THE METALLOGRAPHY OF METEORIC IRON 19 



bands in place of taenite, or alternating with similar bodies of taenite; 

 as larger irregular inclusions of all shapes; and as a dispersion of 

 exceedingly minute particles. Rhabdites originally were described 

 and named as a substance different from other forms of schreibersite. 

 Its crystalline habit, long surmised to be tetragonal, was so estab- 

 lished by Wherry (1917) and confirmed through X-ray analyses by 

 Heide, Herschkowitsch, and Preuss (1932). 



Schi'eibersite is present to some extent in the majority of meteoric 

 irons, and in many it is a conspicuous component. Rhabdites are 

 especiall}^ plentiful in hexahedrites. They may be uniformly scat- 

 tered, or they may appear m certain areas or grouped in parallel 

 zones. Often they appear to be scattered with no definite arrange- 

 ment, but they probably are oriented. In octahedrites Brezina 

 found schreibersite lamellae to be disposed along dodecahedral 

 planes, but that conclusion seems erroneous (p. 81). 



As pointed out in Chapter II, an apparent needle is a cross section 

 of a lamella. It is very unlikely that the plane of the section ever 

 would intersect a true needle lengthwise if it were fine and of any 

 considerable length. True needles, if they exist, would probably 

 be short and stout. It is impossible to determine definitely whether 

 an apparent needle is such, or a section of a lamella, without examin- 

 ing the surface after successive polishings. 



That rhabdites are small crystals, and not cross sections of long 

 prismatic crystals or needles, is apparent from observation and is 

 confirmed by the fact that heating changes them to shapeless rounded 

 bodies and also by the fact that repolishing causes some to disappear 

 and new ones to appear. 



Very fine phosphide particles in iron that has not been reheated, 

 though appearing as dots, are probably not rounded but normally 

 angular. The etching attack around the particles is very rapid, 

 leaving a relatively wide-etched interface between the crystallite 

 and the surrounding kamacite, which causes the more minute ones 

 to appear black even with high magnification. 



In its larger forms schreibersite occurs as apparent needles, some- 

 times several centimeters long; also in rounded, droplike, dendritic, 

 graphic, jagged, and sprangling shapes, sometimes reaching dimensions 

 of an inch or more. It is often associated with troilite, adjoining 

 it or surrounding it, and sometimes with graphite. 



Ov/ing to its brittleness schreibersite, except in small inclusions, 

 is apt to suffer surface chipping in polishing, causing black spots. 

 Minute phosphide particles also pull out in polishing, causing black 

 dots to appear quickly with even the lightest etching. 



Properties of schreibersite. — Schreibersite is tin-white, like taenite, 

 but the eye soon learns to recognize it easily. It is readily distin- 



