462 Hidden and Mackintosh — Thorium Mineral. 



The largest mass found measured l cm through the prism, and 

 it was evidently only part of what had been a long crystal.. 

 The bulk of what we collected is in a very fragmentary condi- 

 tion, down to masses and broken crystals of l mm diameter. 



Our analyses have given the following results : 



H 2 0> 



co 2 f --- 



Si0 2 



PA .... 



ThO,.-.. 



FeA-- 



CaO 



MgO.... 



Al a 3 , etc. 



10-7 



2. 



3. 



4. 



5. 





11-21 







9*88 









1-00 



9'25 



7-64 



8-25 









7*46 



7*59 





69*23 



70-13 







1-42 



]-38 









0-49 











0-29 











1-10 







99-70 



If the amount of C0 2 in analysis number 3 is assumed to 

 be the same as it is in 5, and if we take the amount of H 2 as 

 the difference between the loss on ignition and the C0 2 , the 

 ratio of hydrogen equivalents, of bases and acids (assuming the 

 carbonates to be admixed impurity), is nearly 2:1:2. This 

 gives the formula, 



Th0 » { w?o\ \ 2H *°> 



or a thorite in which part of the silica is replaced by its equiva- 

 lent in phosphoric acid, when 3Si0 2 =lP 2 5 . The ratio of 

 silica to phosphoric acid is variable and is approximately 1 : 1 

 in hydrogen equivalents ; but the P 2 5 tends to be in excess. 



We have considered it possible that this mineral is a mixture 

 of a hydrated thorium phosphate with a hydrated thorium sili- 

 cate, in some respects analogous to the occurrence of zirconium- 

 silicate (zircon) in parallel position with yttrium-phosphate 

 (xenotime),f although there is nothing in the appearance to 

 suggest this — the mineral seeming to be perfectly homogeneous, 

 except on the exposed surfaces. It is more probable, however, 

 that we have here an example of a partial replacement of silica 

 by phosphoric acid, which fact has not yet, to our knowledge, 

 been noticed in the literature of Mineralogy; unless we should 

 so regard the small proportion of P 2 5 (4*17) which EakinsJ 

 observed in the xanthitane (altered titanite) from the same 

 locality. 



This occurrence of a thorium phosphate is the first instance 

 of such a compound existing in nature and seems to have a 



* Including other oxides with traces of thoria. 



f This Journal. Nov., 1888. p. 380. 



% This Journal, vol. xxxv, p. 418, May, 1888. 



