Study On Gemmology And Mineralogy Of Tianhuang Stone

Tianhuang stone, from Shoushan brook, Fujian Province, is the most precious Chinese seal stone. It is not only expensive, but also has a profound cultural heritage. So the systematic research on gemological and mineralogical characteristics of Tianhuang stone has important theoretical value for scientific identification.The mineral composition, spectral characteristics and color genesis of Tianhuang stone have been studied by using the gem microscope, polarizing microscope, X-ray diffraction(XRD), fourier transform infrared spectroscopy(FTIR), Raman, laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS), electron paramagnanetic resonance(EPR) and scanning electron microscope(SEM). It is also discussed the mineralogical characteristics of red crack, turnip vein, wreathed skin and the formation mechanism of Tianhuang based on the above tests.Tianhuang stone can be divided into three types, dickitic type, nacritic type and illitic type according to XRD results. Tianhuang stone usually contains two minerals. Dickitic type Tianhuang can be divided into ordered dickitic and disordered dickitic. The main mineral composition of illitic type Tianhuang stone is2M1illite, while contains a small amount of1M illite.Ordered dickitic type, disordered dickitic type and nacritic type Tianhuang stones have the same IR spectra in the fingerprint area, but have obviously different spectra in functional group area. The characteristic IR absorption peaks of dickitic type Tianhuang stone are at3621cm-1、3653cm-1、3704cm-1/3696cm-1. Compared with ordered dickite, the band assigned to OH3of disordered dickite shifts to low-frequency by8cm-1, and its relative intensity becomes stronger. The characteristic IR absorption peaks of nacritic type Tianhuang stone are at3701cm-1、～3649cm-1、3629cm-1. The IR absorption spectra of nacritic type Tianhuang stone is superimposed strong peaks of dickite, indicating that the intensity of IR absorption bands of dickite are stronger than that of nacrite at3550cm-1～3750cm-1. The characteristic IR absorption peak of illitic type Tianhuang stone is at3631cm-1. The Raman spectra and IR spectra of Tianhuang stone are consistent. The characteristic Raman peaks of dickitic type Tianhuang are at3626cm-1、3648cm-1、3706cm-1/3698cm-1, the characteristic Raman peaks of nacritic type Tianhuang are at3627cm-1、3646cm-1、3703cm-1, the characteristic Raman peak of illitic type Tianhuang is at3632cm"1. Raman spectra can easily provide the information of main mineral composition of Tianhuang stone, and it is a good and convenient method to identify Tianhuang stone, because Raman spectra can do non-destructive and fast identification.LA-ICP-MS and total iron analysises indicate that Fe3+ion causes Tianhuang stone to take on yellow. EPR study shows that iron exists in Tianhuang stone in free state. The author considers that iron oxides and/or iron hydroxides adsorb at the surface of layer silicate minerals grains, which leads to secondary color. The EPR signal at g≈4.21is attributed to Fe3+which replaces Al3+by the way of isomorphous substitution. And the structure iron causes the structure of dickite to be disordered.The micro-characteristics of different types of Tianhuang stones are studied by using SEM. The crystal shape of dickite and nacrite mainly-show flaky with good crystallinity, while some crystals show hypidiomorphic false hexagonal flake. The size of crystal particle varies from a few microns to several tens of microns. The crystals are different in thickness, and are poor in orientation and have dense structure. The crystallinity of illite in Tianhuang stone is good. These crystals have dense structure. The orientations of the lamellae crystals are nearly parallel.Red crack is caused by iron minerals which entered along cracks, and iron solution precipitated in the secondary environment. The iron minerals display three forms:thick film-state, short needles, film or a fine granular.Observed by using gemology microscope, there is no obviously boundary between matrix and turnip veinin in Tianhuang stone. SEM, EDS and Micro-raman spectra results show that svanbergite is the main mineral composition in turnip vein. Svanbergite formed before the mineral composition of matrix of Tianhuang stone. Therefore, turnip vein is a primary structure in Tianhuang stone.It is indicates that the weathered skin and matrix have the same mineral composition by using XRD and FTIR.1CP-MS result suggests that iron content in skin is higher than matrix. The author consideres that weathered skin formed by the microbial weathering in the acidic environment, accompanied by chemical and physical weathering. Surface minerals dissolved in the weathering process, so the structure becomes loose and has voids, accompanied with gradually filled Fe ions.The formation of Tianhuang stone underwent two stages, primary mine generation and secondary alteration. The cause of formation of primary mine is volcanic hydrothermal filling. The main minerals are the product of hydrothermal alteration in the later period of the first stage. The formation of illite is earlier than that of dickite and nacrite. Underwent peeling and buring and tansformation in supergene environment, the primary mine changed to be weathered sedimentary Tianhuang deposit.