E-Mail IMAGE: (a) Schematic drawings of the sample preparation processes. (b) (i) Cryo-EM image of the Ca-Cl crystals in the ultra-thin reduced graphene oxide (rGO) membrane. (ii) Diffraction pattern of a typical... view more Credit: @Science China Press Calcium ions are presented in rocks, bones, shells, biominerals, geological deposits, ocean sediments, and many other important materials. Calcium ions also play major roles in the retention of carbon dioxide in natural waters, water hardness, signal transduction and tissue generation. As one of the alkaline earth metals, the calcium atom has two valence electrons according to the octet rule. Up to now, the only known valence state of calcium ions under ambient conditions is +2, and the corresponding crystals with calcium ions are insulating.