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



It has seemed to the writer that the study of the isometric 

 minerals would be the natural starting point on the general 

 problem, since in this system the crystal structure has the 

 highest possible symmetry, and that, therefore in the study of 

 isometric minerals, one or more of the factors that might 

 modify the relationships would probably be eliminated. In 

 considering the isometric minerals, the Garnet Group naturally 

 suggested itself first for study. Here we have a series of 

 minerals with widely varying percentage composition, embrac- 

 ing a large number of different elements and furthermore one 

 about which considerable data are already available. The fol- 

 lowing study of a series of garnets has produced results that 

 go to confirm some of our fundamental assumptions and lead 

 to other interesting speculations. 



The literature was searched for reliable analyses of the gar- 

 net minerals in conjunction with which indices of refraction 

 were given. After rejecting analyses that showed the pres- 

 ence of unusual radicals like Ti0 2 , CrO, the alkalies, etc., there 

 remained some twenty-three analyses available. In the case 

 of several of these, where the index of refraction was not given 

 in the original description, it happened that material was 

 obtained upon which the needed determination could be made, 

 either from the Brush Mineral Collection or through the kind- 

 ness of various correspondents. These analyses, arranged 

 according to the rise in the index of refraction, are given 

 below. 







1 



2 



3 



4 



5 



6 



7 



8 



Index of refraction 1-7364 



1-7412 



1-7417 



1-7438 



1-7504 



1-7596* 



1-7626 



1-7779 



Specific 



gravity . 



.. 3-506 





3-715 



3-525 





3-837 



3 633 



4-040 



SiO a ... 





.. 40-79 



41-34 



43-37 



38-80 



40-90 



41-59 



38-12 



37-25 



A1 2 3 .. 





.. 21-70 



22-75 



20-99 



22-66 



22-81 



2313 



18-35 



19-43 



Cr 2 O a .. 







2-96 

 12-12 



2-36 

 10-21 



1-75 



1-48 

 13-34 



1-90 

 15-55 



"i-vi 





Fe 2 3 . 





.. 018 



3-29 



FeO... 





.. 0-43 



35-45 



MnO .. 





.. 1-07 



0-36 



0-52 



0-30 



0-38 





0-13 



1-24 



CaO... 





.. 35-63 



5-17 



4-54 



35-00 



470 



0-92 



35-40 



2-51 



MgO... 





.. 0-39 



16-20 



18-42 



0-68 



16-40 



17-23 



0-02 



1-13 





100-19 



10090 



100-41 



99-19 



100-04 



100-32 



99-19§ 



100-30 







9 



10 



11 



12 



13 



14 



15 



16 



Index of refraction 1-7815 



1-7921* 



1-7998 



l-8008*t 



1-8010* 



1-8057* 



1-8074* 



1-8078 



Specific gravity . 



.. 4-025 



4-230 



4-058 



4-255*f 



4093 



4-169* 



4-163 



3-960 



SiO„... 





.. 38-07 



35-66 



35-76 



35-35 



3929 



35-65 



37-61 



37-40 



A1 2 3 ._ 





.. 19-63 



18-55 



21-06 



20-41 



21-70 



20-93 



22-70 



21-08 



Cr 2 3 ._ 





















Fe 2 3 . 





2-16 

 .. 31-58 



0-32 

 14-25 



1-78 



2-75 

 1-75 



30-82 



5-67 



33 : 83 



201 



FeO ... 





28-49 



MnO .. 





.. 1-36 



29-48 



39-40 



38-70 



1-51 



37-21 



1-12 



_ _ 



CaO... 





.. 5-03 



1-15 



1-23 



0-94 



1-99 



0-48 



1-44 



3 05 



MgO .. 





.. 2-77 



0-88f 



1-46 



0-27 



5-26 





3-61 



8-22 



100-60 100-33 99-69 100-17 100-57 9994 100-31 100-25 



