Biotite grains from a variety of gneiss types were analyzed by electron microprobe techniques
for Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe, Zn, F & Cl. TiO_2- and Al_2O_3-contents range continuously
from < 0.1 to 6.0 and 13.9 to 20.6 wt % respectively. Most occur with ilmenite, but a few coexist
with rutile. Biotite in pyribolite contains the least Al, and that in Kfeld-sill gneiss
contains the most, suggesting that Al-content of biotite is related to the bulk composition of the
host rock. For a given rock type, Ti in biotite tends to decrease as Mg/Fe and Al increase
