454 Lindgren and Hillebrand — Minerals from Arizona. 



Chrysocolla from the Modoc open cut appears as mammil- 

 lary crusts of bluish green color on u copper-pitch ore." The 

 latter is isotropic and undoubtedly a distinct mineral from the 

 chrysocolla, of brown color in varying tints, some of it opaque 

 and showing evidence of concentric deposition. On top of the 

 chrysocolla are thin crusts of quartz and some calcite. The 

 chrysocolla has three different structural forms, as seen under 

 the microscope : (1) The dominant mass is a cryptocrystalline 

 to microcrystalline aggregate of particles with high birefract- 

 ing index ; (2) very fibrous and felted aggregates of same sub- 

 stance giving undulatory effects between crossed nicols and 

 medium high colors ; (3) fibrous crusts on top of 1, or also in 

 thin layers between masses of 1, the individuals having such a 

 remarkably parallel orientation that the aggregate of them 

 appears almost like single crystals between crossed nicols, with 

 black shadows sweeping across them when the table is turned. 

 The extinction is parallel to the fibers, double refraction strong, 

 about like augite, character negative. The same optical char- 

 acteristics were repeatedly observed in thin sections of chryso- 

 colla from Metcalf and other places. Reniform deposits were 

 sometimes noted, the center of cryptocrystalline material coated 

 with coarsely fibrous and highly birefringent material. 



Sections from the Coronado and Metcalf mines often showed 

 pseudomorphs of pyrite consisting of a shell of limonite with 

 kernel of fibrous chrysocolla. 



The observations of Jannettaz* on chrysocolla from Boleo 

 Baja, California, Mexico, led to the same results as described 

 above, but seem generally to have been overlooked by editors 

 of text-books. 



Copper pitch ore. — Under this old German name is described 

 a dark brown to black substance, sometimes dull but generally 

 with glassy to resinous luster ; hardness about 4 ; streak dark 

 brown. It occurs among the products of oxidation of the 

 deposits in limestone, as at the Detroit and Longfellow mines 

 and Modoc open out at Morenci, and is associated with azurite, 

 malachite, and chrysocolla, often enclosing these minerals or 

 replacing in branching veinlets, together with azurite, a shale- 

 like mass, probably largely composed of kaolin. In thin sec- 

 tion it is sometimes opaque, but often also translucent, gradual 

 transitions obtaining in the same section, and occurs in irregu- 

 lar or concretionary masses, often containing small embedded 

 crystals of a doubtful mineral, possibly a silicate of zinc. 

 Between crossed nicols the translucent mineral always proves 

 entirely isotropic and, except for varying depth of color and 

 the small crystals mentioned, entirely homogeneous. 

 *Bull. Soc. Min. Paris, 1886, ix, 211. 



