A Comparative Study On The Structural,Micromorphological And Compositional Characteristics In Different Cycles Of Southern Pearl

In recent years,some high-quality Southern pearls have emerged in the market,rekindling the interest of consumers.Due to issues related to the cultivation environment and technology of Southern pearls,the production of high-quality pearls is limited,leading to reduced competitiveness.Currently,there is limited research on the different growth cycle characteristics of Southern pearls.In this study,seawater pearls from a pearl farm on Weizhou Island in Beihai City,Guangxi Province,China were selected as the research objects.Pearl samples in growth cycles of 2,4,6,8,10,and 12 months were collected.The microstructure,structure,and composition characteristics of Southern pearls in different growth cycles were investigated using scanning electron microscopy,laser Raman spectroscopy,Fourier transform infrared spectroscopy,X-ray powder diffraction,X-ray fluorescence spectroscopy,inductively coupled plasma mass spectrometry,and thermogravimetric analysis.The results showed that:(1)The appearance of Southern pearls was circular and white or light yellow in color,with a diameter of 6～8 mm and a smooth surface.Occasionally,minor flaws were observed.The thickness of the nacre layer ranged from 150～419 μm,and the pearl luster increased with increasing growth stage.The average monthly growth rate of Southern pearls in different growth cycles varied.The first two months after nucleation were the fastest growth period,with a growth rate of 75 μm/month,which then stabilized at 30～35 μm/month.(2)The pearl layers of Southern pearls in different growth cycles are composed of prismatic layer and nacreous tablet layer.For pearls in a growth cycle of 10 months,the pearl layer consists of alternating prismatic layer and nacreous tablet layer.For pearls in a growth cycle of 12 months,multiple prismatic layers,alternating with the nacreous layer,appear.The size of the hexagonal nacreous tablets of Southern pearls decreases with increasing culturing time,and the tablets become increasingly closely packed.The thickness of the nacreous tablets of Southern pearls in different growth cycles are related to the season,with thicker tablets secreted during the summer when the secretion rate of the nacreous layer is higher,and thinner tablets secreted during the winter when the secretion rate is lower.When the growth period is less than 8 months,in the growth cycles 10 and 12 months.The prismatic layer is a simple needle-columnar shape,which gradually transforms from needle-columnar to thick plate-columnar.(3)The characteristic peak of aragonite appears in the XRD spectra of Southern pearls in a growth cycle of 2 to 10 months,while the characteristic peaks of both aragonite and calcite appear in those in a growth cycle of 12 months.XRD spectra show that the(012)plane of Southern pearls are preferentially oriented,and the crystal cell parameters of Southern pearls are larger than those of other seawater pearls,making it more susceptible to twinning.(4)Southern pearls contain various elements,including Ca,Na,Mg,Sr,Ba,Zn,Ni,and Li,and the content of each element varies in the growth cycle.Na and Zn increase in the growth cycle,stimulating the activity of alkaline phosphatase(ALP)and increasing the deposition rate of the nacreous layer.Mg and Li decrease with the growth period,promoting the transformation of transient amorphous calcium carbonate(ACC)into calcite,resulting in the appearance of calcite in pearls in a growth cycle of 12 months.(5)The TG-DTG curves of Southern pearls in different growth cycles are similar,but the weight loss rates are different in different cycles,indicating that the calcium carbonate and protein contents of Southern pearls increase in the growth cycle.Particularly,the increase in protein content leads to a tighter connection between the surface aragonite platelets and a stronger luster.The research results provide a reference for analyzing the factors affecting the quality of Southern pearls and guiding the improvement of Southern pearl cultivation methods.These research findings can contribute to the analysis of the factors affecting the quality of Southern pearl and guide the improvement of their cultivation methods.