78 Buddington — Natural and Synthetic Melilites. 



Table XXV. 



Comparison of properties of natural Humooldtilites with those of similar 

 synthetic preparations. 



ABC 



Components Natural Artif. Natural Artif. Natural Artif. 



Gehlenite 10 10 .. .. 8 10 



Akermanite 50 54 37 48 43 45 



3CaO.Al 2 3 .3SiO., .... 24 26 31 39 32 33 



3Na 2 O.AL0 3 .3Si6 2 } - 1n , q 1ft q 10 

 3K 2 O.AL0 3 .3SiO, S ' 



3CaO.Ge 2 3 .3Si0 2 .... 2 3 3 2 2 



Ferrous minerals .... 7 16 6 



no* 1.639 1.635 1.637 1.633 1.637 1.635 



ne 1.633 1.633 1.631 1.630 1.631 1.632 



Birefringence 003 .002 .006 .003 .006 .003 



The nature of the ferrous iron compound is unknown, 

 but for purposes of recalculation the writer follows 

 Schaller in assuming it to exist as a ferrous iron aker- 

 manite (2CaO.Fe0.2Si0 2 ). 



For purposes of comparison, the optical data for arti- 

 ficial mixtures of similar composition, but lacking the fer- 

 rous iron compound, have been computed and are included 

 in Table XXV. 



From an inspection of this table, it is evident that the 

 effect of the ferrous iron compound is to increase both 

 the indices of refraction and the birefringence. 



The comparison of the natural minerals and the arti- 

 ficial mixtures shows an agreement of the optical charac- 

 ters well within the limits of the possible errors involved. 

 As a further check on this matter, several experiments 

 (Table XXVI) were made on the natural minerals, and 

 the results obtained are all confirmatory of the essential 

 identity of the natural minerals and the synthetic 

 preparations. 



This interpretation, however, is conditioned by the 

 facts that the nature and properties of the ferrous iron 

 compound are unknown, and that accurate analyses and 

 indices of refraction of humboldtilites showing a wider 

 range of composition than those now available are very 

 much needed. The amount and effect of ferric iron in the 

 humboldtilites also needs to be studied. 



Table XXVI shows that all three humboldtilites when 

 held for 16 hours at 1150° showed no noticeable change. 

 All three, when completely melted and quenched to form 



