Closed Tube Test

Frequently a small glass tube which has been closed at one end is useful in testing minerals. The tube is made out of soft glass and should have a length of about 3 ½ inches and an internal diameter from 1/8 to 3/16 of an inch. Two closed tubes can easily be made by fusing the center of a piece of tubing 7 inches in length and pulling it apart. The closed tube test is used to determine what takes place w hen a mineral is subjected to heat practically out of contact with the air. Ordinarily there is no mineral will break down into simpler parts if that is possible, but otherwise nothing will take place except possibly a fusion of the mineral. The following table gives a list and brief description of the Important closed tube tests:

Substance Description of Test Water, H2O All minerals containing water of crystallization or the hydroxyl radical will give on moderate heating a deposit of drops of water on the cold upper walls of the tube. Sulfur, S. All sulphides which contain an excess of sulfur will give a sublimate of sulfur, which is red when hot and yellow when cold. Arsenic, As. Native arsenic and some arsenides will give a deposit of metallic arsenic. This consisted of two rings, one being composed of a black and amorphous material, the other lying nearer the bottom of the tube, of a silver-gray and crystalline material. Oxysulphide of antimony Sb2S2O. Sulphide of antimony and some sulphantimonites give this sublimate in the form of a slight coating which deposits close to the bottom of the tube. It is black when hot and red when cold. It is accompanied by a faint deposit of sulfur further up the tube. Sulphide of mercury, HgS A black amorphous sublimate which forms when cinnabar is heated. Mercury, Hg. Gray globules of metallic mercury are obtained when native mercury or amalgams are heated or when the sulphide is mixed with dry sodium carbonate and heated.