370 S. L. Penfield — Spangolite, a new copper mineral. 



in determining the optical orientation which is at once sug- 

 gested by the noteworthy correspondence of riebeckite and 

 segirite. 



The minerals segirite, acmite, arfvedsonite and barkevicite 

 have been described mainly from eruptive rocks rich in 

 alkalies, such as elseolite-syenite, phonolite, leucitite, etc. Rie- 

 beckite occurs in a much altered granite, in large distinct 

 prisms and in microlites included in feldspar. The larger free 

 prisms are regarded by Sauer* as primary, and the latter as 

 secondary, in origin. The occurrences here described suggest 

 the query whether the large individuals of riebeckite in the 

 Socotra granite may not have been originally common horn- 

 blende, which were replaced by riebeckite at the time the 

 secondary needles were formed. The only other occurrence of 

 probable riebeckite known to the writer is in a quartz-porphyry 

 of Wales, described by A. Harker.f Although this rock has 

 undergone structural metamorphosis the blue amphibole is con- 

 sidered to be primary riebeckite. 



If the minerals which have been described be considered as 

 segirite, riebeckite, etc., then they appear in novel associations 

 and were clearly formed under conditions quite different from 

 those attending their origin in other known occurrences. 

 Whatever the minerals in question are held to be there are 

 many interesting points illustrated by their relationships. 

 Perhaps none is more striking than that a pyroxene should be 

 formed in such a manner from the decomposition of an amphi- 

 bole, or that it should be formed at all, as an apparent 

 end product of decomposition, in company with calcite and 

 quartz. The occurrences show plainly that we have much to 

 learn regarding the physical properties, the chemical compo- 

 sition, the genetic relationships, and the conditions of forma- 

 tion of the members of the geologically and mineralogically 

 important groups of the amphiboles and pyroxenes. 



Art. XLYI. — On Spangolite, a new Copper Mineral / by 



S. L. Penfield. 



During the summer of 1889, while visiting Mr. Norman 

 Spang of Etna, Allegheny County, Pa., my attention was 

 called by him to a very beautifully crystallized specimen of an 

 unknown mineral which he had obtained from a man living 

 near Tombstone, Arizona. The original owner had a small 



*Loc. cit. 



f Notes on the Geology of Mynydd Mawr and the Nyntle "Valley, Geol. 

 Mag., v, 1888, p. 221 and p. 455. 



