A new set of composition variables for the representation of reactive-phase diagrams

Abstract
Most natural diamonds contain nitrogen as the main impurity. In some rare diamonds (termed type Ib) the nitrogen is mainly present as single substitutional atoms. However, the large majority of diamonds (termed type Ia) contain the nitrogen atoms in various forms of aggregate. The different types of aggregate are the A centre (two nitrogen atoms), the N 3 centre (three nitrogen atoms) and the B centre (a larger number of nitrogen atoms). These defects give characteristic absorption spectra in the infrared except for the N 3 centre which gives a characteristic absorption in the visible region. In this type of diamond there are usually platelets present in the cube planes and these defects can be examined by transmission electron microscopy and infrared absorption techniques. The paper reports work in which synthetic diamonds containing a high concentration of single nitrogen atoms have been heated in a temperature range of 1500 to 2500 $^\circ$C under various pressures. These heat treatments have resulted in the formation of all the types of aggregate that are found in natural type Ia diamonds. Also some natural diamonds have been heated up to 2700 $^\circ$C under pressure and the ratio of the concentration of the A centres to that of the B centres has been changed. Information has been obtained on the kinetics of the aggregation process. This information has been used to give an approximate estimate of the length of time that natural diamonds spent in the Upper Mantle prior to being ejected to the surface of the Earth. It is suggested that the type Ia diamonds spent between about 200 and 2000 Ma in the Upper Mantle at temperatures of between 1000 and 1400 $^\circ$C. Type Ib diamonds either spent a comparable time in the Upper Mantle at about 800 $^\circ$C or a considerably shorter period if they encountered temperatures in the same range as the type Ia diamonds.