118 



from which by a rather simple transformation may be got an 

 axial ratio : 



a' :b' :c' = ^biaibc = 0'403 : 1 ; 0-341 



having even resemblance to the erikite and especially to colum- 

 bite of which, according to SchraufM the axial ratio is 



a:b :c = 0-4074 : 1 : 0-3347. 



Wöhlerite being monoclinic agrees, as to its crystalline 

 form, rather well with the rhombic minerals; according to 

 Brøgger^) the axial ratio of wöhlerite, when the Tschermak- 

 Groth arrangement is used, is: 



a:b :c = 0-99661 : 1 : 0-35471 

 ß = 89° 19' 



compared with this we have for 



polycrase =*): a : b : с = 0-96289 : 1 : 0-30085 

 and for coliimbite^): a:b:c = 0-818] 9 : 1 : 0-27379 



A somewhat closer resemblance to the above mentioned 

 minerals seems to be obtained, when in the wöhlerite the a-axis 

 is divided by 3, and then replaced by the c-axis ; then we get 



a :b: с = 0-35471 : 1 : 0-3320. 



In the following is given a schematic survey of the mine- 

 rals treated of here, which minerals, according to the preced- 

 ing, are: 



Compounds of titanates, silicates etc. (mostly metatitanates) 

 with niobates^ phosphates etc. (mostly metaniobates). 



A. The ratio between the number of tetravalent groups 

 of acids {BOo) and pentavalent ones (B^Or,) is equal to or less 

 than 1 ; all isometric, generally in octahedra. 



M ßer. Akad. Wien 44, 1861, p. 445. 

 2) Zeitschr. f. Kryst. XVI, 1,S90, p. 364. 



') Both values have been obtained by prolonging the a-axis threefold, 

 and then replacing them by the i-axis; as a starting point has been 



