Study On Composition And Structure Character Of Iridescent Labradorite

Iridescence is frequently used to describe any diffraction and/or thin-film interference-related color phenomena. Labradorite which is a plagioclase feldspar of the triclinic class is well known for the striking iridescence which it exhibits. What is the material which is responsible for the iridescence of labradorite and why is it present? Until 1970s to 1980s, generally believed that the origin of its iridescence is caused by the inner periodic lamellar structures. However, there are several periodic lamellar structures within labradorite such as cleavage, twinning and exsolution. Which one is responsible for the iridescence is not clearly until now.To understand the origin and distributed characteristics of iridescence in labradorite, microscope, XRD, EPMA, SEM, TEM, STEM and SAED have been taken in the paper. The result indicated that inner exsolution lamellar structure of labradorite causes interference that contributes to the iridescent colors. Furthermore, each unit exsolution lamellar structure is made up of calcic-poor and calcic-rich plagioclase feldspar. Only when thickness of the exsolution lamellar structure varies between 128 nm and 292 nm, the interference colors are visible. Because the surface of exsolution lamellar is not absolutely smooth, iridescence always looks like contour line of map. Additionally, the thickness of exsolution lamellar is related to calcic content, the more the calcic content, and the thicker the lamellar, the longer the wavelength of interference color. The selected area electron diffraction (SAED) shows that the direction of exsolution lamellar structure is ( 15 3) which nearly parallel to (010). The uniform interference color may have different calcic content, it varies from place to place and from environment to environment. But it is determined that the relationship between iridescence and calcic content of labradorite is positive during the same place.Based on the research of labradorit's composition and structure, this paper reviews the latest researches of other iridescent gems. As a result, if the iridescence is produced by thin-film interference, not only the thickness of lamellar structure must be defined within a certain nanometer, but also the refractive index need to be different between lamellar structure. In other words, the chemical composition, refractive index and thickness of exsolution lamellae need form to trinity, such as labradorite, albite, andradite and orthoamphibole. If the iridescent color is produced by grating-diffraction, the lamellar structure just needs to be arrayed regularly, the spacing of the edges of lamellar structure needs to be regular and narrow, but not necessarily within the range of the wavelength of visible light, such as shell of abalone, pearl and opal.