Identifying Iron Minerals

Magnetic Test. Any mineral that contains a sufficient amount of iron to permit it to be classified as an iron mineral will readily become magnetic when heated in the reducing part of the blowpipe flame. A comparatively small fragment should be used and the test made with a magnet after it has cooled.

Precipitation with Ammonium Hydroxide. Ferric iron is readily and completely precipitated as ferric hydroxide, Fe (OH)3, from an acid solution by adding an excess of ammonium hydroxide. It is a flocculent precipitate with a reddish brown color. If there is any doubt as to the state of oxidation of the iron in the original solution a few droops of nitric acid should be added and the solution heated in order to make certain that there iron is ferric.

Cyanide Tests for Ferrous And Ferric Iron. Occasionally it may be important to determine whether the iron in a mineral is ferrous or ferric in its valence. This can be done only when the mineral is soluble in a nonoxidizing acid like hydrochloric and when it is not a sulphide. If these conditions can be fulfilled, then divide the solution into two parts. To one add a few drops of a dilute solution of potassium fericyanide, and if the solution contains any ferrous iron a heavy dark blue precipitate will form. If, on the other hand, it contained only ferric iron, these would be no precipitate but only a darkening of the color of the solution. To the second portion of the solution add a few drops of a dilute solution of potassium ferocyanide, and if their is any ferric iron present a heavy dark blue precipitate similar to the one in the previous case will form. But I the solution contained only ferrous iron; a light blue precipitate would be formed. The characteristic dark blue precipitate must contain both valences of iron and will only form when cyanide is added containing the opposite king of iron to that already in the solution.
Ammonium or potassium sulphocyanate is also used in making the ferric test. A few drops of one of these reagents added to a ferric iron solution will give it a deep red color. All of these tests are extremely delicate and will give good results if only a trace of iron is present. They should never be used to determine the presence of iron in a mineral but only to differentiate ferrous from ferric iron.