Is the mineral selenite a kind of kryptonite for the extraterrestrials? Does it confer any protection against extraterrestrial beings or manipulation? Is there a mineral that will work better?

This is not the case. The idea that selenite has such protective powers is because it has a type of consciousness that can more readily attune to darker beings and interact with them. That is not the same as having the ability to exert control to limit their influence or exclude their presence. It is simply a characteristic of the energy they express, and is a misinterpretation of its meaning and its utility. Unfortunately, there is no natural repellant that can discourage, deflect, defeat, extraterrestrial incursion. This is why a divine solution is needed. There is no other good defense. All of the extraterrestrial beings are vulnerable physically and can be killed, the problem is they will not present themselves as targets. If they want to destroy humanity, it will be done indirectly and primarily through human hands by recruiting humans as mercenaries to carry out the deed through mind manipulation, and then these too, will be destroyed. And then, the deed will be done with no personal risk to the extraterrestrials themselves. In the same way that human consciousness cannot stand up to the extraterrestrial power by itself, any natural formation, animal, vegetable or mineral, so to speak, will not do so either. Only divine consciousness has greater power sufficient for this task.