Abstract:
Garnet deposits are widely distributed and it is an important gemmological and mineralogical research object. However, systematic studies on green garnets from the Luobusha area of Xizang are limited. In this paper, green garnet samples from the Luobusha area were tested by conventional gemmological instruments, electron microprobe, laser ablation inductively coupled plasma mass spectrometer, laser Raman spectrometer, scanning electron microscope-energy spectrometer, microscopic UV-visible-near infrared spectrometer, and single-crystal X-ray diffraction analyses, aiming to obtain the gemmological and mineralogical characteristics of garnets from this area. The results showed that the green garnet samples from Luobusha, Xizang, exhibit a colour range from light green to dark green with a relative densitie of 3.46-3.73. A large number of fibrous inclusions can be seen under magnified observation, which were confirmed to be tremolite by Raman spectroscopic analysis. The garnets contain a variety of inclusions such as diopside and actinolite, which indicated that the green garnets from Luobusha, Xizang, are produced in skarn deposit. The average crystal chemical formula of the green garnet, from electron microprobe test data calculation, is (Ca
2.581Mn
0.332Fe
0.094Mg
0.005)
Σ3.011(Al
1.560Cr
0.350Fe
0.097Ti
0.036)
Σ2.014Si
2.976O
12, suggesting that its main mineralogical component is grossular. The single-crystal X-ray diffraction analysis results showed that its lattice parameter is
a0= 1.184 3(2) nm. The ultraviolet absorption spectra showed that the main chromogenic elements are Cr
3+ and Fe
3+, in which the 438 nm absorption broad band is caused by the
6A1g(
6S)→
4A1g,
4Eg(
4G) leaps of Fe
3+ and the d—d electron spin-allowed leaps of
4A2g(
4F)→
4T1g (
4F),
4T2g(
4F) of Cr
3+. And the broad absorption band at 605 nm is generated by the corresponding leaps of Cr
3+ at the octahedral site.