Some optical properties of biominerals

Abstract

Calcium carbonates are materials found abundantly in the nature as minerals and biominerals. In both cases calcite and aragonite isomorphic forms can be found, where the calcite form is the most stable. In calcium carbonates (in the calcite form), calcium can be replaced by Mn and Fe and, in the aragonite form, calcium can be replaced by Sr, Pb and Zn [1]. In this work, oyster shell, coral, tiger shell and mother-of-pearl samples with predominantly aragonite isomorphic forms were studied. X-ray fluorescence measurements showed the presence of CaO, Na2O, SrO, SO3, SiO2 and Fe2O3 elements in all samples. The results of a neutron activation analysis showed, besides of the Ca element, the presence of Mn, La, Ce, Cr, Ba, Fe, Zn elements. The presence of Mn in the coral and mother-of-pearl samples was confirmed by the electronic paramagnetic resonance technique due to Mn2+ transition characteristics. Optical absorption measurements were taken for all samples in the wavelength range of 190 nm to 900 nm. All of the Mn signatures are transitions from the fundamental states 6A1g(S) to the energy states of 4T1g(G); 4Eg(G); 4A1g(G); 4T2g(D) or 4Eg(D); 4A2(G). The spectrometer UV-2120PC, model SHIMADZU Company, was utilized with two light beams in this study. The optical absorption measurements were carried out in powder samples with 0.177 mm of diameter, and with vegetal oil, transparent in the region of interest; they were placed between two quartz plates with 89% of transmittance in the ultraviolet region. The results show that the studied samples present amazing optical properties with good ultraviolet absorbance and with a great potential to be used even as radiation detectors.