Study on Gemological Characteristics of Changbai Jade

Cao Miaocong,Tang Bin

(Changchun Institute of Technology,Changchun,Jilin 130011)

Abstract:In this paper,the refractive index,density,hardness,transparency and luster of Changbai jade were measured by gemological conventional instruments,scanning electron microscope and energy disperse spectroscopy,and the microstructure and chemical composition of Changbai jade were analyzed.It is considered that Changbai jade is mainly composed of kaolinite minerals,with cryptocrystalline texture and pseudohexagonal sheet is present under high magnification;the Changbai jade’s refractive index range is 1.56～1.57,density is 2.52～2.81,hardness is 2.5～2.7,and the steatite stone is mostly translucent,the colored stone is more transparent,with the gloss of grease,earth and wax;Changbai jade is rich in color,mild and moist in structure and moderate in hardness.It is an excellent seal stone,which is suitable for carving,development and promotion.

Key words:Changbai jade;gemological characteristics;mineral composition;microstructure

Changbai jade is produced in the area of Malugou,Changbai County,Jilin province.It was discovered by the geological exploration team in 1981 that the proved reserves were 1.9 million tons,4 million tons in the periphery,with abundant reserves[1].The mined Changbai stone mostly is taken as ceramic materials,a few beautiful stone is taken as carved seal stone,also known as Malu jade or Changbai jade.Changbai jade,as the second largest jade stone in Jilin province,has fine texture,bright color,complete variety and moderate hardness.It is another precious seal stone after Shoushan stone in Fujian,Qingtian stone in Zhejiang and Balin stone in Inner Mongolia,with great economic value.But for a long time,restricted by the traditional resource view and the level of gemological knowledge,the systematic gemological research on Changbai jade is relatively lacking.In this paper,20 specimens and samples of Changbai jade were collected,and gemological characteristics of Changbai stone were studied in detail and systematically,the mineralogical characteristics were studied preliminarily.

I.Sample Source and Sample Description

Changbai stone is composed by steatite stone and colored stone,the steatite stone’s structure is generally delicate,with translucence and strong sense of frozen;the colored stone is generally micro transparent or opaque,with delicate structure and bright colors.In this paper,14 pieces of Changbai jade specimens were collected from different mines in Changbai County,covering two categories of steatite stone and colored stone[2].The varieties of steatite stone have graphite steatite,sorghum pigment,lamplight steatite,horn steatite,old pit steatite;the colored stone have calligraphy,colored stone,with rich impurity type and has certain research value.Among them,DG sample is lamplight steatite,NJ is horn steatite,LK is old pit steatite,QHD is graphite steatite,GLH is sorghum pigment,and CS is colored stone series.The appearance description of the sample is shown in table 1.

II.Gemological Analysis of Samples

Gemstone microscope,abbe refract meter,hardness pen,hydrostatic weighing and other methods were used to conduct gemological tests on 14 samples,among which the relative density was measured by hydrostatic weighing method,and the average value was obtained after 4 times of measurement;hardness is quantified by the difficulty of texture characterization;the refractive index requires polishing the sample first and obtained by point measurement.The results are shown in table 2.

After summarizing and sorting out the above data,it was found that the refractive index of Changbai jade ranged from 1.56 to 1.57,but some sample data were fuzzy,the reason was that the low hardness of Changbai jade led to poor polishing effect;the relative density of Changbai jade is 2.48～2.81,among which 3 samples of colored stone are slightly higher than steatite stone,and the reason is that the impurity content is dense;the hardness of Changbai stone is 2.5～2.7,and each sample can make a very light mark with the nail(nail hardness 2.5),the other one is Changbai jade with darker color,it is harder to grind than the long white jade of a lighter color,this may be because the dark Changbai jade con-tains more impurities,resulting in a higher hardness;the transparency of Changbai jade is translucent～ subtranslucent,among which the steatite stone is translucent,and the colored stone is subtranslucent.

Table 1 Description of the Surface Features of the Changbai Jade Specimen

Table 2 Gemological Characteristics of Changbai Jade Samples

III.Test Analysis of Scanning Electron Microscope of Samples

Three representative samples were selected:DG-1、QHD-1、MB-1,and tested by scanning electron microscope to analyze the microstructure and chemical composition of Changbai jade.SEM is Ya-0019 environmental scanning electron microscope(EDS),and the test was carried out in Changchun Institute of Applied Chemistry.The fresh port of the Changbai jade sample was adopted in the experiment,and the test was conducted after the metal spraying.The voltage was 20kV.The analysis of microstructure test results is shown in figure 1-1 to figure 3-2.

As can be seen from SEM characterization,the sample DG-1 presents cryptocrystalline texture,which is allotriomorphic sheet shaped with small particle size and close arrangement,when it at 5,000 times magnification,the particles appear to accumulate in a semi-idiomorphic layer,with a small number of pseudohexagonal crystal particles ranging from 5 to 50 μm.The overall crystallinity is high,the crystal arrangement is dense.The sample QHD-1 is lamellar and arranges in an amorphous manner.When magnified to 2000 times,allotriomorphic texture,irregular arrangement and pseudohexagona crystal particles could be seen,when at 5,000 times magnification,full grain with the size of 3～5 μm can be seen.At 500 times,the sample MB-1 showed microscopic cryptocrystalline flake and granular shape with different grain sizes,disordered arrangement and loose structure.At 2000 times magnification,the crystal scales are stacked together and various impurities are distributed in different shapes,among which the perfect crystal shape impurity of pentagonal dodecahedron can be seen.

Pseudohexagonal sheet can be seen in all three samples,which is inferred to be kaolinite mineral[3],the highest degree of crystallization is lamplight steatite,the second is the horn steatite,and the calligraphy is the lowest.The transparency of the three samples is consistent with their crystallization degree,so it can be inferred that the transparency of the Changbai jade sample is related to the crystallinity,the higher the crystallinity,the better the transparency.Because of the impurity of calligraphy,the transparency of it is low.

Chemical composition test was carried out with energy disperse spectroscopy,and impurity parts of sample DG-1,sample QHD-1,sample MB-1 and sample MBZ were tested respectively.The following data were obtained as shown in table 3.

Figure 1-1 DG-1 sheets closely parallel arrange(500x)

Figure 1-2 DG-1 from idiomorphic to semi-idiomorphic pseudohexagonal sheet(2000x)

Figure 2-1 QHD-1 grains vary in size and are loosely arranged(500x)

Figure 2-1 QHD-1 pseudohexagonal sheet and allotriomorphic shapes irregular sheet(2000x)

Figure 3-2 MB-1 the scale of the sample varies in size and has loose structure(500x)

Figure 3-2 MB-1 sample showed pentagonal dodecahedron impurity(2000x)

Table 3 Chemical Composition List of Changbai Jade Sample

According to the kaolinite mineral chemical formula Al4[Si4O10](OH)8,the tested elements were chemically coordinated according to the percentage of atoms,in sample DG-1,sample QHD-1 and sample MB-1,w (Al2O3)was 42.7%～46.5%and w (SiO2)was 52.5～54.3%,with a basic proportion of 1:1,consistent with the proportion of kaolinite,silica and aluminum,which was speculated to be kaolinite mineral[4].The proportion of oxygen is obviously insufficient,but the specific reasons need to be further studied.In sample QHD-1,besides aluminum and silicon,a high proportion of C also appears.If C is in the form of carbonate([CO3]2-),the oxygen content is obviously not enough for the distribution of kaolinite mineral and carbonate,so C is considered to be in the form of elemental substance[5].There are a lot of P elements in MBZ-1,and it is speculated that phosphate exists.There are Fe and S elements in MBZ-2 and MBZ-3,which can be calculated as FeS from the chemical formula ratio.

IV.Conclusion

(i)The color of Changbai stone is mostly gray,light yellow,yellow brown,light red brown,gray black;the refractive index range is 1.56～1.57,the density is 2.52～2.81,and the hardness is 2.5～2.7,and the steatite stone is mostly translucent,the colored stone is more transparent,with the gloss of grease,earth and wax.

(ii)Changbai stone is mainly composed of kaolinite family minerals and contains phosphate,graphite,pyrite and other impurities.

(iii)Changbai stone is mostly cryptocrystalline texture,with the crystal particles ranged from 0.2～50 μm,presenting granular,flake,pseudohexagonal crystal shape can be seen under high power electron microscope.Some crystals with good crystal orientation are obviously arranged,while those with poor crystal degree are larger in diameter,loose in arrangement and disordered.