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



Litchfield Lepidomelane. — In a previous paper upon the min- 

 erals of Litchfield,* I gave an analysis of this mica, together 

 with some data as to its occurrence. It is scattered abundantly 

 through the elseolite syenite of the locality, in black, brilliant 

 plates or scales, which are transparent only in very thin laminae, 

 and then transmit light of a dark green color. It is one of the 

 least transparent micas I have ever seen. According to Mr. 

 Diller, who kindly made a microscopic examination of the 

 mineral, it is so nearly uniaxial that a measurement of its optic- 

 axial angle is impracticable. Some specimens show evidence 

 of two micas ; one being blacker and more brittle than the 

 other; so that a new analysis was made upon very carefully 

 selected material. Only the more transparent and elastic foliee 

 were chosen, and the results are given on the preceding page (II). 



The ferrous iron and water determinations were made by Mr. 

 Riggs. Titanium was carefully sought for, by two methods, in 

 three separate tests; but only a doubtful trace was discovered. 

 The formula of the mica, reckoning water with the alkalies, is 

 very nearly Bf$."&"'$\ t O w 



This mica, on account of its low percentage of silica and its 

 basic nature, has peculiar interest. Among published analyses 

 of micas I find only two which approach it in these particulars, 

 namely, the micas from Brevig and Miask described by Ram- 

 melsberg.f These gave respectively 32*97 and 32*49 per cent 

 of silica, but they also contained, the first 2*42 and the second 

 4*03 of titanic oxide. This constituent, as has been stated, is 

 lacking in the Litchfield mineral. Notably all three of these 

 low-silica micas are from localities of elseolite syenite; although 

 Rammelsberg says nothing of the actual associations of the 

 specimens discussed by him. The question at once arises, to 

 be settled by future investigation, whether micas of this peculiar 

 character are representative of elseolitic rocks, and whether 

 there is any genetic relation between them and elseolite. I 

 hope that observations upon this point may soon be multiplied. 



Some months ago I published some analyses of the Rockport 

 micas, which were made by Mr. Riggs in this laboratory.*}: 

 One of these, the so-called annite, seems to have a serial rela- 

 tion with the analogous micas from Baltimore and Litchfield, 

 which may be expressed in formulae. These, stated in em- 

 pirical form, are as follows : 



Rockport R' 6 R" 4 R'" 2 Si 5 O 20 



Baltimore R' 6 R" 3 R"' 4 Si & 22 



Litchfield R' 6 R" 2 R'" 6 Si 5 24 



Here we have R' and silicon constant, while a gain of R"- a O a 



♦This Journal, April, 1886. f Zeitschr. Geol. Gesell., xxxi, 676. 



% This Journal for Nov. 1886. 



