82 Buddington — Natural and Synthetic Melilites. 



A specimen of the mineral supplied by Dr. Larsen was 

 heated for 20 hours at 1150 °C. and exhibited no altera- 

 tion ; heated for 20 hours at 1175°C, it dissociated. The 

 synthetic material dissociates at about the same tem- 

 perature. The mineral, called simply melilite by Larsen, 

 is here more specifically regarded as the humboldtilite 

 variety. 



The agreement between the optical properties of the 

 humboldtilite minerals and homogeneous preparations 

 of similar composition formed above 1000°C. is very close 

 and within the limits of error involved. Assuming that 

 the pure solid solutions are essentially true equivalents 

 of the corresponding humboldtilites, it follows by analogy 

 that the compound 3CaO.Al 2 3 .3Si0 2 enters into the hum- 

 boldtilites with the effect of a negative, moderately 

 birefringent, uniaxial compound, for which a mineral 

 equivalent as a separate entity is unknown. 



The present explanation obviates the difficulty of 

 conceiving the formation of a negative uniaxial mineral 

 from an isomorphous mixture of two positive uniaxial 

 end members, as suggested by Schaller. 



The humboldtilites, according to the interpretation 

 offered here, are essentially isomorphous mixtures of 

 positive uniaxial akermanite (2CaO.Mg0.2Si0 2 ) and a 

 negative uniaxial, moderately birefringent form of 3CaO. 

 Al 2 3 .3Si0 2 with minor amounts of gehlenite, a ferrous 

 iron compound, and 3RO.R 2 3 .3Si0 2 compounds. 



Ferric Iron-rich Melilite. 



Among the specimens of melilite from Capo di Bove, 

 obtained by Dr. H. S. Washington from Professor F. 

 Millosevich, was one which proved to be a new member 

 of the group. It occurs in pockets, about 1% inch in 

 diameter, of massive crystalline granular material asso- 

 ciated with nephelite and pyroxene, and as minute crys- 

 tals coating druses in a melilite-leucitite lava. The 

 crystals are yellowish-brown and coated with a needle- 

 like unidentified mineral. Their habit is tabular to 

 pseudo-cubic, or a combination of the first order prism 

 and base modified by the second order prism. 



Under the microscope they are found to show a marked 

 zonal growth. The cores are isotropic for Na light and 



