Geochemical and Mineralogical Investigation into Causes of Color in Pearl and Shell Materials

Abstract

Shells materials of mollusks, Mercenaria mercenria, Haliotis fulgens, Haliotis corrugate, and Perna canaliculus were employed in this thesis to develop feasible techniques to detect the causes of color in costly pearls. Samples of shell materials were prepared in the form of powders, extractions, and fragments for different analytical methods. The results of Infrared (KBr disc), X-ray Diffractometer (XRD) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectroscopic (EDS) analyses indicate that the major chemical composition of nacre of shells is CaCO3, mainly in the form of aragonite. The XRD results also indicate that there are calcite and dolomite present in the nacre layer of M. mercenaria shell. The EDS and Inductively Coupled Plasma-Mass Spectrometry (ICPMS) results indicate that the purple nacre layer of M. mercenaria contains lower concentration of calcium carbonate than the white layer. The Infrared and UV-Visible spectra of extracted solutions of purple and white M. mercenaria indicate that the compositions of extractions are similar except some varieties of amide molecules. But no specific molecular structures related to common organic pigments can be inferred from the spectra. The results of ICPMS and EDS analyses show that concentrations of transition metal and rare earth trace elements in the purple and white M. mercenaria have no significant differences within the limits of detection (ppm). The SEM images (magnifications up to 50000X) of the ridge and valley areas of M. mercenaria shell indicate that different sizes of grains make up the purple and white layers of M. mercenaria, and the sizes of particles in purple layers are two times larger than the sizes of particles in white layers (about 2.5 µm). The heat testing (L.O.I.,~2.5 Wt%) and the different shrinkage and expanding phenomena of the sample fabric of purple and white M. mercenaria under the various water vapor pressure in environmental SEM support the idea that the purple nacre layers in the M. mercenaria have 0.58 Wt% more organic content than the white ones. The different colors in the shells are not caused by the different concentrations of certain metal elements or the organic pigments. Optical effects caused by the structure may be the cause for the different colors in shells, but whether the different optical effects are caused by the crystal sizes of calcite and aragonite or the different content of organic proteins in the shells has not been established.