The Closed Tube Test

In the closed tube test, a powdered mineral sample is placed at the bottom of a glass test tube and heated. The following characteristics must be reported  

Some hydrayed minerals containing hydratation water release their water and gives the filling of boiling. Zeolithes are the most common examples. 

Minerals containing liquid inclusions may explode owing to the evolution of steam during heating. Other such as baryte break into smaller crystal while milky quartz breaks into very fine powder or dust. 

Several minerals having a lamellar structure, including many phyllosilicates delaminate or exfoliate. 

Only minerals having a low melting point melt in the closed tube. 

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Table 12.9. Mineral changes during the closed tube test...

The main feature of the Carius tube test, which was developed by ICI as the so-called 10 g tube test, is the simplicity of its set-up (cf Figure 3-11) [21]. As the name already says, a 10 g sample is filled into a special Carius tube, which has a dip in the bottom to accommodate a thermocouple. The top may be closed using a metal/glass fitting, which contains a pressure sensor. The tube is placed into an oven, which may be heated up in a controlled linear way. In analogy to the DTA-set-up the temperature difference between sample and oven temperature s well as the pressme are reeorded. 

The liquid becomes progressively darker in colour, and then effervesces gently as ethylene is evolved. Allow the gas to escape from the delivery-tube in T for several minutes in order to sweep out the air in F and B. Now fill a test-tube with water, close it with the finger, and invert the tube in the water in T over the delivery-tube so that a sample of the gas collects in the tube. Close the tube again with the finger, and then light the gas at a Bunsen burner at a safe distance from the apparatus. If the tube contains pure ethylene, the latter burns with a clear pale blue (almost invisible) flame if the ethylene still contains air, the mixture in the test-tube ignites with a sharp report. Allow the... 

Proceed as follows for both solid and liquid compounds. By means of a test-tube holder, hold the tube in a horizontal position, and by means of a small flame gently heat the mixture at the open end. Increase the size of the flame gradually until the mixture is red-hot at this end. Now slowly and cautiously extend the heating towards the closed end until the whole of the mixture is red-hot. (If during the extension of the heating, the mixture tends to be pushed out of the tube by the evolution of gas, stop the heating momentarily, and rotate the... 

If preferred, suitable ignition tubes may be prepared by the student from soft glass tubing it is important that the thickness of the glass at the closed end be tmiform, otherwise the tube is likely to crack when heated. The simplest procedure is to blow a small bulb of uniform wall thickness at the end of the tube. The small test-tubes are available from all laboratory supply dealers... 

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