Penfield and Sperry — Mineralogical Notes. 331 



Tschermak regards this excess of acid as comprised in the 

 molecule S = Si 10 H 8 O 24 or the fluorine compound Si 10 O 8 F 24 . 

 The ortho-silicate which we find in this analysis, however, 

 HjSi0 4 , seems a more natural compound to be associated with 

 the other ortho-silicate molecules K and M. 



If we do not follow Professor Tschermak in discussing the 

 results of our analysis, as we have done above, we can detect a 

 certain relation between the constituents. By taking one- 

 seventh of the Si0 2 in the above ratio as unity we obtain 



Si0 2 : Al 2 O s 

 7 : 0-99 

 nearly 7 : 1" 



(Mg, Fe)0 

 6-80 

 7 



(K, Na, Li) 2 : H 2 

 0-92 : 3-31 



1 : 3 



and if the excess of the H 2 above 3*00 is regarded as replac- 

 ing MgO and K 2 0, the ratio would be almost exactly whole 

 numbers. This ratio and the composition of the mineral may 

 be expressed by the somewhat elaborate formula H 6 K 2 Mg 7 Al 2 

 Si 7 28 . The acid radical Si 7 28 shows that the mineral is an 

 orthosilicate, while the following percentage composition cal- 

 culated from the above formula shows how closely it corre- 

 sponds to the results of the analysis : 





Si0 2 



A1 2 3 



MgO 



K 2 



H 2 



Theory 



44-16 



10-83 



29-44 



9-89 



5-68 = 100-00 



Analysis 



4481 



10-87 



29-06 



9-25 



6-38 = 100-37 



The above analysis is like the original in our article except 

 that the FeO has been calculated to an equivalent of MgO and 

 the Na 2 and Li 2 to an equivalent of K 2 0. Although it will 

 not do to make any general statement regarding the chemical 

 composition of phlogopite from one single analysis we offer 

 the above results as a contribution to the chemistry of the 

 micas, feeling assured that they are derived from exceptionally 

 pure material. 



In closing, we wish to express our thanks to those persons 

 who have assisted us in our work by furnishing the material 

 for carrying on these investigations, and whose names have 

 been mentioned in our article. 



Mineralogical Laboratory, Sheffield Scientific School, 

 July 19th, 1888. 



