680 CROSS, IDDINGS, PIRSSON, WASHINGTON 



CaO to diopside, the remaining (Mg,Fe)0 to hypersthene, and 

 the remaining Si0 3 to quartz. From the respective molecules 

 the percentage weights are found by multiplying each by the 

 molecular weight. 



The classification of this rock from the data given in Table 

 VII has been described on a previous page as an example of 

 an intermediate rock. 



fr) To calculate the mode from the chemical analysis of the 

 rock we have to introduce biotite and hornblende into the calcu- 

 lation, and we have the chemical analysis of each given in Tables 

 XIII and XIV. We will assume that they contain all the mag- 

 nesia (MgO) and all the iron oxide (FeO and Fe 3 3 ), except 

 that which goes into pyrite, magnetite and ilmenite. For 

 these are the only ferromagnesian minerals observed in thin sec- 

 tions of the rock except a small amount of pyroxene intergrown 

 with the hornblende. This was probably included in the horn- 

 blende material analyzed so that the analysis represents the 

 mixture and may for the present calculation be treated as the 

 composition of one mineral. 



In Tables XIII and XIV are found the following ratios : 

 For Butte liornblende : 



MgO_ FeO_ A1 2 3 _ Fe 2 3 _ 



CaO" - T ' 53 ' CaO - ' 71 ' ~C^O~- 32 ' "CaO""' 15 ' 



For Butte biotite . 



MgO_ FeO _ A1 2 3 _ Fe 2 Q 



3- 12 > w-^ = l -9° > ^ ^ = i-3 > T ^ /V = -3 2 



K 2 J ' K 2 ' y ' K 2 ° ' K 2 



H a O_ 



K 2 "'' ' K 2 " ' K 2 



•44 



If ^=molecules of CaO in hornblende, y — molecules of K 3 

 in biotite, 3 = molecules of Fe 3 O s in resulting magnetite, w= 

 molecules of Ti0 3 in resulting ilmenite, 



Then 2.24a:-|-5.o8jy-f-z+Z£/ = 88 — (Total (Mg,Fe)0 less FeO in pyrite). 

 •15*+ -3 2 y+ z =i-3 = (Total Fe 2 3 ). 



. 44 y+w = 8 = (Total Ti0 2 ). 

 . 32a" -|- 1.3 y = i2X-\-y-\-(x — 3)— (transferred A1 2 3 ). 



