no A METEORITE FROM ZULULAND. 



room for a field of " plessite." the taenite appears to merge into 

 the latter, as seen in photo 1 1 . . 



Taenite has been stated to be a solid solution of iron in a 

 Nickel, containing 13 to 35 per cent. Nickel, and according to- 

 this, the difficultly soluble portion, photo 4, may be regarded as 

 taenite ; this .however, is in conflict with the statement of another 

 authority that Taenite is TCgNi. 



Under high power (Xi/O) and vertical illumination 

 (photo 13) this constituent is seen as white bands between two 

 kamacite bars, but whenever it broadens out a dark constituent 

 appears in the centre, as is well shown in the photo and wherever 

 it enlarges so as to become of sufficient size to be apparent ac 

 lower powers, or to the naked eye. they apparently constitute 

 the fields of plessite. 



This plessite is of the greatest interest. Apparently the 

 kamacite crystallised first, the mother liquor became continually 

 richer in nickel, and finally caught between two nearly approach- 

 ing faces, solidified as taenite. But when more space was left 

 and a cell formed, the material enclosed in the cell would not 

 be sufficiently rich in nickel to crystallise as taenite, and further 

 segregation would occur. 



The amount of segregation and nature of the resulting 

 structure would vary with the size of the cell, and consecjuent 

 composition of the mother liquor entrapped, and this appears 

 to be the reason for the variable appearance presented by the 

 "plessite '" fields in different cases. 



This difference in appearance has. of course, been noted by 

 other observers in other meteorites, such terms as " dark plessite " 

 and " light plessite." " plessite with metallic sheen." etc., etc., 

 having having been used, but I have not seen any accounts of 

 the examination of plessite fields under high powers. 



In the present specimen, to the unaided eye, the small fields 

 of plessite are dark, and as the size increases, the centre becomes 

 lighter, till, with the larger fields, the i>lessite appears to consist 

 of a lig"ht area with a peculiar orientated metallic sheen. 



Under high power (X 170), each plessite area, dark or 

 light, is seen to be surrounded by a thin line of " taenite," inside 

 that occurs a zone of a dark constituent (photograph No. 13), 

 resembling in structure pearlite, i.e., it shows a true eutectic 

 structure of alternating ]3lates ; in the small areas this makes up 

 the whole section, but as the size increases the centre shows 

 segregation till in the largest areas only a comparatively narrow 

 zone of dark eutectic is seen inside the taenite, the whole interior 

 being occupied by a crystalline aggregate resembling very much, 

 on a microscopic scale, the macroscopic structure of the meteorite 

 as a whole. 



This is shown in photograph No. 14. and it appears probable 

 that these small crystals are also of kamacite surrounded by 

 taenite. the taenite. however, being relatively more abundant than 

 in the whole meteorite. 



