PHYSIOGRAPHY. 



Plumbago. 



PLUMBAGO. Rhombohedral Graphite-Mica. 



MOHS. 



Primary form. Rhomboid, of unknown dimensions. 

 Secondary forms. 1. Six sided prism. 2. Six sided, 

 prism, with terminal edges truncated. The crystals inva- 

 riably posses-s a tabular appearance. 



Cleavage, perpendicular to the axis of the rhomboid, (or 

 parallel with the bases of the hexagonal tables,) perfect. 

 Fracture uneven, scarcely observable. Surface, bases of 

 the prisms generally smooth, or faintly striated parallel to 

 their edges of combination ; the rest of the faces rough. 



Lustre metallic. The highest degrees of lustre are found 

 upon the perfect faces of cleavage, and upon the bases of 

 the hexagonal tables. Color iron-black, dark steel-grev. 

 Streak black, shining. Opake. 



Sectile. Thin lamina are highly flexible. Hardness 

 = 1-0... 2-0. Sp.gr. =2-0891. 



Compound Varieties. Massive : composition granular, 

 tbe individuals flat and scaly, of various sizes, frequently 

 impalpable. Of the latter, the fracture is conchoidal or 

 even. 



1. In a high degree of heat, it is combustible, and leaves a residue of 



oxide of iron, tt is infusible. 



2. Analysis. 



ByScHEELE. By VAUQUELIN By SAUSSURE. 

 Carbon - 81-00 - - 9200 96* 



- 10-00 - - 8 00 4-00 



9-00 - 000 - 000 



Oxygen 



3 The varieties of this species are found in beds, or form beds by 

 themselves, in slaty and ancient trap-rocks. They seem often to replac 

 Mica and Talc in certain rocks. It is particularly in beds of limestone 

 that the crystallized variety of Plumbago ^occurs. It is likewise 

 in the coal formation. 



