Pleochroic Halos. 



633 



by the ionization range of RaC in the particular mineral in 

 which the halo is formed. In the second the radius is 

 limited by the range of ThC. In the third it will again be 

 limited by the range of ThC. It might, in general, be 

 difficult to distinguish the last two cases from one another ; 

 but if the halo is not oyer exposed we would expect in the Ra-Th 

 halo a corona with inner and outer radii corresponding to the 

 radii of RaC and ThC. Thus if formed in biotite the radius of 

 the pupil would be about 0*033 mm. and of the corona or iris 

 0*040. There is no difficulty in distinguishing uranium from 

 thorium halos, however blackened up they may be. In biotite 

 the outer radius of a good uranium halo is never very different 

 from 033 mm. and that of a thorium halo from 0*040. 

 The difference, unless the originating radioactive particle is 

 large enough to exaggerate the dimensions of the former, is 

 such as cannot escape detection. In these cases if the halo 

 is not over exposed the radius of the pupil will be respee- 

 tivelv about 0*022 and 0*026; the former being due to RaA, 

 the latter to ThX. 



The table which follows gives the radii (in millimetres) in 

 the case of the commoner halo-bearing minerals. Only some 

 of the figures in the above table are based on an accurate 

 knowledge of the density and the chemical composition. Both 

 quantities require to be known for the particular mineral. 

 The more reliable values will be recognized by the accurate 

 figures given for the specific gravity. 



Uranium 



RmC. RaA. 



Thorium. 



X. 



Common 

 'Hornblende 



3-0 00340 

 8-1 0-0336 



00237 

 00230 



0-0422 0-0280 (MgFe) 2 Si,O e (ALFe) 4 O s . 

 0-0409 00272 



Piopside ...3492 0-0281 00193 0-0343 0-0227 Hedenbergite. 



Biotite 28 0-0326 0-0222 00397 00203 (HK^MgFe^l^SiO,),. 



Anouiite 2-846 0-0321 0-0219 00390 00259 Analysis selected bv Tschermak. 



Haughtonite. 2 934 00334 00229 0-0407 0270 ., see p. 638. 



Muscovite ... 

 Lepidolite ... 

 Zinnwaldite. . 



28 00304 

 2-830 00310 



2-885 0-0307 

 2-8 1 0-0275 



I 

 2-9 0-0322 



29 00329 



00208 

 0-0212 



00210 

 0188 



0-0221 

 00225 



00370 0024: Al 3 KH 2 Si 3 )2 . 



0-0378 00250 Analysis >elec:ed by Tschermak. 



0-0374 0-0248 Analysis selected bv Tschermak. 

 0-0335 00222 KLiAl>VJISi 3 O 10 . 



00391 00260 inalvsis selected bv Tschermak.; 

 0-0400 00265 20 Li, 20 Al, 34 Si, 990, 20 Fe. 



