert of Inner Mongolia,which can be roughly divided into two different colors:red to orange and green.The agate produced in this area has dense structure and high color saturation,and belongs to the new jade resources active in the jewelry industry market in recent years.Based on the existing research results,Through conventional gemological instruments,polarizing microscope observation,scanning electron microscope observation,X-ray powder crystal diffraction analysis,infrared spectrum analysis,electron probe analysis,laser Raman spectrum analysis,ultraviolite-visible spectrum analysis,determination of colorimetric parameters and other research methods,the metallogenic model of Alxa agate,gem mineralogy characteristics,microscopic morphology,phase composition,element composition and other issues The color origin and colorimetric characteristics of Alxa agate were further explored.Through naked eye observation,the specific colors of Alxa red-orange agate are maroon red,red,pink,orange red,brown yellow,etc.The transparency is mostly opaque,a few are micro transparent to sub transparent,and is weak glass luster,the surface is often cracks,pitted pits and color spots and other surface characteristics.The specific colors of Alxa green agate are dark green,blue-green,yellow-green and so on.Most of them are opaque,showing earth-like luster or waxy luster,and the surface is characterized by stains,cracks,punctated pits and so on.The index of refraction was about 1.54 by conventional gemological tests.The relative density is about 2.60～2.75.Mohs hardness is mostly 7,a few areas test is 3;The absorption spectrum varies with the color of specific samples.The fluorescence is inert under ultraviolet fluorescent lamp.By observing the microstructure,the chromogenic inclusion hematite in the red-orange agate is dotted,dispersed or disseminated,and the color is red to orange yellow.The particle size is about 1～10μm in diameter.The chromophore of green agate was enriched in the surface layer of the sample and grew radially to the interior.The chromic inclusions mainly come from the iron minerals on the pore wall surface of the parent rock.After the hydrothermal filling rich in Si O₂,alteration occurs and crystallization occurs.In addition,both quartz and calcite have cryptocrystalline structures.By scanning electron microscope analysis,the quartz in the two kinds of color agate showed mylonitic micromorphology,and the green agate contained calcite in the shape of fiber sheet.For red-orange agates,the content of iron minerals directly exposed from the test section is very low;For green agate,a small amount of chromogenic minerals are distributed in the agate in granular form.Compositional analysis indicates that the red-orange agate is mainly composed of quartz,while the green agate is mainly composed of quartz and a little calcite.Green agate can be determined to be chloropyrophyllite phase by stripping the surface green substance test.Analysis of spectral characteristics of molecular vibrations,the results are basically consistent with the X-ray powder crystal diffraction analysis.The red-orange yellow agate corresponds to the vibration of related groups of quartz,but reflects the vibration of related groups of hematite or goethite to a certain extent,and the surface material of green agate belongs to the vibration of related groups of chloropyleophyte.By analyzing element composition showed that chlorophyllite is mainly composed of Si,Fe,Al,K and Mg,and contains trace elements such as Cr,Mn,Ca and Ti.Through chromogenic inclusion analysis,the chromogenic inclusions in red-orange agates are hematite,and the peak intensity of hematite related Raman spectrum is weak due to the serious influence of quartz basement.Alxa red-orange agate is mainly caused by hematite.In the UV-visible absorption spectrum,it shows the crystal field transitions of Fe³⁺ions in various modes,the charge transfer between O^2-and Fe³⁺,and the main absorption between the direct band gaps of hematite.Alxa green agates are colored by chloropylesquite minerals.The UV-VIS absorption spectrum test results mainly show charge transfer between Fe²⁺-Fe³⁺,and there may be charge transfer between Fe²⁺and Ti⁴⁺.According to the determination of colorimetric parameters,the main color of the Alxa red-orange yellow agate samples studied is from grade 5 to 6,medium brightness to grade 7,medium color to high color orange-red to orange-yellow.The main color of Alxa green Gobi agate samples shows the medium brightness of grade 4-5,and the yellow-green to blue-green of low color to low color.In addition,there is a certain relation between the position of the maximum point of the first derivative spectrum of the UV-VIS spectrum and the hue Angle of the Alxa red-orange agate.The common position of the maximum point in the visible range of the samples with the main color of red is 570,574,582,592nm,with the increase of the red hue,the position of the first derivative maximum is redshifted accordingly.In the range of tonal Angle 10～51°,the best red tone is about 27°tonal Angle,and the maximum position of the first derivative is the largest.The main peak of maximum value in the visible range of samples with orange hue is commonly located within 564～574nm,and the secondary peak at 435nm can be seen in some samples.Moreover,when the hue Angle is in the range of 51～63°,the position of the main peak of maximum value of the first derivative of the absorption spectrum will also be redshifted with the increase of orange tone.For yellow samples,when the tone Angle is about 63～97°,the main peak of the first leading maximum is basically stable at 535nm,and there are secondary peaks at 435nm.