Barium partition coefficients for phlogopite crystallized from a natural lamproite at 10-30 kbar and 900-1150°C show a three-fold decrease in partition coefficient (D) with increasing pressure from 10 to 30 kbar (e.g., from 0.75 to 0.23), and 35-50% increase in D with increasing temperature from 1000° to 1100°C at 10 and 15 kbar (e.g., from 0.51 to 0.75). The variation with temperature is contrary to most recorded variations of D, and is attributed to a compositional control in the phlogopite. D correlates with substitution of titanium in the phlogopite. Thus as the titanium content increases with increasing temperature, the barium content, and hence D, increases. Application of partitioning data for barium in models for lamproite genesis suggests that phlogopite of metasomatic origin, and possibly some barium-rich accessory phases, are present in the source rocks for lamproite, and phlogopite plays a major role in the generation of lamproitic magma.