There are three kinds of Nickel Compounds in nature,Namely,Nickel Oxide,Nickel Sulphides and Nickel Arsenides.The Oxide includes NiO,Ni3O4 and Ni2O3.Ni2O3 is stable only at low temperature.It dissociates into Ni3O4 when heated to 400-500Deg,and if continue to be heated, Ni2O3 can become NiO. The melting point of NiO is 1650-1660℃,so it is easy to be deoxidized by C and CO.Like FeO or Co,NiO can form NiO·SiO2 and the silicate compound of NiO·SiO2,among them,the NiO2·SiO2 is not stable.Nickel Protoxide can be dissolved in sulphuric acid,sulfinic acid,hydrochloric acid and nitric acid and ect,it will be green nickelous salts.
