LUMINESCENCE OF ZIECONS 



By p. p. Vexable 



It has long been known that zircons from certain localities, as Nor- 

 way and Expailly in France, phosphoresce when heated to low red- 

 ness. Many of the zircons under this treatment become colorless and 

 transparent, also in most cases the density is permanently increased. 

 A number of investigations have been made into the cause and possible 

 inter-relation of these phenomena. 



Damour (Compt. Rand. 58:154) has shown that there is slight and 

 often inappreciable loss of w^eight in heating ; that the index of refrac- 

 tion is changed as well as the density and that the increased density 

 remains unchanged even when the heating is pushed to fusion (except 

 in one or two of the cases examined). He suggests that it may be a 

 matter of allotropism, the action of heat bringing about the change 

 into the second allotropic modification. Fizeau (Compt. lieud. 66: 

 1005) has shown that heat causes a lasting expansion of form. 



The density of zircons varies from 4.0-4.74. This may be due in 

 part at least to the presence of impurities driven off in the heating 

 and to small variations in composition such as arc revealed in the 

 large number of published analyses. Zircons with a density under 

 4.7 may be raised to that density on heating. Tho>e with a density 

 of 4.7 show no material change. 



The luminescence in question appears according to llenncberg 

 (J. prakt. Chem. 38: 508) at a temperature lower than that at which 

 the color is lost or changed. The loss of weight may be from practi- 

 cally nothing to 0.45% and the change of density from 2.5% to 1%. 

 The brown red color is lost at a temperature of about 300 ' C. Spezia 

 (Ber. deutsch. chem. Ges. 10: 295) has stated that according to his 

 experiments the change of color is due to the reduction of the tcM-ric 

 compounds present, and that heating in a stream of oxygen restores 

 the color. The experiments of Hermann (Z. Anorg. Cheni. 60: 3(J"J) 

 were more detailed and exhaustive and he agrees with Spezia tbat the 

 color is ];ir,iic]\- due to iron in diU'crcnt stages of oxidation and in the 

 case of gi'ccn zircon to :in admixture with chrominin. 



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