The phosphorus contents of alkali feldspars of 80 pegmatite veins from the Chinese Altay, NW China, were determined by electron microprobe in this study. The alkali feldspars of the barren pegmatites are poor in phosphorus and range from <0.01–0.05 wt% P2O5. Alkali feldspars from the fertile (rare-metal mineralized) LCT-class pegmatites are characterized by P2O5 in the range 0.10–0.40 wt% P2O5 for K-feldspars (Kfs), and 0.10–0.30 wt% P2O5 for albites (Ab). In the Qunkuer Be-Nb-Ta mineralized pegmatite, a value of 2.89 wt% P2O5 in K-feldspars was determined, the highest value of P2O5 in alkali feldspars reported.

According to the statistical results, 0.1 wt% P2O5 in alkali feldspars separate the fertile pegmatites from the barren pegmatites, and more than 0.1 wt% P2O5 in the alkali feldspars, especially in K-feldspars, can be used as an indicator for prospecting for LCT-class pegmatite-type rare-metal ore deposits.

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