Is it possible to react Rb + Al2O3?

I am wondering if the Rubidium ($\ce{Rb}$) in gas form can react naturally with Sapphire crystal ($\ce{Al2O3}$)?
The temperature is $500\ \mathrm{K}$ and the pressure is around $1\ \mathrm{Pa}$ to $10\ \mathrm{Pa}$.


One of the reaction that comes into my mind is
$$\ce{4 Rb + Al2O3 -> 2 Rb2O + Al2O}$$


I was trying to calculate the Gibbs free energy of the reaction, but I could just find the Gibbs free energy for Rubidium and the Sapphire, which is $\Delta G\_{\ce{Rb}}=53.1\ \mathrm{kJ/mol}$ and $\Delta G\_{\ce{Al2O3}}=-1582.3\ \mathrm{kJ/mol}$. I can hardly find the Gibbs free energy for the products.


Is there any other reaction that can occur naturally?


