# What's the partition coefficient (D) of Nb for apatite?

Was there any experimental study that determined $D$ for apatite and silicate liquid?

The GERM Kd database only lists one study with carbonatite liquid, but that's not what I'm looking for.

Yes, in the paper Trace element partitioning between apatite and silicate melts (Prowatke and Klemme, 2006), where they present experimental apatite/melt trace element partition coefficients for a large number of trace elements, including $\ce{Nb}$. The experiments were performed at pressures of 1 GPa and at a temperature of 1250 C.
• An average of $D$ = 0.0018 for $\ce{Nb}$ (this value is an average of Table 5: Trace element partition coefficients between apatite and different silicate melt of the linked document).
• $D_{Nb} \approx D_{Ta}$