西藏“象牙玉”的矿物学及谱学特征

doi:10.3964/j.issn.1000-0593(2020)09-2908-05

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Abstract There is a new kind of magnesite jade which is white mostly and some part of it present red color in veins named ivory jade from Tibet. The structure is compact,and color is attractive.Thus it is an ideal stone to carve. The mineralogical and spectroscopic characteristics of the ivory jade are studied with the help of conventional gemological instruments, slice observation, scanning electron microscopy, X-ray powder diffraction (XRD), infrared spectroscopy (IR) and Raman spectroscopy. The gemological testing results show that specific gravity of ivory jade are 2.72~2.94, and reflection index is 1.68 (distant vision technique), strong white fluorescent light is observed under the ultraviolet lamp. There have not extinction characteristics of magnesite exists under the cross-polarized microscope, due to its granularity is too tiny. Quartz vein interpenetrates magnesite and magnesite cracks are filled by hematite. Backscattered electron imaging shows that thematrix is made of magnesite and microcrystalline quartz. There is also some larger size quartz (approximated 25 μm) spread in magnesite. Quartz and hematite are filled in cracks of magnesite as microcrystal. And few calcites are founded in magnesite. XRD testing confirms 3.34 and 4.25 Å which are characteristic diffraction peaks of quartz and 2.74，2.10 and 1.70 Å are characteristics diffraction peaks of magnesite. The FTIR spectrum present 886, 1 453 and 748 cm^-1 which are attribute to carbonate minerals characteristics absorb peaks and the frequency of ν₄（in-plane bending vibration）is inversely proportional to the radius of positive ion, therefore, 748 cm^-1 indicated the sample is magnesite; 1 089，1 165，798，779，694，515 and 465 cm^-1 are distinct peaks of quartz. According to Raman results, the white matrix is the mixture of magnesite and quartz. Transparency vein mineral is made of quartz. In the Raman spectrum of quartz, there are Raman shift 500 cm^-1 which attribute to moganite. In group minerals of quartzite jade, as the main component mineral is α-quartz, if it were contained a great amount of moganite, it should find low crystalline. In another way, the value of I₅₀₀/I₄₆₅ is inversely proportional to crystalline. Thus, both are cryptocrystalline quartz and crystalline in magnesite is lower than in the quartz vein. 1 317，655, 608, 492, 460, 406, 292, 242 and 222 cm^-1 are attributes to Raman shift of hematite. Because there are no biological indication exists, its cause of formation is not related to biogenic. Speculating it may be formed related to weathering-leaching react to ultrabasic.

Key words：“Ivory Jade”; Magnesite; Mineralogical characteristic; Spectroscopic characteristic; Dingqing county

Received: 2019-07-31 Accepted: 2019-12-06

ZTFLH:

P575.4

Corresponding Authors: CHEN Tao E-mail: summerjewelry@163.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)09-2908-05 OR https://www.gpxygpfx.com/EN/Y2020/V40/I09/2908