Under　ambient　conditions,　the　only　known　valence　state　of　calcium　ions　is　+2,　and　the　corresponding　crystals　with　calcium　ions　are　insulating　and　nonferromagnetic.　Here,　using　cryo-electron　microscopy,　we　report　direct　observation　of　two-dimensional　(2D)　CaCl　crystals　on　reduced　graphene　oxide　(rGO)　membranes,　in　which　the　calcium　ions　are　only　monovalent　(i.e.　+1).　Remarkably,　metallic　rather　than　insulating　properties　are　displayed　by　those　CaCl　crystals.　More　interestingly,　room-temperature　ferromagnetism,　graphene-CaCl　heterojunction,　coexistence　of　piezoelectricity-like　property　and　metallicity,　as　well　as　the　distinct　hydrogen　storage　and　release　capability　of　the　CaCl　crystals　in　rGO　membranes　are　experimentally　demonstrated.　We　note　that　such　CaCl　crystals　are　obtained　by　simply　incubating　rGO　membranes　in　salt　solutions　below　the　saturated　concentration,　under　ambient　conditions.　Theoretical　studies　suggest　that　the　formation　of　those　abnormal　crystals　is　attributed　to　the　strong　cation-　interactions　of　the　Ca　cations　with　the　aromatic　rings　in　the　graphene　surfaces.　The　findings　highlight　the　realistic　potential　applications　of　such　abnormal　CaCl　material　with　unusual　electronic　properties　in　designing　novel　transistors　and　magnetic　devices,　hydrogen　storage,　catalyzers,　high-performance　conducting　electrodes　and　sensors,　with　a　size　down　to　atomic　scale.