昌化田黄的矿物学特征及其与田黄的区别

对昌化田黄和田黄进行了红外光谱（IR）、X射线粉末衍射（XRD）、扫描电子显微镜（SEM）和激光剥蚀等离子质谱（LA-ICP-MS）测试,以获得两者的矿物学和地球化学特征,并对比研究它们之间的差别.研究表明,昌化田黄主要由地开石或者高岭石组成,而田黄主要由地开石、珍珠陶石或者伊利石组成.田黄和昌化田黄样品中均可含有少量的硫磷铝锶矿.有的昌化田黄中存在微量明矾石,其石皮中含有微量石英.昌化田黄中的地开石晶体颗粒为自形半自形结构,集合体呈书本状叠置排列；田黄中的地开石大多为他形结构,晶体颗粒在三维空间无序堆叠.田黄中P,V,Cr,Ga和Sr元素的质量分数低于昌化田黄,而Rb元素的质量分数明显高于昌化田黄.V-Rb,Cr-Rb,Ga-Rb元素投点图可区分昌化田黄和田黄品种.另外,昌化田黄石皮中Mg,Cr,Fe和Sn元素的质量分数高于其石肉.昌化田黄和田黄皆为轻稀土富集,重稀土亏损.昌化田黄比田黄的REE富集程度高.当所测主要矿物组成为非地开石,如珍珠陶石或者伊利石时,可能为田黄；如为高岭石时,可能为昌化田黄.当所测主要矿物组成为地开石时,两者皆有可能.矿物组成测试结合微量元素的质量分数和微形貌特征可对田黄和昌化田黄进行产地鉴别.

Mineralogical Characteristics of Changhua Tianhuang and Its Differences from Tianhuang

Changhua Tianhuang produced in Zhejiang Province is considered to be a new spe- cies of Tianhuang, which has recently appeared on the market. According to our tradition, Tianhuang is a special Shoushan stone which is produced in Shoushan brook in the north sub urbs of Fuzhou, Fujian Province. The price of Tianhuang and Changhua Tianhuang has great differences, and therefore it is very important to identify the origin of the stones. The gemmological and mineralogical characteristics of Tianhuang have been widely studied, while the study of Changhua Tianhuang is much less. In this paper, six pieces of Changhua Tian- huang samples （two light-yellow ones and four yellow ones）, and thirteen pieces of Tian-huang samples （seven yellow ones, two white ones, three studied by using the infrared spectrometer （IR）, X-ray （XRD）, scanning electron microscope （SEM） and laser ab mass spectrometry （LA-ICP-MS）. All of the Changhua T black ones, and a gray one） are powder diffraction spectrometer ation inductively coupled plasma anhuang samples are raw stones with different thickness of the stone skin. The psephicity of these raw stones is poor, and they are of subangular shape. In the tests some of Tianhuang samples are fragments of raw stones, and some are only small pieces or a handful of powder obtained from carved Tian- huang crafts. Most of Tianhuang samples have no stone skin, a few have thin stone skin. With an overall observation, the yellow part of Changhua Tianhuang distributes heterogene- ous, and the body of the stone looks relatively dry. However, the yellow part of Tianhuang body distributes homogeneous, and the texture is fine. According to the IR results of Changhua Tianhuang and Tianhuang samples, both of them are mainly composed of kaolin minerals, while absorption peaks in the fingerprint re- gion have a little difference from kaolin minerals, which is not fit for polymorph identifica- tion. However, these polymorphs are easy to be distinguished by differences in the position, relative intensity, and splitting degree of OH--stretching bands. Based on the characteristics of OH--stretching bands, it can be deduced that the main mineral component of Changhua Tianhuang is dickite or kaolinite, without naerite, while the main mineral component of dral texture. The stacking crystals in the fresh fracture look like fragments of different si- zes. However, diekite in Tianhuang is anhedral texture. The crystals stack randomly in the three-dimensional space. Nacrite in some Tianhuang samples crystalizes to be lamellae stac- king randomly. Illite in Tianhuang is anhedral texture and stacking randomly. Therefore, the micromorphology of dickite is different between Changhua Tianhuang and Tianhuang. The contents of trace elements and rare earth elements （REE） are tested by LA-ICP- MS. According to chemical compositions of Changhua Tianhuang and Tianhuang, the con- tents of P, V, Cr, Ga, and Sr in Changhua Tianhuang are higher than those in Tianhuang, hut the content of Rb is obviously lower than that in Tianhuang. The element contents can be used in origin identification synthetically. Plots of V--Rb, Cr--Rb, Ga--Rb can also be used to distinguish Changhua Tianhuang from Tianhuang. On the other hand, the contents of Mg, Cr, Fe, and Sn in the skin of Changhua Tianhuang are higher than those in the body. Fractionation of light rare earth element （LREE） and heave rare earth element （HREE） are obvious in both Changhua Tianhuang and Tianhuang. Two species are rich in LREE, and areabsent in HREE. The degree of REE enrichment in Changhua Tianhuang is always higher than that in Tianhuang. From the analyses above, it can be concluded that absorption peaks in functional group region of IR can distinguish kaolin polymorphs. This method can be used to identify the ma- jor mineral components of Changhua Tianhuang and Tianhuang rapidly and accurately. Therefore, when the major mineral components is not dickite, for example nacrite or illite, the stone may be Tianhuang, but when it is kaolinite, the stone may be Changhua Tian- huang. When the major mineral component is dickite, the stone may be either of them. Af- ter analysis of mineral components, it needs to study the contents of trace elements by LA- ICP-MS, and micromorphology by SEM to distinguish origin of Changhua Tianhuang and Tianhuang.