454 Hillebrand, Turner and Clarice — Roseoelite. 



species of mica various molecules must sometimes be assumed 

 to exist in different proportions and the general formula for 

 such a species can only be arrived at by comparison of a series 

 of analyses of different varieties. Hence, in view of the lack 

 of any simple ratios, the deduction of a definite and final 

 formula from my data is not justifiable. Further analyses are 

 needed of new and very pure material from other locations 

 even if these be not far removed from the source of the 

 present material. Nevertheless, in the hands of an expert very 

 unpromising data may often be made to afford positive indica- 

 tions, and that this is true in the present case the following dis- 

 cussion by Professor F. W. Clarke clearly shows. 



Chemical Constitution op Rosco elite, by F. W. Claeke. 



The ratios given in the foregoing new analysis, used directly, 

 lead to the following empirical formula for roseoelite : 

 H 4M K JS0 Fe 82 Mg„Al ! „V 3ie Si 758 O ! „ 1 . 



Here H to K, and Mg to Fe are as 2 to 1. Between O and Si, 

 however, the ratio is not simple, and lies below the orthosili- 

 cate and above the trisilicate proportion. Since in many micas 

 the groups Si0 4 and Si 3 8 are replaceable, that suggestion may 

 be followed out here ; and then the formula reduces to 



R' 6;8 R"„Ai ! , 6 v !18 (Si0 4 ) sl6 (Si 3 o B ) n . 



From this expression, applying Clarke's mica theory, the min- 

 eral may be regarded as a molecular mixture of the three 

 compounds 



1. 2. 3. 



^Si0 4 =FeK ^Si 3 8 EEKH 2 ^Si0 4 =KH 3 



Al-Si0 4 EEMgH Al— Si 3 8 = KH 2 Al— SiO = V 

 ^Si0 4 EEMgH, \Si,0 8 = Al, \Si0 4 =V, 



in the ratio 21 : 22 : 159, or nearly 1:1:8. Upon reducing the 

 analysis to 100 per cent, after throwing out the Ti0 2 and the 

 water lost below 280° as extraneous, we get the following com- 

 parison between the results found and the theoretical compo- 

 sition. 



Found. Reduced. Calculated. 



Si0 2 .__ ...45-17 45-88 45'52 



Ti0 2 _. -78 



V 2 3 _ 24-01 24-39 2464 



A1 2 G 3 11-54 11-73 11-62 



MgO ._ 1-64 1-66 1-72 



FeO 1-60 1*63 1-55 



K 2 _ 10-37 10-53 10-81 



H 2 0, 280°- .... -57 ■ .... 

 " 280°+ 4-12 4-18 4-14 



99-80 100-00 100-00 



