Study on the Unique Mineral Microstructure of Seawater Cultured Gray Akoya Pearl by SEM, FTIR and Reflection Spectroscopy

FANG Biao; YAN Xue-jun; SUN Qing; WU Jing-yi; LI Shu-hua; YAN Jun

doi:10.15898/j.cnki.11-2131/td.201908200124

Rock and Mineral Analysis > 2021 > 40(1): 42-49. > DOI: 10.15898/j.cnki.11-2131/td.201908200124

FANG Biao, YAN Xue-jun, SUN Qing, WU Jing-yi, LI Shu-hua, YAN Jun. Study on the Unique Mineral Microstructure of Seawater Cultured Gray Akoya Pearl by SEM, FTIR and Reflection Spectroscopy[J]. Rock and Mineral Analysis, 2021, 40(1): 42-49. DOI: 10.15898/j.cnki.11-2131/td.201908200124

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Study on the Unique Mineral Microstructure of Seawater Cultured Gray Akoya Pearl by SEM, FTIR and Reflection Spectroscopy

1.
Zhejiang Fangyuan Testing Group Co., LTD, Hangzhou 310013, China
2.
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
3.
Wenzhou Institute of Science and Technology, Zhejiang University of Technology, Wenzhou 325024, China

More Information

Received Date: August 19, 2019
Revised Date: January 30, 2020
Accepted Date: May 12, 2020
Published Date: January 27, 2021

Abstract

HIGHLIGHTS
(1) The discovery of a brown organic transition layer between the nacre and the nucleus.

(2) Amorphous structural morphology composed of aragonite and calcite were found in nacre near the nucleus.

(3) The reflection spectra of the inner concave surface of nacre with or without a brown interlayer are almost the same, which is consistent with the reflection spectrum of the whole pearl.

ABSTRACT

BACKGROUNDSeawater cultured gray Akoya pearls have become popular as jewelry in the recent years. In the early stage, some research focused mainly on investigating the cultured environment of seawater or freshwater pearls, element occurrence characteristics of each structural unit, irradiation treatment and the identification method of irradiated pearls.
OBJECTIVESTo further study the gemological characteristics and fine microstructure of a type of gray pearl with a white nucleus.
METHODSUltraviolet-visible reflection spectrum, micro-infrared spectrum and scanning electron microscope methods were used.
RESULTSA brown transition layer of organic matter between the nacre and nucleus was discovered, which measures several microns in thickness. A layer with no fixed morphology composed of calcite and vaterite in the nacre near the brown transition layer was also discovered. Quasi plates of aragonite exist in the nacre near the surface of the pearl. The morphology of these aragonite tablets in the middle area of the nacre was more regular, the thickness of individual aragonite plate gradually decreased in the direction from the nucleus to the surface of the pearls. The reflectance spectrum of the entire pearl surface was consistent with the spectral characteristics of the outer single nacre. The brown transition layer had no direct effect on the UV-Vis reflectance spectrum of the entire pearl. Therefore, whether or not the brown transition layer affected the gray appearance of the pearl needs further discussion.
CONCLUSIONSThe research work has important guiding significance for the coloring mechanism of gray Akoya pearls and the identification of the formation attributes. It can also aid in the recognition of the fine structure and mineralization characteristics of pearls with a thin layer of nacre of 0.3mm to 0.6mm.
- gray pearl,
- microstructure,
- transition layer,
- nacre,
- UV-Vis reflectance spectroscopy,
- scanning electron microscope

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