F. W. Clarke — Studies in the Mica Group. 135 



corresponds in each step to a loss of one atom of R". If now 

 we assume that B/'^O,, represents really two of the recognized 

 univalent groups — A1=0, we shall find that all ihree of the 

 micas reduce to the true orthosilicate type, containing the 

 nucleus B/'^SiO,,)^ which appears to be characteristic of many 

 iron micas and of the phlogopite group. We thus have, unit- 

 ing all the monoxide bases, the following general formulae : 



Rockport R' u R'" 2 (SiO.) B 



Baltimore . . . _R' 12 (A10) 2 R ? " 2 (Si0 4 ) 5 



Litchfield R'JAlO^R'^SiOJ, 



In short, the three micas are built upon the same fundamental 

 plan, and exhibit a new and highly suggestive order of varia- 

 tion. 



3. Iron-Biotite from Auburn, Maine. 

 Among the minerals occurring at the well-known lepidolite 

 locality at Auburn, a brilliant black biotite is common. Some 

 time since, in looking over a collection of the Auburn material 

 in the possession of Mr. N. H. Perry, of South Paris, I observed 

 that certain broad foliae of this mica were bordered by a 

 grayish, distinct margin, about five millimeters wide, which was 

 made up of micaceous scales. Inasmuch as the lepidolite of 

 the locality forms similar borders upon the muscovite, I thought 

 that this bordering might have interest, and Mr. Perry kindly 

 placed his best specimen at my disposal. Analyses of both the 

 biotite and the margin were made, but unfortunately the amount 

 of the latter available was too small for complete investigation. 

 In the study of the border, therefore, ferrous iron and water 

 had to be neglected, and that analysis is so far incomplete. The 

 results are given below, with determinations of water and 

 ferrous iron in the biotite, made by Mr. Riggs : 



Biotite. Margin. 



H 2 _.. 4-64 undet. 



Si0 2 34-67 56-44 



A1 2 3 __ 30-09 16-01 



Fe 2 3 2-42) 



FeO. 16-14 f 15Ui 



MnO -85 undet. 



MgO._ 1-98 -97 



CaO none none 



Na 2 1-67 1-92 



K 2 7-55 615 



F -28 -71 



100-29 

 Less oxygen -12 



100-17 



