THE METALLOGRiAPHY OF METEORIC IRON 25 



intergranular development. The contrasts in appearance are shown 

 in plates 67-70. Intrusive oxide is apt to invade bodies of schreiber- 

 site because of its low fusibility. 



A further discussion of intrusive oxide will be found in Chapter 

 XIII. 



Silicates. — A number of silicates are associated with meteoric iron. 

 In mesosiderites (ironstones), transitional between stony and iron 

 meteorites, they may form half or more of the mass. In pallasites, 

 composed of nickel-iron and the silicate chrysolite, the silicate may 

 form a groundmass in which bodies of iron are dispersed, or the hon 

 may form a spongy mass filled with the silicate in the interstices, 

 showing as a network upon a polished surface. In siderites (meteor- 

 ites consisting wholly of iron) sUicates occasionally occur as inclusions. 



Of the silicates thus found in siderites the most common are chrys- 

 olite (olivine), anorthite, and some members of the pyroxene group. 

 These also are found in mesosiderites. Forsterite occm-s in the 

 Tucson ataxite (pi. 20), but according to Cohen it has not been ob- 

 served in other irons. In the unique Fom* Corners octahedrite 

 there are abundant inclusions of pyroxene. Blebs of glass are 

 occasionally observed in meteoric irons. 



Minor constituents. — The foregoing are the more important acces- 

 sory constituents of meteoric irons, but there are numerous others 

 that are of interest from the standpoint of the general science of 

 meteoritics. These include the diamond, of which small crystals 

 and particles have been observed, notably in Canyon Diablo; chi*o- 

 mite, which occurs not infrequently as grains and sometimes in nod- 

 ules; and the ferrous cliloride lawrencite (FeCl2), which is reported 

 to have been observed in the solid state but which is deliquescent 

 and usually manifests itself in an exudation of clear greenish drops. 



Lawrencite is not a factor in the structure of meteoric irons except 

 by reason of its destructive action in causing rust and disintegration. 

 By the action of the air it turns to ferric chloride (FeCls) and ferric 

 hydroxide (limonite). The ferric chloride in contact with the still 

 unaltered u*on is reduced again to the ferrous chloride. Thus the 

 process is continuous, and as it is accompanied by an increase in 

 volume the hon may become wholly altered and disintegrated unless 

 it is kept under oil. Lawrencite is the bare of curators and col- 

 lectors. If an iron definitely contains lawrencite, no amount of 

 lacquering seems to protect it; the only recourse is immersion in oU. 



V. ARTIFICIAL IRONS 



A brief outline of the metallography of artificial irons is a neces- 

 sary preliminary to the study of the structures of meteoric irons — 

 partly because of the important analogies involved and partly be- 



507394—44 3 



