C. H. Warren — Mineralogical Notes. 117 



tions somewhat uncertain, and this same difficulty has been 

 experienced by others who have studied crystals of melanotekite. 

 The crystal which was finally selected as best, however, gave 

 excellent reflections from the pyramid faces, and it is believed 

 that the axial ratio derived from the measurements marked 

 with an asterisk in the accompanying table is very nearly 

 correct. The axial ratio is given below, together with those 

 obtained by Nordenskiold and Flink from melanotekite and 

 kentrolite, from Swedish localities, and by vom Rath from 

 kentrolite from southern Chili. 



Melanotekite* \ Author a:b:c = 



Melanotekite j Nordens ki6ld... a ; b : c = 



0-6338 : 1 : 0*9126 

 : 0-6216 : 1 : 0-9041 



( Flink a : b : c = 



0-6328 : 1 : 0*8988 



Kentrolite •< Nordenskiold ... a : b : c = 



0-6314 : 1 : 0-8793 



( Yom Rath . a : b : c = 



0*6334 : 1 : 0-8830 



The measurements and the calculated values of the melan 



tekite from Hillsboro are as follows : 





Measured. 



Calculated. 



a 0, lll/vlll = 55° 00'* 





0^0, Ill/sill = 119 13 * 





a 0, 111/vlll = 93 31 



.93° 32' 



o A m, 111 /si 10 = 30 24 



30 23J 



o ^ b, 111^010 = 62 32 



62 30 



vi^m, 110/sllO == 64 55£ 



64 44 



m^n, 110^130— 29 34 to 55 



29 53 



m^k, 110/sl50 = 16 52 



17 30 



The chemical formulas that have been assigned to melanote- 

 kite and kentrolite respectively are Pb Q Fe 2 Si 2 9 and Pb 2 Mn 2 

 Si 2 9 , but as the analyses do not agree in a very satisfactory 

 manner among themselves, nor with the theoretical composition, 

 it seemed very desirable to make a new analysis, and some of 

 the very best material was carefully selected for that purpose. 

 The material was treated for a few minutes with a little warm 

 dilute nitric acid to remove a trace of cerussite which could not 

 be avoided, and further than this no visible impurity could be 

 detected. The specific gravity was taken with great care on 

 the chemical balance and found to be 5-851. 



The method of analysis was as follows. The mineral was 

 dissolved in hydrochloric acid, and the silica separated in the 

 usual manner. In the filtrate from the silica lead was precipi- 

 tated as sulphide, and eventually converted into sulphate and 

 weighed. In the filtrate from % the lead the iron was precipitated 

 with ammonia, and weighed as oxide. It was subsequently 

 dissolved and determined by potassium permanganate giving a 

 slightly lower result, but the nature of the difference, X, has 

 not been ascertained. 



