Prof. J. P. Cooke on the Vermiculites. 253 



the plates become opaque at a very moderate thickness ; and for 

 this reason it is impossible to measure the angle with great pre- 

 cision. Some of the plates are apparently uniaxial; but this 

 may result from the blending of the two hyperbolas, due to the 

 thinness of the plate. The dispersion of the axes is but slight, 

 and only perceptible in the thicker laminae when p<v. It is 

 obvious, therefore, that the crystallographic characters of the 

 mineral are identical with those of mica. 



The plates are generally, if not invariably, twinned ; and the 

 twinning is the cause of this most remarkable variation in the 

 optical angle, as will be explained at length in connexion with 

 our description of Culsageeite. On this last mineral the same 

 phenomena are more marked, owing chiefly to the greater trans- 

 parency of the plates. 



In order to illustrate the chemical relations of the mineral to 

 the Biotite micas, we give below : — (1) the results of the analysis 

 of Jefferisite by Professor Brush ; (2) the same results, calculated 

 for the anhydrous mineral ; (3) the results of an analysis of a 

 Biotite mica, from Pargas, Finland, by Svanberg. In each 

 case I have added the amounts of oxygen in the several oxides 

 to show the atomic ratios : — 



(1) 



Si. 11. Fe. 



37-10 17-57 10-54 

 19-78 8-18 3-16 



' v ' 



19-78 11-34 

 5 3 



Si. H. Fe. 



42-94 20-33 12-20 

 22-90 9-47 3-66 



Fe. Mg. Ca. K. fi. 



1-26 19-65 0-56 0-43 13-76 = 100-87 

 •28 7-86 -16 -07 12-23 



(2) 



8-37 12-23 

 2 3 



Fe. Mg. Ca. K. 



1-46 22-75 -66 -50 =10084 

 •32 910 -19 -08 



(3) 



22-90 13-13 

 5 3 



Si. M. Fe. 

 42-58 21-68 10-39 

 22-71 10-10 3-12 



9-69 

 2 



Mn. Mg. Ca. K. H. F. 



•75 10-27 1-04 8-45 3-35 -51 = 9902 

 •17 4-11 -30 1-43 2-98 



22-71 13-22 8-99 



5 3 2 



The general symbol of Jefferisite deduced from (1) would be 



II VI 



R 4 ,E 2 .O 20 .Si 5 .6H 2 O. 



A comparison of the results given in (2) and (3) will show 

 that the anhydrous Jefferisite corresponds very closely in its 

 chemical constitution with the Biotite mica from Pargas. The 

 chief difference is to be found in the fact that the mica contains 



