106 



If the erikite be really a pseudomorph after a mineral of 

 a quite different composition, its peculiar crystalline form that 

 cannot be identified with any one else, shows, however, that 

 the original mineral must be hitherto unknown. As one of 

 the most nearly allied minerals britholite ^) occurs to the 

 thought as being somewhat similar to the erikite with regard 

 to composition and crystalline form. It is also rhombic and 

 has the axial ratio : 



a:b:c = 0*620 : 1 : 0*423 

 while that of the erikite is 



a:b:c = 0-675522 : 1 : 0-757961 (= 2 • 0-378980) 



These values , however, are too far distant to admit of 

 taking the minerals to be isomorphous, so much the less, as 

 the britholite always crystallises in pseudo-hexagonal twin 

 crystals. 



Systematic. 



As is seen from the preceding , the erikite is not nearly 

 allied to any other mineral, which is also the case with several 

 others of the more rarely occurring Greenland and Norwegian 

 pegmatitic minerals. By an examination of the chemical com- 

 position of these minerals we find, however, the remarkable 

 fact that in most of them some corresponding features are 

 found, which are found in almost no other mineral; therefore 

 I think it to be natural to form a special group of these 

 minerals, Avhich group will have to be designated as 



Compounds of silicates^ tkanates^ etc. with phosphates^ 

 niobates^ tantalates etc. 



that is, upon the whole containing at once tetravalent and pen- 

 tavalent acids. 



In the annexed table are entered the compositions of the 

 minerals containing both kinds of acids to so great an amount 



>) Described by Chr. Winther in Medd. om Grönland XXIV, 1901, p. 190. 



