904: Radioactive Minerals in Common Rocks. 



rocks. There is, however, an unforeseen difficulty owing to 

 the small size of the crystals in which the minerals occur. 

 A large number of these were measured under the micro- 

 scope, and their mean dimensions obtained. They are given 

 in the following table : — 



Mineral. 



Average greatest 

 dimension. 



Average least 

 dimension. 



Allanite from 1 

 Dalbeattie granite j '" 



Rutile from j 

 Cornwall granite J '" 



Zircon from 1 

 Lewisian gneiss J " * 



Zircon from 1 

 Mourne granite J 



0-083 mm. 

 0-130 „ 

 0-140 „ 

 0-108 „ 



0055 mm. 

 0082 „ 



0-052 „ 

 0-033 „ 



The range of a particles in these minerals is about O04 mm. 

 It will be seen that the dimensions of the crystals are of this 

 order of magnitude. If the helium in radioactive minerals 

 consists of a particles shot out by the radioactive substance, 

 the greater part of it will therefore be found, not in the 

 crystal, but in the ground-mass surrounding it. (This is 

 independent of the gaseous diffusion that takes place in the 

 course of long periods of time.) The helium ratio for 

 minerals occurring in small crystals isolated by this method 

 will therefore be smaller than we should be led to expect 

 from fcheir age. 



Prof. Strutt has kindly determined the quantity of helium 

 present in some of the minerals, and finds the following 

 values : — 



Helium in 

 c.c. per gm. X 10~ 4 . 



Sphene from Dalbeattie granite 3'4 



Rutile from Cornish granite 5*3 



Zircon from Lewisian gneiss 36*0 



As might be expected, these values are much lower than is 

 usual for such minerals. As any deductions of the age of 

 rocks based upon radioactive measurements for minerals 

 isolated in this way are unreliable, the experiments have 

 been discontinued. 



