W. F. Hillebrand — Composition of Yttrialite. 151 



Hbd. R/' 61 R'^3 8 6 ^""fl ( S !2 t)24 7 



Mack. R" 65 R'" 373 R"" 4 8 ( Si 2 <?)243 



there being a slight deficiency of oxygen atoms in each case for 

 the radical Si 2 7 , which is increased by allowing for the C0 2 

 and P 2 5 . 



In so far then as the character of the acid radical is con- 

 cerned the results of Mackintosh's analysis are fully confirmed 

 and there is absolutely no ground for accepting Benedicks' 

 basic formula, which as I have already shown (p. 14:5) is based 

 on a palpable error. Bnt the ratios are not at all such as to 

 lend themselves to ready resolution into isomorphous salts of 

 the acid H 6 Si 2 7 . By doing quite unwarranted violence to the 

 analytical data the above formulas might be reduced to 



R" R'" 6 R"" (Si 2 7 ) 4 



which can. be readily represented structurally as a single com- 

 plex molecule or as a mixture of molecules like 3R" /, 9 Si 3 7 + 

 j^//j£////g^Q^ j n nether caS e 5 however, is the type of the 



rowlandite molecule approached, which requires an altogether 

 different ratio of monoxide, dioxide, and trioxide bases, nor, if 

 the second be accepted, is it at all clear that the two molecules 

 would be mineralogically equivalent, that is, isomorphous. 



An alternative hypothesis is to regard the mineral as a mix- 

 ture containing the anhydrous thorite molecule. Proceeding 

 on this assumption and deducting all thorium and uranium and 

 the proper amounts of silicon and oxygen, the crude empirical 

 formulas become 



Mack. R'^ 5 R'"„ s S i4 3 8 O l5 01 



Hbd. R" 61 R"' 38G Si 447 1533 

 which may be interpreted as basic salts of metasilicic acid : 



R "65 R '"l8 5 ( R/ " )'l87 ( Si0 s)438 



R" 61 R'" 194 (R'"0)' 192 (SiO s ) 44 , 

 or R" R'" 3 (R"'0)' 3 (SiO s ), 



this last being easily susceptible of symmetrical representation 

 in graphic form. 



On the whole I prefer to leave the constitution of yttrialite 

 unsettled until further evidence can be gathered, either from 

 analyses of allied minerals or from yttrialite itself of more cer- 

 tain purity than any that has yet been discovered. 



It must not be forgotten that the gases other than C0 2 con- 

 tained in the mineral may be the cause of the inability to 

 arrive at satisfactory conclusions in the case of this and all 

 other minerals which contain them, as I have already pointed 

 out on p. 147. 



