Identifying Arsenic Minerals

The test to be used for arsenic depends upon whether the mineral contains oxygen, and then test 1,2,3 and 4 will serve. If, on the other hand, the mineral is an oxygen compound, test 5 must be used.

1. Oxide Coating on Charcoal. When an arsenic mineral is heated in the oxidizing flame on charcoal, a white coating of arsenious oxide, As2O3, is deposited on the charcoal last some distance from the mineral. The coating is very volatile. Its formation is usually accompanied by a characteristic odor of garlic.
2. Sublimate on Plaster Tablet. When mixed with a mixture of potassium iodide and sulfur arsenic minerals yield in the oxidizing flame an orange-yellow coating of ASI₃.
3. Open Tube Test. When an arsenic mineral carefully heated in the open tube a colorless or white crystalline sublimate of arsenious oxide, AS2O3, forms in a ring on the inner wall of the tube at a considerable distance above the mineral. It is very volatile. When examined with a lens the coating will usually show well-defined octahedral crystals. If the mineral is heated too rapidly, metallic arsenic may sublime instead of the oxide (see the next test).
4. Closed Tube Test. Many arsenic minerals when heated in a closed tube yield  a sublimate of metallic arsenic, known as the arsenic mirror. This sublimate shows an amorphous black band above and a silver-gray crystalline band below. If the bottom of the tube be broken off and the metallic arsenic volatilized by heat, the characteristic garlic odor will be obtained.
5. Closed Tube Test for an Arsenate. When arsenic occurs in a mineral in the form of an arsenate, i.e., an oxidized compound, none of the above tests will serve. In this case place the mineral in a closed tube with a splinter o f charcoal and then heat. The charcoal will act as a reducing agent and set metallic arsenic free, which will condense on the wall of the tube as an arsenical mirror similar to that described under test 4.