410 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OP MONTANA. 



Pyroxene. — Of all the ferro-magnesian minerals this is by far the most 

 important, determining with the orthoclase the essential character of the 

 rock. Owing to the ease with which it may be de- 

 tached from the matrix, excellent specimens may 

 be obtained for crystallographic study. In general 

 they present the common form of angite bounded 

 by the planes a (100), b (010), m (110) and s (111), 

 and somewhat tabular on a (100). The form o (221) 

 has also been observed. Twinning on a (100) oc- 

 curs, and a crystal of this type having the form 

 ■Twinned Pyroxene q (221) in addition is showu in figure 7. This 

 ^^*^" ■ crystal was measured on the reflecting goniometer 



with the following results : 



Theory. Measured. 



a^m (100 a 110) 46° 25^ 46° 27^ 46° 23^ 46° 42, 46° 51^ 



SAnlllAlU) 59 11 



m A (110 A 22T) 35 29 



SAs"(llI A 111 twin) ..26 52 



Figure 7.- 



Measured. 











46° 



27^ 



46° 



23^ 



46° 



42, 



46 



59 



8 













35 



40 













26 



24 



26 



35 









The reflections of the signal were only moderately good, and the meas- 

 ured angles are therefore of value only in determining the faces. 



As this variety of augite is very common and persistent, not alone at 

 Square butte but generally throughout the High wood rocks, at times, 

 however, passing into varieties which have a narrow mantle of material 

 rich in the aegirite molecule, as, for example, aegirite-augite, it has been 

 deemed important to investigate it chemically, especially since the Square 

 Butte rock presents such excellent material. The analysis yielded the 

 following results : 



Analysis" of Pyroxene."^ 









Oxygen ratios. 



SiO...... 



TiOg. .. 





49.42 ... 



55 .. 



•;; S}.8303 



AI2O3 . . 





4.28 ... 



:: :^tJ^}-05«^ 



Fe„0, 





. . . 2 86 . . . 



FeO.... 





5.56 ... 



.. .0772) 



MnO... 





10 ... 



.. .0014 L 4181) 



MgO.... 





13.58 ... 



.. .3395 J (1.04) L8176 



CaO.... 





22.35 ... 



.. .3995 .3995 J 

 (1.00) 



Na^O . . . 

 K,0... 





1.04 ... 



38 ... 



:: :^J^a-o207 



H2O (at 



110°) 



otal 



09 





T 



...... 100.21 





* By L. V. Pirsson. 



