Mineralogical Characteristic and Colour Zonation of Malachite in "Yudian" Turquoise from Xunyang, Shaanxi Province, China

A novel type of "Yudian" turquoise samples(turquoise with raindrop-like patterns) obtained from the jewelry market were investigate by a series of modern analytical techniques, including Raman spectroscopy, micro-X-ray diffraction (μ-XRD) and scanning electron microscopy (SEM). The results revealed 3 distinct types of growth ring structures within the raindrop-like pattern areas, distinguished by the number, width, and colouration of the areas they occupy. Micro-area XRD analysis at the growth ring sites confirmed the presence of turquoise and malachite. Raman spectroscopy verified the blue matrix of the growth rings as turquoise, with characteristic peaks at 421, 644, 814, 1 041 cm^-1 and 3 470 cm^-1. The green regions exhibiting additional Raman shifts at 178, 1 495 cm^-1 and 3 377 cm^-1, consistent with malachite. Backscattered electron imaging under SEM showed that turquoise displays regular columnar and flaky morphologies, while malachite appears as irregular granular and amorphous aggregates filling the interstices between the turquoise flakes. Energy-dispersive X-ray spectroscopy (EDS) further confirmed that the Cu content in the botryoidal malachite reaches 68.67%. The findings indicated that the growth ring structure represents a symbiotic assemblage of turquoise and malachite. The formation of these growth rings is attributed to variations in the crystallinity and compactness of the turquoise matrix. In regions that turquoise is well-crystallized and densely structured, malachite content is low and its grain size is small, resulting in light green to blue bands. In contrast, areas with a relatively loose flaky structure contain more malachite with larger exposed grains, leading to darker green colouration.