F. W. Clarice — Studies in the Mica Group. 133 



0'3165 gram in all, and made a partial analysis of it as fol- 

 lows: 



Ignition 20-50 



Si0 2 31-16 



Al 2 O s 8-06 



Fe 2 3 35-86 = 32-28 FeO. 



MgO 5-43 



101-01 

 The iron is undoubtedly present chiefly, but not wholly in 

 the ferrous state ; and the alumina is ascribable to traces of 

 admixed mica. Still, with all its defects, the analysis clearly 

 defines the mineral as a member of the obscure hisingerite 

 group, which has rarely been noted in this country. Its 

 association with pyrite is similar to some of its occurrences 

 abroad, and here, as observed elsewhere, it may originate 

 in the alteration of that species. I hope to give it a fuller 

 examination sometime, with a larger supply of material. For 

 the specimens so far studied I am indebted to the kindness 

 of Mr. Wm. Earl Hidden. 



2. Lepidomelane from "Baltimore, Md., and Litchfield, Maine. 

 Baltimore Lepidomelane. — This mineral, the characteristic 

 black mica of the Jones Falls pegmatite, was sent me for analy- 

 sis by Dr. Gr. H. Williams of the Johns Hopkins University. 

 The specimen submitted was in broad, brilliant foliations, 

 black by reflected, but dark smoky green by transmitted light. 

 According to Dr. Williams it is very nearly uniaxial. In the 

 following analysis (I.) the direct estimation of water and the 

 determination of ferrous iron were made by Mr. R. B. Riggs. 

 Reckoning the water with the alkalies, the analysis gives ap- 

 proximately the formula E / 6 R" s R /// 4 Si B !1!! . It is, therefore, 

 slightly too basic for an ordinary orthosilicate, and it seems to 

 occupy an intermediate position, as doits ratios, between annite 

 and the Litchfield lepidomelane. 



I. Baltimore. IT. Litchfield. 



H 2 0.. 4-48 4-67 



Si0 2 35-78 32-35 



A1 2 G 3 16-39 17-47 



Fe 2 3 14-55 24-22 



FeO 11-02 13-11 



MnO 1-08 1-02 



CaO * none -89 



MgO .. 8-67 none 



K 2 0... 7-76 -70 



Na 2 . -56 6-40 



TiO, — . none none 



100-29 100-83 Fluorine none. 



