Vol. 70.] COMPOSITION OF KOCKALI/ITE. 301 



The Kockall pyroxene resembles other aegirite-acmites in general 

 features, but is notably lower in ferrous oxide, especially in 

 comparison with the older analyses. It is also markedly lower in 

 lime, except in the ease of the Quincy aegirite (VIII). The 

 amount of titanium oxide is comparable with those in the Quincv 

 and Brevik minerals. As the presence of this constituent was 

 strongly suspected also in the Magnet Cove aegirite, Dr. Steiger 

 not having determined it in his analysis, I obtained, through his 

 kindness, a portion of the material analysed by him, in which I 

 determined titanium oxide and zirconia. The results are given in 

 No. VII, the original figure for alumina (2'76) beinp- corrected 

 for their presence. 



There are apparently only five analyses in the literature of 

 brown acmite, 1 and while these are of early date, incomplete, and 

 not wholly satisfactory, they all differ consistently from the analyses 

 of aegirite in their much lower proportion of lime. iEgirite and 

 acmite unquestionably are closely related chemically, ervstallo- 

 graphically, and optically in many respects ; but the marked 

 differences in colour, character, and strength of pleochroism, and 

 apparently also, but to a less extent, in extinction-angle, indicate a 

 constant chemical difference between the two. This matter has 

 been discussed both by Prof. Dcelter and by Prof. Brop-ger, but 

 without either investigator arriving at any definite conclusion. 

 This may be attributed in part to the difficulty of obtaining 

 pure acmite substance for analysis, as it is commonly intergrown 

 with aegirite ; and in part to the faulty character of the analyses 

 which have been made of this mineral. 



The presence of ceria and other rare earths and the large amount 

 of zirconia found in the rockallite, which can be present only or for 

 the greater part in the pyroxenes, suggests that herein is to be 

 found the essential difference between aegirite and acmite : that is. 

 that these constituents are characteristic of acmite, but not of 

 aegirite. All the silicates which contain much cerium, as melano- 

 cerite, allanite, cerite, mosandrite, and rinkite, are yellow or 

 yellow-brown, with weak pleochroism. Similarly, it is well known 

 that the zirconia-bearing pyroxenes, — rosenbuschite, lavenite, wohl- 

 erite. hiortdahlite — are all yellowish and have generally weak pleo- 

 chroism. It may. therefore, be considered possible or probable that 

 ceria and zirconia form an essential part, and are characteristic, 

 of the molecule of the brown acmite, while they are wanting, 

 or are present only to a slight extent, in the green aegirite. 



This is, as yet, merely a suggested hypothesis, and, in order to 

 decide the question, complete analyses are needed both of brown 

 acmite and green aegirite, especially that associated with tin- 

 former. The smallness of the amount of zirconia found bv me in 

 the Magnet Cove aegirite (which was the only appropriate material 

 available), would seem to favour the view here advanced ; but, of 

 course, a single determination is quite inadequate to settle the matter. 

 The chemical study of these minerals is to be undertaken in the 

 near future. 



1 C. Hintze. : Handbuch der Mineralogie,' vol. ii (1897) p. 1134, nos. i-v. 



