^■s.] 371 t G Cnth . 



Neither specimen showed on examination with a magnifying glass any 

 corundum, but both b and c\ and 2 contained small quantities of chlorite- 

 I shall leave out the analysis b, which was made with material of not 

 sufficient purity to base any calculation upon the results, but only for the 

 purpose of ascertaining the character of the black mineral. This ana- 

 lysis, however, established the very important fact of the mechanical 

 admixture of corundum. Taking into consideration the analyses of a 

 and cl and 2, and d, we arrive at the following data : 



a. The oxygen in the mean results of the two analyses of the spinel 

 from Hindostan is, in the sesquioxides, 4.22 percent, in excess over that 

 required by the formula RO,R,0. ; , showing that besides the 4.31 per 

 cent, of corundum, which was not dissolved by dilute sulphuric acid, 

 9.05 per cent, had gone into solution, and that the most carefully selected 

 material still contained a mechanical admixture of 13.36 per cent, of 

 corundum. The composition of the pure spinel, together with that 

 of the pure spinels from Culsagee, I shall give below. 



cl. After deducting for 1.14 percent, of Si0 2 , 0.93 A1A» 0.09 Fe,0 3 , 

 0.22 FeO and 1.18 MgO, corresponding with the foliated chlorite from 

 this locality, the oxygen ratio between RO and R_,0, required by the 

 spinel formula is 6.67 per cent, in excess in the sesquioxides, represent- 

 ing an admixture of 14.74 per cent, of corundum. 



c2. The 1.56 per cent, of SiO, require, for making the foliated chlorite 

 associated with the siunel, 1.28 A1,0, 0.14 Fe,0 3 , 0.32 FeO and 1.61 MgO, 

 which being deducted, leaves the oxygen ratio between RO and Kfi. i = 

 7.92 : 31.34, being for the sesquioxides 7.58 per cent, more than that re- 

 quired by RO, R.,0 : , ; this corresponds with a mechanical admixture of 

 16.27 per cent, of corundum. 



d. The decomposition by dilute sulphuric acid gave 0.73 per cent, of 

 silica and a trace of corundum, which were deducted from the quantity- 

 taken for analysis. The oxygen ratio 1 : 3.15 is nearly that of spinel. 

 The excess of 1.57 per cent, of oxygen would represent an admixture of 

 3.37 per cent, of corundum. 



After deducting these mechanical admixtures, the composition of the 

 pure minerals is represented by the following : 



A1,0 3 = 



CrA 



Fe 2 3 



FeO 



MgO 



CuO 



NiO 



These analyses show that the spinel, pseudomorphous after corundum, 

 from Hindostan, and the green spinel from Culsagee d, are mixtures of 

 the spinel varieties pleonaste and hercynite, whilst the spinels from Cul- 

 sagee c (1 and b) contain besides an admixture of the variety picotite. 



