. Bulletin of the Geological Society of America. Geology. 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 augite bounded by the planes a (100), b (010), m (110) and s(Ill), and somewhat tabular on a (100). The form o (221) has also been observed. Twinning on a (100) oc- c
. Bulletin of the Geological Society of America. Geology. 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 augite bounded by the planes a (100), b (010), m (110) and s(Ill), 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 o (221) in addition is shown in figure 7. This crystal was measured on the reflecting goniometer with the following results :. Figure 7.— Ticinned Pyroxene Crystal. a Am (100 a 110)... 5 AS (HI A III).... m a o (110 a 221) s as (111 a 111 twin) Theory. Measured. 46° 25' 46° 27', 46° 23' 59 11 59 8 35 29 35 40 26 52 26 24 26 35 46° 42, 46° 5P 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 Highwood 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.* Si02 Ti02 55 A1A Fe,Os FeO MnO 10 MgO CaO K20. H20 Oxygen ratios. .. .8236) So0o .. .0067/ 'b6{)S 04151 .. .0178 }-°o93 .. .0772) .. .0014 \ .4181) .. .3395 J () [ .. .3995 .3995 J () .. .01671 09Q7 .. .0040/ •UiU/ at 110c .38 .09 8176 Total *By L. V. Please note that these images are extracted from scanned page i
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