THE METALLOGEiAiPHY OF METEQiRIC IROflST 87 



in droplike form and crystals, can have separated only from the 

 liquid state. The same conclusion applies to Reichenbach lamellae. 



Reichenbach lamellae.— The fact that Reichenbach lamellae 

 (Chapter IV) are oriented according to cubic planes is not readily 

 explainable from the iron-sulphide and iron-nickel-sulphide diagrams, 

 except upon the assumption that they separated in the delta phase, 

 which like the alpha phase is cubic in its crystallographic planes. 

 Their separation obviously must have been complete long before 

 the gamma-alpha transformation began, because even the very 

 fusible iron-iron-sulphide eutectic solidifies at 970°; whereas the 

 gamma-alpha transformation, even in the lowest nickel octahedrites, 

 did not begin until about 700°. 



The hexagonal crystals observed in a few irons also probably 

 separated in that phase, slow cooling through the higher temperature 

 range affording time for them to perfect their crystalline form. 



Troilite has a structure of polyhedral grains, similar to that of 

 ferrite and nickel-iron in the alpha phase (pi. 49). 



Expansion of troilite. — The very considerable expansion of troilite 

 at its beta-alpha transformation at 130°- — sufficient to burst porcelain 

 tubes used in experiments with the artificial sulphide — is cited by 

 Rinne and Boeke, and also by Vogel and others, as a possible expla- 

 nation for the disruptions evidenced by cracks often observed in 

 meteoric irons. To the author such cracks seem adequately explain- 

 able upon other grounds (see Chapter XVI), apart from the fact 

 that they occur in irons that contain no troilite. 



Swathing kamacite. — The character and occurrence of swathing 

 kamacite were mentioned in Chapter III, and in this chapter it has 

 been referred to as bemg rejected from cooling and shrinking masses 

 of troilite. That mechanism seems to explain quite satisfactorily 

 the bands surrounding nodular masses. The fact that they are 

 likely to vary in thickness according to the size of the nodule, and 

 are lacking around small inclusions, is also consistent with that 

 explanation. 



We find, however, that thin needles (lamellae) of troilite in some 

 octahedrites, especially Reichenbach lamellae, are sometimes sur- 

 rounded by broad zones of kamacite, unrelated in direction with 

 the structure of the mass but having angular outlines not exactly 

 corresponding with those of the lamellae. Their thickness may be 

 much more than that of the lamellae, which would preclude the 

 assumption that they were wholly a product of rejection from the 

 latter during cooling. In such cases a narrow band of rejected 

 kamacite may have acted as a nucleus for the precipitation of addi- 

 tional kamacite as the Widmanstatten structure developed. 



Swathing kamacite also is found surrounding needles of schi'ei- 



