38 W. E. Ford — Chemical, Optical and other Physical 



tain three or more isomorphous molecules. It would be pos- 

 sible, therefore, in any series composed of three different 

 molecules to have considerable variation in composition with- 

 out changing the index of refraction. The problem would be 

 still further complicated if four different molecules were 

 present. 



The problem in the case of a garnet containing three differ- 

 ent molcules is simplified by the fact, recently pointed out by 

 Boecke,* that the miscibility of the different garnets with 

 each other has definite limits. In any series only a certain 

 restricted portion of the possible combinations are found. 

 The results of Boecke's investigation were not given, how- 

 ever, in a form that made them available for the present dis- 

 cussion, so it became necessary to make an independent study 

 along similar lines. 



All the readily available garnet analyses were collected and 

 studied. All analyses were rejected which contained unusual 

 elements or molecules, the study being confined to the five 

 common members of the. group, namely, pyrope, grossularite, 

 spessartite, almandite, and andradite. Further, any analysis 

 which on calculation did not show a reasonably close agree- 

 ment to the theoretical garnet formula was discarded. There 

 remained after such elimination nearly two hundred analyses 

 available for study. The percentages of the different com- 

 ponents in each of these garnets were obtained and tabulated. 



It was found that nearly 15 per cent of the total number 

 contained four or more constituents, with the molecule in 

 smallest amount forming more than 5 per cent of the total. 

 The remaining 85 per cent were garnets in which two or three 

 components formed more than 95 percent of the total. Nearly 

 17 per cent of all the analyses studied had but two constit- 

 uents. In other words, approximately 60 per cent of all 

 garnets contain three isomorphous molecules which together 

 form more than 95 per cent of the total constituents. Of the 

 garnets containing three constituents the average percentage 

 of the molecule present in smallest amount was 4*7. The 

 highest percentage observed of the third molecule was 20 and 

 86 per cent of these garnets show the third molecule forming 

 less than 10 per cent of the total. It is evident, therefore, that 

 in any given series formed by the mixture of three garnet 

 molecules, two of them must predominate while the third is 

 present in very subordinate amounts. 



Interesting data concerning the frequency of occurrence of 

 the different possible combinations were available. Of the 31 

 garnets containing but two components the following combi- 

 nations were observed : grossularite + andradite = 14 ; alman- 



*Zs. Kr., liii, 149, 1913. 



