158 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by Brogger. As a rule the smaller the crystal the better the quality 

 of its faces ; the best ones were minute needles of clear green color. 

 Larger crystals are dark green to blackish gi-een in color and often occur 

 in subparallel groups, sheafe or rosette forms ; many show fractures 

 more or less healed or extreme bending. 



Twinning on the orthopinacoid is common in larger crystals but is 

 invariably associated with rounding and irregularity of the terminal 

 planes to a degree that entirely precludes measurement. 



The basal plane was not observed. The form series as a whole is 

 much more like that of augite than like that described as typical for 

 either aegirite or acmite. None of the forms supposed by Brogger to 

 be peculiar to those species were discovered. On this account and be- 

 cause several of the forms determined have not been recorded for 

 aegirite, since, moreover, this aegirite is shown by the analysis to be 

 nearer to the theoretical aegirite molecule Na2Fe2Si40i2 than any pre- 

 viously described, it was deemed advisable to calculate the angles of 

 the forms found on the basis of the axial ratio derived from these 

 measurements, and they are accordingly presented in the following 

 table together with the observed angles. 



The axial ratio calculated from fifty faces of six forms on eight 

 crystals gives the values of the first line below, with which may be 

 compared the ratios of aegirite and acmite as determined by Brogger. 



a b P 



Aegirite, Quincy 1.1044 : 1 : 0.G043 72° 27' 



Aegirite, Norway, Brogger 1.0975 : 1 : 0.6009 73 09 



Acmite, " " 1.0996 : 1 : 0.6012 73 11 



Table of Angles of Aegirite, Quinct. 



po = .5572 qo = .5762 e == .3015 ^ m = 72°27' ^^ ^^ 



Calculated. Measured. Limits. ' Faces, ity." 



<t> p <t> P 4> P 



a 100 90°00'90°00' 89°57' 90°00' 89°37'-90°04' 5 poor 



b 010 00 00 90 00 00 37 90 00 1 poor 



mllO 43 35 90 00 43 33 90 00 43 00 -43 55 27 good 



f 310 70 42 90 00 70 26 90 00 69 56 -70 56 5 poor 



u ni 55 50 47 06 55 54 47 08 55 17 -56 16 46°54'-47°31 11 good 



s 111 -23 15 33 20 -23 40 33 24 1 fair 



w 331 48 21 69 52 48 29 69 50 47 45 -49 13 69 36 -70 00 4 poor 



A 331 -37 47 66 27 -37 46 66 42 1 poor 



5 r)51 46 31 77 10 44 46 77 57 44 02 -45 31 77 30 -78 25 2 v. poor 



T Tl2 -5 32 16 53 - 5 51 16 50 5 16 -6 19 16 40 -16 57 5 good 



8 311 -66 44 56 50 -66 40 56 45 66 37 -66 44 2 v. poor 



d 131 26 09 63 39 26 40 63 48 26 19 -26 52 63 34 -64 03 3 good 



