Study on Petrological and Mineralogical Characteristics of Laos Stone by EPMA-XRD-SEM

Since the newly-discovered Laos stones have highly similar appearances with Shoushan stones, it is difficult to correctly identify and evaluate them.Electron Microprobe(EPMA), X-ray Powder Diffraction(XRD) and Scanning Electron Microscopy(SEM) were used to study the chemical composition, mineral components, and microstructure of the Laos stone.Results show that the Laos stone is mainly composed of kaolinite, dickite and their transitional minerals with minor nacrite.There is a positive correlation between the red color and the yellow color and the amount of impurity element Fe, which indicates that these colors are possibly caused by Fe.Laos stone's crystals are mostly irregular platy or uninterrupted pseudo-hexagonal platy.The smaller size the crystals have, the worse porosity of its crystals, leading to the finer texture and better seal engraving experience and therefore a higher value.The density(2.58-2.61 g/cm³), refractive index(1.559-1.570) as well as other conventional gemological characteristics of Laos stone are similar to todickite-type Shoushan stones.It is suggested that Laos stone with bright color and fine texture should be used as a new source of high-quality seal stones.