[johnston-ellsworth] annaheim meteorite 87 



Metallography 



Metallographic study of polished surfaces of the Annaheim iron 

 reveals remarkable complexity of structure and composition. At 

 least four distinct constituents are seen to be present in appreciable 

 amounts, each having a definite arrangement with respect to the 

 others. Of these the most prominent and the most remarkable are 

 the structures shown in PI. Y, Figs. 1, 2; PI. VI, Figs. 3, 4; PI. YU, 

 Figs. 5, 6, which consist of an iron-nickel alloy containing 38% of 

 nickel, cobalt and copper (Analysis 2, page ). This alloy occurs 

 either as single lamellae (PI. V, Fig. 1 ; PI. YI, Fig. 3 ; PI. XI, Figs. 13, 

 14) or as complex lamellar structures somewhat rese.mbling in appear- 

 ance the cementite-ferrite eutectic perlite. The complex structures 

 consist of alternate lamellae of 38% nickel alloy and 6.87% nickel- 

 cobalt ground mass, the whole surrounded by a layer of the high 

 nickel alloy (PI. \T, Fig. 4; PI. VII, Figs. 5, 6). They have evidently 

 been built up by the high nickel alloy in its eutectic capacity envelop- 

 ing numerous relatively small areas of ground mass. When the 38% 

 nickel alloy occurs as thin single lamellae it appears to be very pure 

 and homogeneous, but the larger complex structures usually contain 

 minute inclusions, which in some cases resemble the ground mass 

 (best detected in heat-tinted specimens). Somtimes others that 

 appear to be partially resorbed crystals of (FeNiCoCu)3P are present 

 (Analysis 4, page 83). On etching with bromine water the larger 

 structures sometimes develop dark areas (PI. X, Fig. 11) which the 

 writer had at first taken to indicate the presence of carbon or minute 

 disseminated graphite particles. On reading Stead's work, however, 

 it appears that the black deposit might be due to the presence of 

 dissolved phosphide (FeNi)3P so that it is uncertain which it is. 



Occupying situations, like the 38% Ni alloy, between grains of 

 ground mass are irregular, elongated masses of uncrystallized phos- 

 phide (Analysis 3). This phosphide is usually, if not always, much 

 fractured (PI. VIII, Fig. 7; PI. X, Fig. 12), showing its brittle char- 

 acter and indicating that forces of deformation either internal or 

 external have been at work subsequent to cooling. Certain masses 

 of this phosphide obtained by dissolving out the ground mass (page 

 13) showed some indications of what might be rough crystal faces, 

 but these surfaces probably have resulted from contact with the 

 grains of the ground mass. 



This phosphide and the 38% Ni alloy together appear to play 

 the part of a eutectic, to the 6.87% nickel ground mass, each con- 

 tributing to the envelope which surrounds the areas of ground mass. 



