The Study On Gemological Characteristic And Color Genesis Of Orange To Red Series Of Garnets

The same series of garnets have a widespread phenomenon of isomorphismphase replacement, so it is difficult to rely on the color of garnets to accurately distinguish the subspecies of them. According to that, the orange to red garnets which often appear in the market are selected in this paper, and the nondestructive IR spectra testing, Raman spectra testing, UV-Vis spectra testing, magnetic susceptibility testing and the small-invasive electron microprobe testing are used to research and summarize spectrum and magnetic susceptibility characteristics of this series of garnets. Nondestructive testing of the color characterization of the orange to red series garnets is proceed by Color i5 spectrophotometer based on CIE 1976 L*a*b* uniform color space according to which this paper has the preliminary research and discuss on this series garnets’ color genesis and characteristics.The testing results show that, with the increase of the major divalent cations’ radius of the orange to red series of garnets, the infrared vibration peak of the Si-O bonding shifts to short-wave direction in the range of 500 cm^-11100cm^-1. Combined with electron probe data, the height of the Spy/Alm or Spy/（Alm+And） value determines whether the garnets will appear the D peak in the range of 620 cm^-1630cm^-1. The relationship between the above two is: the vibration intensity of D peak increases with the decrease of Spy/Alm or Spy/（Alm+And）ratio.In the orange to red series of garnets, different subspecific garnets have different Raman spectra. Through the summary, the Si-OStr bonding and Si-OBend bonding characteristic vibration peak of different subspecific garnets would shift to the shortwave direction with the increase of the crystal unit cell parameters. In the range of 1000 cm^-11100cm^-1, the correlation coefficient between the unit cell parameters and the SiO bonding non-bridge oxygen stretching vibration mode A1 g left side vibration peak is-0.993 which is a significant negative correlation. Therefore, this peak can better help to distinguish the garnets subspecies.Spessartine, Almandine, Pyrope and Grossular are all paramagnetic minerals, and their change trend of mass susceptibility data is: χMn>χFe>χMg>χCa. Therefore, in addition to the traditional methods of testing garnet subspecies, the magnetic susceptibility can also be introduced to distinguish the subspecies of the orange to red series of garnets.In the compositions of pyrope and almandine, there is just a low negative correlation between the content of Fe²⁺ and ho. So, the color performance of pyrope and almandine may is the result of Fe²⁺, a little Fe³⁺ and Mn²⁺ forces. In the compositions of Grossular, there is a positive correlation between the content of Fe³⁺ and L*, a*, b*, C*. In the compositions of Spessartine, with the increase of Mn²⁺ content and the decrease of Fe²⁺ content, the b*, C*, ho of Spessartine all increased. After changing the light source, if the radiation of the light source increases the warm color tone, garnet samples with more red tone degree will have a higher color matching. Otherwise, the color mismatch is more obvious. And the orange to red series of garnets have no obvious correlation between color and magnetic susceptibility. That is to say, garnets color do not vary with the change of the magnetic susceptibility.