132 F. W. Clarice — Studies in the Mica Group. 



less dusted over with a dark-green chloritic coating, which 

 together with some ferruginous staining, gives the mica a some- 

 what bronzy appearance. An analysis made upon carefully 

 purified material gave the following results: 



Ignition 5 -46 



Si0 o 45-40 



TiO"„ l-io 



A1 2 3 ._ 33-66 



Fe o 3 2-36 



MgO 1-86 



CaO none 



Li Q trace 



Na s O . 1-41 



K„0 8-33 



F. -69 



100-27 

 Less oxygen -29 



99-98 



This, except for the presence of titanium, is the composition 

 of an ordinary muscovite. Possibly the titanium may be due 

 to minute inclusions of rutile, but none such, were detected. It 

 is more probably present as a replacement either of silica or of 

 alumina. 



One of the smaller plates of this mica was examined micro- 

 scopically by Mr. J. S. Diller, who has kindly furnished the 

 subjoined notes: 



" The group of muscovite scales shows well-defined crystal- 

 lographic outlines, most prominent among which are the clino- 

 pinacoid and the unit prism, although a small clinodiagonal 

 prism is also common. Parallel with the latter there are in- 

 distinct cleavage lines, which in the basal plane make an angle 

 of 31° with the edge between the basal plane and the clino- 

 pinacoid. The angle of the optic axes, measured in oil in a 

 plane perpendicular to the plane of symmetry, is 35°. 



" The muscovite is partially coated by a greenish dust, which 

 in its arrangement follows crystallographic lines, giving to the 

 scales a streaked appearance. It appears most abundantly upon 

 the prismatic edges, but to a considerable extent also upon the 

 clinopinacoid, and in traces on the clinodiagonal. Thin laminae 

 occur between the folise- of the muscovite, to, which it imparts 

 a yellowish tinge in reflected light. It appears to be weakly 

 doubly refracting, but its structureless character and want of 

 definite optical properties render its determination a matter of 

 difficulty." 



With some trouble I collected a little of this chloritic dust 



