Hidden and Hillebrand — Description of Rowlandite. 211 



In calculating the molecular ratios a brief consideration 

 shows that CaO and C0 2 may be neglected, and likewise, so 

 far as their effect on the general formula is concerned, the 

 ThO, and the gronp denominated X ; also the trace of ferric 

 oxide and the alkalies. For the present the water may also be 

 disregarded. There result then the following molecular ratios : 



•4340 3-91 

 •2223 2-00 



SiO„- 



_. 26*04 - 



- 60 = 





Ce 2 O s 



_. 5-06 



328-4 



•0154 



La group . - 



.. 9-34 



336-8 



•0277 



T group 



._ 47-70 



266-2 



•1792 



FeO 



.. 4-39 



72 



•0610 



MnO 



-67 



71 



•0094 



MgO 



._ 1-62 



40 



•0405 



Fl 



,. 3-87 



19 





■1109 1-00 



•2037 1-84 



These furnish the following empirical formula, after deduct- 

 ing 10 atoms of oxygen for 20 of fluorine : 



Si„'"R 11 "B; 1 F„0 ]M 



The silica is in all probability as much as half a per cent too 

 low through loss occasioned by the presence of fluorine. 

 Furthermore, it is possible that the fluorine, estimated by the 

 Berzelian method, is a little too low ; or, if not, that the small 

 amount of water found is in part at least derived from hy- 

 droxyl replacing fluorine. The presumption is then strong 

 that the composition of rowlandite is represented by the 

 formula 



Si^'RJ'R^O^, or in simplified form Si/"R/'RF 2 14 



Structurally this formula may perhaps be written in a number 

 of ways, that which most readily suggests itself being 



"RF 



SiA^''R 



^Si./X^'R 



II 

 "'RF 



in which the mineral is to be regarded as a derivative of the 

 acid H 6 Si 2 7 . Until, however, the whole group of rare earth 

 silicates is carefully studied and their relations among them- 

 selves and to other better known silicates are ascertained, it 

 would be premature to pronounce in favor of this or any other 

 structural formula. 



Reducing all the other earths to a hypothetical one having 

 the molecular weight of the yttrium group as found, and the 

 manganese and magnesia to their equivalents in iron, the com- 



