phalen] 



NEW OCCURRENCE OF UNAKITE 



309 



Hypcrsthcnc-akeritc'^ 



Akerite, the country rock of the unakite, is a very coarsely grained, 

 dark grayish green aggregate of feldspars and black pyroxenes with 

 here and there an occasional speck of limpid quartz. The greasy 

 appearance of some of the constituents is strongly suggestive of 

 the mineral nephelite, though the chemical and microscopic analyses 

 show that none of this mineral can be present. The albite twinning 

 of the plagioclase feldspar is plainly seen on many of the fresh frac- 

 tures and an occasional Carlsbad twin is also evident. 



In thin section the following minerals were noticed : Orthoclase, 

 plagioclase, orthorhombic and monoclinic pyroxene, quartz, micro- 

 cline, iron ore, apatite, and zircon, with the decomposition or altera- 

 tion products, epidote, chlorite, and 

 sericite. Hornblende is absent in 

 most of the slides studied. In a 

 light-colored segregation situated 

 near the upper boundary of the 

 akerite (a single instance, it may 

 be observed), large masses of horn- 

 blende crystals were noticed. The 

 microscopic features of the horn- 

 blende will be alluded to in the sub- 

 sequent descriptions. The accom- 

 panying drawing (fig. 39) repre- 

 sents the crystallization of the horn- 

 blende in the light-colored segrega- 

 tion of the country rock. 



The minerals are arranged in a 

 hypautomorphic mosaic, which is 

 strikingly clear and beautiful when 

 fresh. By far the most abundant 



mineral is feldspar, which shows evidences of strain in the bent 

 albite lamellae and frequency of wavy extinction. It is often asso- 



^ Classified as harzose according to the quantitative scheme of Cross, Iddings, 

 Pirsson, and Washington. For a revision of the calculation of the analyses 

 according to the quantitative scheme, and for examining the proof, I am 

 indebted to Dr. H. S. Washington. Dr. Washington was also kind enough to 

 point out a discrepancy in a former paper by me on " The Rocks of Nugsuaks 

 Peninsula, Greenland" {Smithsonian Miscellaneous Collections, xlv, p. 183), 

 in which, in the calculation of the rangs, percentages instead of molecular 

 proportions were employed. This necessitates changing omeose (p. 209) and 

 dellenose (p. 212) to positions in the next subrangs in each instance. They 

 are, therefore, liparose and toscanose, respectively. 



Fig. 39. — Hornblende in lighter phase 

 of akerite. (Scale i"^i'.) 



