Study on Petrological and Mineralogical Characteristics of the Ningxia Helan Stone by X-ray Fluorescence Spectrometry and X-ray Diffraction

The Ningxia Helan Stone is one of the well-known stones that are used for making inkslab in China. The development and application of Helan Stone is becoming more and more widespread, and therefore, it is necessary to carry out a systematic study. At present, there are only preliminary artificial experiments and systematic study of mineralogy is lacking. The conventional gemological test method is combined with spectroscopic analytical methods such as optical thin section identification, X-ray Fluorescence Spectrometry (XRF), X-ray Powder Diffraction (XRD) to study the gemological characteristics, chemical components, rock structure, and the mineral compositions of the Ningxia Helan Stone and is reported in this paper. The results show that the minerals of the Ningxia Helan Stone include major hydromica and sericite, and minor ferric oxide, sandy clastics, chlorite, and trace amounts of hematite, rutile, and tourmaline. The Ningxia Helan Stone has chemical components including SiO₂, Al₂O₃, TFe₂O₃, K₂O, MgO, TiO₂, CaO, P₂O₅, a refractive index of 1.56-1.57, a density of 2.81-2.86 g/cm³, and a mohs hardness of 3-4. The main color-causing factor of the Ningxia Helan Stone is also discussed. Based on the correlation between the morphology and content of Fe element and the color of the Helan stone, it is considered that Fe is the main color-causing element. The most characteristic parameters of Ningxia Helan Stone have been determined in this study, which provides technical support for follow-up accurate nomenclature and identification method for the Helan Stone.