Mineralogical Characteristics of Changhua Tianhuang and Its Differences from Tianhuang

Changhua Tianhuang produced in Zhejiang Province is considered to be a new species 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 suburbs 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 Tianhuang samples (two light-yellow ones and four yellow ones), and thirteen pieces of Tianhuang samples (seven yellow ones, two white ones, three black ones, and a gray one) are studied by using the infrared spectrometer (IR), X-ray powder diffraction spectrometer (XRD), scanning electron microscope (SEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).All of the Changhua Tianhuang 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 Tianhuang 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 heterogeneous, 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 region have a little difference from kaolin minerals, which is not fit for polymorph identification.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 nacrite, while the main mineral component of Tianhuang is dickite, nacrite or illite. XRD is used to further analyze the mineral components of Changhua Tianhuang and Tianhuang.The major purpose of XRD in this study is to identify the small quantity of minerals in the samples.It is found that, in Changhua Tianhuang, a little amount of svanbergite exists in the skin and body of some samples, and alunite exists in some samples, and quartz exists in the skin of some samples.A little amount of svanbergite is found in three pieces of Tianhuang samples by XRD. The micromorphology of Chuanghua Tianhuang and Tianhuang are observed by SEM.It is found that dickite in Changhua Tianhuang grows well, and has euhedral-subhedral texture.The crystalline aggregate is book-like stacking, and arranges randomly in three-dimensional space.Dickite in some samples grows to be pseudohexagonal lamellar crystal.The crystal of kaolinite in Changhua Tianhuang has different grain sizes, and has subhedral-anhedral texture.The stacking crystals in the fresh fracture look like fragments of different sizes. However, dickite in Tianhuang is anhedral texture.The crystals stack randomly in the three-dimensional space.Nacrite in some Tianhuang samples crystalizes to be lamellae stacking 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, but 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 are absent 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 major 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 Tianhuang.When the major mineral component is dickite, the stone may be either of them.After analysis of mineral components, it needs to study the contents of trace elements by LA-ICPMS, and micromorphology by SEM to distinguish origin of Changhua Tianhuang and Tianhuang.