This rare metal usually occurs with tantalum, in the form of an oxide (Nb205), in the following uncommon minerals:- 51 1/2 to 78 percent. in columbite, 47 to 51 Per cent. in pyrochlore, 45 Per cent. in fergusonite, 42 Per cent. in yttrotantalite, 35 Per cent. cent in euxenite, and 7 1/2 to 29 Per cent. in tantalite. Roscoe has obtained the impure metal (containing small quantities of hydrogen, and chloride and oxide) as a steel-grey crust by repeatedly passing the vapour of the pure chloride with hydrogen through a red-hot tube, which was then more strongly heated in a porcelain tube traversed by hydrogen. Niobium is dissolved by concentrated sulphuric acid, being scarcely affected by even heated nitric or hydrochloric acids or aqua-regia; it ignites and oxidizes with vivid incandescence on heating in the air at a low temperature.