南疆和田玉戈壁料的宝石学特征

Gemmological Characteristic of Gobi Nephrite from South Xinjiang,China

和田玉戈壁料是和田玉按产状分类中的一种重要的品种,是原生矿经风化剥落搬运至戈壁滩,暴露于地表、经历长期风吹雨打风化吹蚀形成的,表面凹凸不平,油脂光泽极强,主要产区为南疆地区的若羌县、策勒县、叶城县、莎车县及喀什地区。选取了36件产自南疆的和田玉戈壁料样品为研究对象,采用常规宝石学测试、激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)、红外光谱,拉曼光谱等现代分析测试手段进行系统的研究。常规宝石学测试结果表明,南疆和田玉戈壁料(简称"戈壁料")为多晶质集合体,致密块状,块度一般不大。颜色多样,有白色、青白色、青色、黄绿色、浅褐色等,颜色分布均匀,部分玉质内部杂有白色和黑色斑点,常有暗色杂质沉淀裂隙或凹坑底部。样品为不透明-微透明,光泽较好,油脂光泽-玻璃光泽,折射率为1.61～1.62(点测),相对密度范围2.97～3.02,平均为2.97。戈壁料常呈次棱角状,有一定的磨圆度,形状大多为片状、薄板状,长厚比范围(2.11～8.85)变化范围很大,与其片状、薄板状的扁平状外形一致,长宽比范围为1.04～7.52,与和田玉子料长宽比范围较小(1.0～3.0)不同,戈壁料长宽比可以达到7.52,故可以见少量戈壁料为锥形、棍形外观。显微硬度仪测试结果显示,戈壁料的摩氏硬度为5.55～6.60(平均为6.03),受戈壁料表面凹凸起伏和不同位置硬度差异的影响,测试结果可能存在着一定偏差。超景深显微镜对戈壁料表面微观特征观察结果显示:和田玉戈壁料表面呈现独特的凹凸不平的形貌,跟子料相比高低起伏大,可存在较深的风化凹坑。大量的大小、形态各异的风化坑及被风沙打磨的光泽强的表面共同呈现出如经络般的外表。戈壁料表面棱线清晰,圆滑,面与面间有一定的弧度,棱角有一定的磨圆,呈现次棱角状。风化坑壁与接触面的过渡自然,有光滑的过渡面,光泽基本一致。风化坑内常有杂质矿物的沉积,并存在于裂隙之中,有水草状的侵染现象。存在扇形、阶梯状的风化面,有大凹坑里套小凹坑的现象。戈壁料独特表面形貌的形成与风沙的裹挟及摩擦等作用有关。LA-ICP-MS微区测试结果显示,和田玉戈壁料的主要化学成分为SiO2,MgO及CaO,其中SiO2的质量分数为57.6%～59.6%(平均58.6%),MgO的质量分数为23.4%～25.2%(平均为24.6%),CaO的质量分数为13.9%～15.1%(平均为14.6%),与透闪石的理论质量分数基本一致。微量元素包括Na(568.6～928.3μg/g),Al(2 815.2～6 286.3μg/g),K(286.4～905.1μg/g),Fe(771.3～16 972.8μg/g),Mn(201.5～1 741.2μg/g)等,其中,Na,K的质量分数要略低于山料,可能与戈壁料所产出的环境有关。随着Fe质量分数的增高,样品颜色变深。稀土元素的总量ΣREE低,轻稀土LREE和重稀土HREE分异不明显,曲线呈水平海鸥状,重稀土略微富集。有明显的Eu负异常,未见明显Ce异常,出现较杂乱的配分曲线的原因可能与戈壁料来源复杂有关系。南疆和田玉戈壁料的红外光谱显示了900～1 150cm^(-1)和400～760cm^(-1)范围内透闪石的特征吸收谱带。其中,与O-Si-O的反伸缩振动及O-Si-O对称伸缩振动有关的峰位位于1 142、1 090、1 039、995、918cm^(-1),与Si-O-Si对称伸缩振动有关的峰位位于756、680、661、640cm^(-1),因Si-O弯曲振动及M-O的晶格振动及OH平动产生的峰位为542、511、461、415cm^(-1)。由于受晶体结构中Fe^(2+)对Mg^(2+)的类质同象替代的影响,红外吸收谱带的峰位会出现小幅度波数的偏移。拉曼光谱谱峰主要出现在3 672、1 061、1 030、932、675、530、417、395、372、225、179、123cm^(-1)处,与和田玉山料的拉曼散射峰峰位较为一致。其中因M-OH伸缩振动引起的主要拉曼峰位为3 672cm^(-1),随着Fe质量分数的增加,在M—OH伸缩振动区会出现除3 672cm^(-1)峰之外的3 661cm^(-1)吸收峰。1 061、1 030、932cm^(-1)处吸收峰是因Si-O伸缩振动引起,675cm^(-1)是因Si-O-Si伸缩振动引起的,530cm^(-1)因Si-O弯曲振动产生,417、395、372、225、179、123cm^(-1)为晶格振动峰值。3 500～3 800cm^(-1)范围内反映了OH^-与金属离子成键信息。

Gobi nephrite was a significant type of nephrite.After weathering,exfoliation and transportation process,original deposit mines were carried to the Gobi desert,the Gobi nephrite surface was uneven with intense greasy luster by withstanding long-term weather near surface.The major production areas included Ruoqiang county,Cele county,Yecheng county,Shache county and Kashgar region.36 pieces of Gobi nephrite samples from south Xinjiang were chosen as research objects.Conventional tests,LA-ICP-MS,FTIR spectrum and Raman spectrum were applied to conduct a systematic study.From the conventional gemmological method results,the Gobi nephrite samples from south Xinjiang were polycrystalline aggregation.The structure was compact and the vast majority size was small-to-moderated.The colour of Gobi nephrite diverses,from shade of white,cyan-white,cyan,yellow-green to light grey and so forth.The colour distribution was even.Part of samples mixed with white and white spot inside and the dark inclusions were common in the botton of holes or in cracks.The samples are subtranslucent to opaque with greasy to glassy luster.The refractive index is 1.61^(-1).62(point measurement)and the relative density gravity is2.97-3.02,average 2.97.The Gobi nephrites are often subangular and have a certain roundness,and the shapes mainly were flaky and thick plate-like.The length-thick ratio of Gobi nephrite ranges from 2.11 to 8.85,which varied entirely.The length-width ratio of placer nephrite was restricted in a small range(1.0-3.0).In comparison to placer nephrite,the length-width ratio of Gobi nephrite could reach as far as 7.52.A minority of Gobi nephrite shows stick-like or awl-shaped appearance.According to microhardness system results,the Mohs hardness of Gobi nephrite ranges from 5.55 to 6.60(average 6.03).Due to the impact of uneven surface and hardness difference of Gobi nephrite,the testing results may exist some deviations.The microscopic characteristics on the surface of Gobi nephrite observed by the high resolution digital image technique show that the surface of Gobi nephrite presents a distinctively uneven appearance,which is more undulating than the nephrite placer,and there are deep weathering pits.A large number of weathering pits of different sizes and shapes and the surface with strong gloss polished by wind and sand present an appearance of meridian.The surface of the Gobi nephrite presents clear and smooth ridges,a certain radian between the surfaces,and rounded edges to a degree,showing a subangular shape.The transition of weathering pits and contact surface was natural with smooth surface and basically consistent gloss.Impurity minerals are often deposited in weathering pits,and impurity minerals often exist in fissures as well as a phenomenon of water-grass-like infection.There are fan-shaped,stepped weathering surfaces,and a phenomenon of big pits covering small pits.The formation of unique surface of Gobi nephrite is related to entrapment and friction of the wind and the sand.The results of the LA-ICP-MS testing results are shown that the main chemical compositions of Gobi nephrite were SiO2,MgO and CaO.The content of SiO2 is 57.6%-59.6%(average 58.6%),MgO is 23.4%-25.2%(average24.6%)and CaO is 13.9%^(-1)5.1%(average 14.6%),which are basically consistent with the theoretical content of tremolite.The trace elements include Na(568.6-928.3μg/g),Al(2 815.2-6 286.3μg/g),K(286.4-905.1μg/g),Fe(771.3^(-1)6972.8μg/g),Mn(201.5^(-1) 741.2μg/g),etc.,and the content of Na and K are slightly lower than that of mountainous nephrite,which may be related to the production environment of Gobi nephrite.As Fe content increases,the sample colour becomes darker.The amount of rare earth elementsΣREE is low,and light rare earth elements(LREE)and heavy rare earth elements(HREE)differentiation is not obvious,as the curve is like horizontal seagulls and heavy rare earth elements are slight enriched.There is obvious Eu negatively anomaly,no obvious Ce anomaly,and the reason for the disordered distribution curve may be related to the complexity sources of Gobi nephrite.Infrared spectra display the typical tremolite absorption between 900^(-1) 150 cm^(-1) and 400-760 cm^(-1).O-Si-O asymmetrical stretching vibration and O-Si-O symmetrical stretching vibration attribut to 1 142,1 090,1 039,995,918 cm^(-1).Besides,756,680,661,640 cm^(-1) peaks related to Si-O-Si symmetrical stretching vibration,as well as absorption bands at 542,511,461,415 cm^(-1) are resulted by co-operative work from Si-O bending vibration,M-O lattice vibration and OH translation.Infrared spectrum absorption band peak appears minor shifts as a result of the isomorphous substitution of Fe and Mg in crystal lattice.Raman spectrum mainly demonstrated typical peaks at 3 672,1 061,1 030,932,675,530,417,395,372,225,179,123 cm^(-1),in accordance with mountainous nephrite.M-OH stretching vibration mainly caused peak at 3 672 cm^(-1).The peak at 3 661 cm^(-1) in MOH stretching vibration region appears with the Fe concentration increasing.Peak at 1 061,1 030 and 932 cm^(-1) related to Si-O stretching vibration.Peak at 675 cm^(-1) is led by Si-OSi stretching vibration.Peak at 530 cm^(-1) is due to Si-O bending vibration.Peaks at 417,395,372,225,179,123 cm^(-1) are caused by lattice vibration.Bonding information of OH^- and metal iron revealed in 3 500-3 800 cm^(-1).