The Study On Chemical Composition And Spectroscopy Characteristics Of Color-Banding Tourmaline

Tourmaline is one of the most colorful monocrystal gems in the nature whose chemical composition and crystal structure is complex, and its isomorphous substitution is plentiful. Its different color-banding show the change of physical and chemical conditions in its process of growth. The color-bandings of tourmaline is concentric rings around the c-axis or along the c-axis, and the different colors’ partition is a clear dividing line or a gradual transition. Rely on gemology routine testing, microscopic observation, X-ray diffraction analysis, electron microprobe, LAICP-MS, infrared spectroscopy, Raman spectroscopy and UV-Vis-NIR, This thesis makes a comparison study on the components and spectral characteristics of colorbanding tourmaline, also explores the growth process of color-banding tourmaline.The tourmaline samples have the obvious color bandings, which are bicolor, multicolor and concentric rings around the c-axis. They strongly display the pleochroism and developed longitudinal grains. There are growing stripes and other micro-morphologies on the surface and fissures and rich inclusions inside.Electron microprobe analysis and triangular diagram of tourmaline components classification show that most samples belong to elbaite, while some data points show the corresponding tourmalines belong to liddicoatite. Tourmaline chemical composition and diagenetic environment relationship triangle diagram suggested that the color-banding tourmalines might grow in the zone of Li-rich granitoid, pegmatites and aplites. The approximate relationship of MnO and FeO is as follows, the content of FeO of green and black tourmalines are higher than MnO, while the content of MnO of pink tourmalines is higher than FeO. The percentage content change between MgO and FeO is roughly accord with synchronization relationship, while the percentage content change between MnO and TiO2 is nearly accord with the negative correlation, the content ratio of FeO/(FeO+MgO) is high, which reflects that the growth of tourmaline samples might be related to granitoid.X-ray diffraction analysis shows that the lattice parameters of tourmaline samples are the same as elbaite, but their c0 are lower than the ideal value. Infrared spectra confirmed that the sample is elbaite. Raman spectra shows that the increasing of the content of the element Fe might lead to the Si-O stretching vibration peak splitting, or suppress the [OH]- stretching vibration peak around at 3650cm-1.UV-vis spectroscopy shows that the color of pink tourmalines is related to Mn3+ d-electron transition. Fe3+ and Mn3+ cause the color of brown-red ones. Green tourmaline’s color is related to Fe, Cr, Ti and plasma valence, content, occupation and so on. The color of blue tourmalines may be causes by Fe2+-Fe3+ charge transfer, Fe2+,Cu and other ions. Yellow tourmaline’s color may be caused by Mn2+-O-Ti4+ charge transfer, Fe3+ or Mn2+.Due to the discontinuous changes of physical and chemical conditions, the growth process of "watermelon" tourmaline is discontinuous. Their growth stages include: growth- stagnation(dissolution)- particle adsorption- re-growth and so on. The color zones of color-banding tourmalines are related to the chromophorous elements,species, content, valence of chromophorous elements, their selective placeholder in the crystal structure, as well as the different placeholder and coordination.