80 Buddington — Natural and Synthetic Melilites. 



tion of the base and first order prism modified by the 

 ditetragonal prism. « u t= 1.636 m = 1.629. The other 

 specimen is from Monte Albano, Latium, Italy, and is 

 a leucite lava with medium sized tabular crystals of hum- 

 boldtilite partly enclosing leucite. n a = 1.639 n £ = 1.637. 

 Humboldtilite from Latium. — In a recent article on 

 melilites F. Millosevich 21 describes melilite (humboldt- 

 ilite) obtained from blocks in peperino associated with 

 pyroxene, leucite, hauyne, and yellow garnet. In Table 

 XXVII are given the original chemical analysis, the 



Table XXVII. 

 Humboldtilite from Latium. 



Original Analysis Calculated Differ- Mixture 



analysis reduced. Mol. molecules ence weight percent. 



Si0 2 41.07 41.24 6839 6802 —37 40 2CaO.Mg0.2Si0 2 



A1 2 3 10.47 10.51 1030 1055 +25 38 3CaO.Al 2 3 .3Si0 2 



Fe 2 3 3.80 3.82 239 236 —3 12 3CaO.Fe 2 3 .3Si0 2 



FeO None 10 (Na,K) 2 O.AL0 3 . 



CaO 33.92 34.06 6071 6157 +96 3Si0 2 



MgO 6.02 6.05 1501 1465 —36 



Na,0 3.25 3.27 527 527 



K 2 6 1.04 1.05 111 111 



99.57 100.00 



analysis reduced to 100, the constituent molecules, and for 

 comparison the calculated molecules for a mixture of 

 assumed components most closely approximating the 

 natural mineral in composition. 



The indices of refraction for the natural mineral are 

 given by Millosevich as n u = 1.633 and n e = 1.629 with a 

 birefringence of 0.004. The indices of refraction for a 

 synthetic preparation of similar composition are n 9 = 

 1.639 and n E = 1.635 with a birefringence of 0.004. The 

 difference is greater than might be expected, but the 

 agreement may still be considered good. 



Colorado humboldtilite. A humboldtilite from Gun- 

 nison County, Colorado, has been analyzed by Schaller 

 and described by Larsen and Hunter. 22 It forms two- 

 thirds of a rock called uncompahgrite, and is associated 



21 Bend. Accad. Lincei, vol. 30, pp. 80-84, 1921. 



22 E. S. Larsen and J. F. Hunter, J. Wash. Acad. Sei, vol. 4, p. 473, 1914. 



