SUMMARY

Structural state (Al-Si order) of green feldspar in gem- bearing pegmatite from Lục Yên, Yên Bái

Ngụy Tuyết Nhung, Nguyễn Thị Minh Thuyết

Twenty feldspar samples with different clarity and color varying from green to blue were collected from Lục Yên gemstone bearing pegmatite body to determine the Al-Si order by X-ray powder diffractometer (XRD). The results show that the order of feldspar represented by the value of Σt1 varies in width from 0.71 to 0.95, the triclinicity ranges from 0.23 to 0.91, corresponding to orthoclasse and microcline, which means that the Lục Yên green feldspar exists simultaneously in two polymorphic forms as monoclinic and trinclinic. The chemical composition of four orthoclasse and microcline samples determined by electron microprobe analysis (EPMA) indicated that the content of Pb and rare alkaline elements such as Rb and Cs in orthoclasse was higher than that in microcline. These ions prevent Al during the ordering process. Gem quality (including the color and transparency) of orthoclasse was higher than that of microcline. Green color is expressed both in orthoclasse and microcline, therefore the authors of this paper recommend that the name of amazonite should be used for both of these minerals, as well for the green feldspar in Lục Yên area.

Keywords: green feldspar, pegmatite, Lục Yên, Yên Bái.