160 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In many crystals the whole or a part possess a hrownish-yellow or 

 even reddish-yellow color. This is often most pronounced about black 

 oxide (ilmenite) grains and is believed to be due to a pigment stain of 

 ferruginous character. There at least appears to be no regularity in 

 the distribution of the brownish and reddish colorations, a makes 

 an angle of 6° with c', and other optical properties are the usual ones 

 for this mineral. Twins parallel to a, 100 are common. A careful 

 examination of aegirite from various portions of the diflFerent pegma- 

 tites indicates that it is identical in chemical composition throughout. 



Chemical Composition. — The almost universal contamination of the 

 aegirite with other minerals made the obtaining of suitable material 

 for chemical analysis very difficult. By means of magnetic and heavy 

 solution separations combined with hand picking under the microscope 

 about three grams of material was finally obtained which showed as 

 impurities only a little ilmenite and a trace of octahedrite and quartz. 



The analysis made in duplicate averaged as follows : 



Aegirite, Fallon Quarry, North Common Hill, Quincy, Mass., U. S. A. 

 Analyst Warren. 



Per cent. Molecular Ratios. 



SiOz 51.73 0.862 0.862 



TiOi 0.64 0.008 0.008 



AI2O3 1.91 0.018 0.217 



FeA 31.86 0.199 



FeO 0.87 0.012 



MnO 0.60 0.008 



CaO 0.87 0.015 0.226 



MgO 0.14 0.003 



NaaO 11.43 0.184 



K2O 0.40 0.004 



H2O 0.20 



F none 



Total 100.65 



Specific gravity at 25° C. 3.499. 



Although a portion of the TiOa was probably present as TiOa (octa- 

 hedrite) most of it is combined with RO as ilmenite, and after deduct- 

 ing the TiOa and the proportionate amount of RO as ilmenite, the 

 combined ratios are, SiOs =0.838, RaOs = 0.217, RO + R20= 0.218 ; 

 or very nearly SiOa : R2O8 : RO + R2O = 4:1:1. This is almost ex- 

 actly the ratio of the compound (R"2R) Fe2Si40i2. So far as known to 

 the writers the ferrous iron is lower in this aegirite than in any hitherto 

 analyzed, and the composition approaches very closely to the theoretical 



