20 BULLETIN 18 4, UNITED STATES NATIONiAL MUSEUM 



guished from taenite by its great hardness (6.5), a needle making 

 no impression on it while producing a deep scratch on the adjacent 

 taenite or kamacite. It is magnetic and readily fusible, its melting 

 point being about 1,000° C. 



It is insoluble in cold dilute acids and therefore is unchanged with 

 ordinary etching. It is insoluble in copper ammonium chloride and 

 does not reduce copper from copper salt solutions — differing in both 

 respects from taenite, and also from cohenite, which presently will 

 be mentioned. After boiling in nitric acid the addition of ammonium 

 molybdate produces a yellow precipitate, which also is not the case 

 with cohenite. 



An unfailing test for schreibersite is etching with boiling neutral 

 sodium picrate, which blackens it but does not affect cohenite or 

 the metallic components. 



Schreibersite inclusions are often surrounded or invaded by oxide. 

 Sometimes aureoles of oxide around rhabdites or needles might 

 appear to be due to diffusion of the phosphide as a result of reheat- 

 ing; but if such were the case the rhabdites would not be likely to 

 show perfect crystalline forms, and the aureoles if due to diffused 

 phosphide would be invisible with ordinary etching. The oxide, 

 however, appears black with any etching and is also visible on an 

 unetched suface. Rhabdites thus surrounded by oxide are shown 

 in plate 53. 



Composition of schreibersite. — The composition of schreibersite, 

 as indicated by analyses, varies greatly. It is essentially a nickel- 

 iron phosphide, with usually some cobalt. The commonly accepted 

 formula is (Fe-Ni-Co)8P, although some analyses indicate the ratio 

 of Fe-Ni-Co to P as 4, 5, 7, and even higher. 



Higher ratios than 3 to 1 might possibly be explained on the 

 supposition that the material analyzed was an iron-phosphide eutectic 

 (pp. 79, 80), which without metallographic examination would not be 

 distinguished from the pure phosphide. Such eutectics often contain 

 droplets of rejected iron (pis. 58, 59), which would further increase 

 their iron content. 



Schreibersite is peculiar to meteorites. Terrestrial phosphides 

 do not exist, as they become phosphates by the action of oxygen. 



Occurrence of schreibersite. — The quantity of schreibersite in some 

 irons is very considerable; thus pieces of Glorieta Mountain were 

 found to contain 3 to 8 percent. This might seem inconsistent 

 with the phosphorus content of that iron, which analyses indicate 

 is not more than 0.36 percent. But inasmuch as the atomic weight 

 of phosphorus is about 31, while the atomic weights of iron, nickel, 

 and cobalt are between 55 and 59, an iron with 1 part of phosphorus 

 would contain about 6K parts of the "pure" phosphide. If the 



