Pure phase reference intensity ratio determinations were completed on eleven biotites ranging in Fe composition from χFe = 0.058 to χFe = 0.695, where χFe = Fe/(Fe + Mg). The crystal chemistry of biotites requires that the intensity of diffraction for the 00l reflections, and thus the corresponding reference intensity ratio, correlate with the Fe, Mg and Al in the octahedral sites. As Fe occupancy increases in the octahedral site, the Lki for the 00l reflections increases as does the value of ∣F∣2 computed using the Takeda and Ross (1975) biotite model. The ∣F∣2 trend closely parallels the experimental regression of Lki = 6.359χFe + 1.513 (R = 0.91). Observed mass absorption measurements completed by X-ray transmission show a similar trend with μo = 69.36χFe + 47.85 (R = 0.97). Calculated values of μ from biotite chemical analyses agree well with the measured values. These relationships may be used to predict the appropriate Lki needed for quantitative analysis of biotite-bearing specimens if the Fe and Mg content of the biotite can be determined.