Bornite and its Relation to Other Sulpho-Minerals. 545 

 Borcrite 5-248 



Cu 14 Fe 2 S 10 

 Cu 18 Fe 2 S 12 



5 



2 

 1 



Cu 34 Fe 2 S 20 



1 



Cu 40 Fe 2 S 23 



1 



Cu 76 Fe 2 S 4I 



1 



Chalcocite 5*51 Cu 2 S 



It appears then that there is here a regular progression in 

 the chemical composition, by the addition of Cu 2 S, from the 

 end member Fe„S 3 , through to chalcocite, the other end mem- 

 ber. The specific gravities, as far as determined, seem also to 

 show a steady increase from the lower value to the highest, 

 namely to that of chalcocite. Furthermore, the composition of 

 any member of the series can be expressed by the general 



formula Cu x Fe 2 S y , where y == "— 4- 3. 



The criticism may be raised that the evidence for the exist- 

 ence of some of the compounds included in the above list is not 

 sufficient, especially for those having large molecules. Further, 

 that there is no positive evidence that the material used for 

 analysis was always homogeneous, not having been examined 

 metallographically. In reply to these criticisms, it may be 

 stated that there are many sulpho-minerals the chemical com- 

 positions of which have not been questioned and which have 

 not been studied metallographically. In fact, there are few 

 minerals which have been investigated in this way. Also, 

 there is no doubt in our minds but that the analysts in each 

 case endeavored to get as homogeneous material as possible, 

 and that Cu 18 Fe 2 S ]2 , for example, with but only one analysis at 

 present as a basis, must, nevertheless, be considered as a definite 

 compound, being analogous in composition to the well-estab- 

 lished polybasite, Ag 18 Sb 2 S 12 . The composition of many minerals 

 is based upon more meager evidence than this, for example, 

 that of prolectite. Although this mineral has never been 

 analyzed, Mg[Mg(F,OH)] 2 Si0 4 is assumed as its formula since 

 its elements of crystallization are such as to place the mineral 

 in the humite group.* This question of the composition of the 

 various members of the above series will be discussed again. 



There are few crystallographic data at present available with 



*Groth, loc. cit., 119-120; Sjogren, Bull. G. Inst., Upsala, 1892, 1, 40, 

 also 1894, 2, 99 ; Dana, loc. cit., 1st Appendix, 1899, 55; Doelter, Handbuch 

 der Mineralcnemie, 1913, II, 313-324. 



