Buddington — Natural and Synthetic Melilites. 79 



Table XXVI. 

 Thermal data for Humboldtilites from Monte Somma. 



Humboldtilite A. 



1150° 16 hrs. Clear and unaltered. 



1170° 40 hrs. Trace of dissociation. Poikilitie rods in some 



grains, but most grains unchanged. Altered 

 grains are mottled in appearance, unaltered 

 grains, are fresh and clear. 



1200° 16 hrs. Part of material is dissociated, and traces of 



glass are present; part is unaltered and with- 

 out traces of glass. Indices of refraction of 

 altered material are higher. 



1225° 16 hrs. Crystals plus a small amount of interstitial 



glass. 



1320° 1 hr. Glass plus few crystals. The crystals exhibit 



anomalous bluish and brownish yellow inter- 

 ference colors. 



1313° 1 hr. All glass. 



Humboldtilite B. 

 1150° 16 hrs. Clear and unaltered. 



1170° 20 hrs. Dissociated. For the major crystalline material 



no = 1.642 and n € = 1.635. 



Humboldtilite C. 

 1150° 16 hrs. Glass formed by melting at higher temperature 



and quenching was recrystallized and the 



crystals had the following indices 



n<a = 1.639 n e = 1.633. 

 1170° 20 hrs. Unaltered; clear and fresh. 



1200° 16 hrs. Most grains are clear without a trace of glass. 



A few grains with impurities show traces of 



glass. 

 1235° 16 hrs. Interstitial glass noticeable. 



1313° 1 hr. Crystals with gray interference colors and glass. 



1335° 1 hr. All glass. 



glass, and when subsequently recrystallized at 1150° for 

 16 hours, exhibited indices of refraction which are within 

 the limits of error for those of the original material. 



A and B are also similar to the synthetic preparations 

 inasmuch as they dissociate at similar temperatures, most 

 of the material having higher indices of refraction after 

 dissociation. No trace of dissociation was noted in 

 experiments with C. 



The indices of refraction alone were determined on 

 two other Italian specimens with the following results. 

 The first specimen is a limestone fragment, with hum- 

 boldtilite crystals occurring in pockets with biotite and 

 pyroxene. The crystals are short prismatic, a combina- 



