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UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-22 1:49 |
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Conference: Bucharest University Faculty of Physics 2015 Meeting
Section: Atomic and Molecular Physics; Astrophysics
Title:
EPR Evidence of Crystallographic Ordering in the Pearl Nacreous Layer
Authors:
V. V. GRECU(1), Maria Nicoleta GRECU(2), Mihaela Raluca SLAVE(3)
Affiliation:
1) Physics Faculty, University of Bucharest, 2) National Institute of Materials Physics, Magurele-Bucharest, 3) Naval Academy “Mircea cel Batran”, Constantza
E-mail
vvgrecu@gmail.com
Keywords:
Electronic Paramagnetic Resonance
Abstract:
Pearls, produced by a number of mollusk species, are widely used in fashion industry. Most of them are bead cultured pearls. They consist of a bead nucleus surrounded by a nacreous layer. Its structure is mainly formed of aragonitic platelets, arranged in a brick like lamella interwoven by an organic matrix, conchinolin [1,2]. XRD and electronic microscopy have demonstrated that the aragonite platelets are 0.35-0.5 µm thick and 3-10 µm wide. The crystallographic c-axis points approximately perpendicular to the shell surface (a random orientation of about 100-200 of vertical is also observed). In this paper EPR spectrum of Mn(II) ions, present as impurity in the nacreous shell, is used to prove the texture of aragonite platelets [3]. Manganese ions are very sensitive to local crystallographic structure; in aragonite they occupy substitution position having an orthorhombic symmetry [4]. In the aragonite component of pearls Mn(II) was observed, in powder samples, in the form of complex spectrum [5,6, and references there in]. The fine structure parameters, D and E, were determined using forbidden line intensities. Evidently, no information on c-axis orientation was accessible. We have used a sample cut from the pearl in the form of a slightly curved sheet. The aragonite microcrystals have an almost perpendicular orientation to the sheet surface. The Mn(II) EPR spectrum shows resolved fine structure terms. Its angular dependence can be explained in terms of the EPR z-axis orientation in single crystal, and a random orientation of the other axes in a plane perpendicular to c-axis. This result, for the first time mentioned in literature, opens the possibility to use EPR technique to study the pearl homogeneity in correlation to its quality as a gemstone.
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