A very rare example of a mineral I had not heard of until now, Copper Chloride pure and simple: CuCl. This is according to MINDAT a rare anhydrous copper chloride, known only from arid environments in its natural form. The description goes on to say that it is "colorless and transparent when fresh. Unstable in humid air, and transforms slowly into greenish secondary Cu-hydroxchlorides" which overlay the natokite now, on its surface. However the crystal cores should remain nantokite, and there should also be material inside this rather large study specimen. From the type locality!