428 



PROCEEDINOS OF THE NATIONAL MUSEUM. 



VOL. 41. 



TWIN CRYSTAL. 



A twin crystal was observed under the microscope among the 

 crushed aggregate. The twinning phxne is (102) (first orientation) as 

 the inclination of the twinned part of the complex to the untwinned 

 part was measured as 31° and the angle (001):(102) is calculated as 

 31° 28'. Similar twins were noted on phosphosiderite, again showing 

 the striking crystallograpMcal closeness of variscite with phospho- 

 siderite, and emphasizing the probable identity of phosphosiderite 

 with strengite. 



RELATION TO ANALOGOUS MINERALS. 



It may be stated that the angular values for the minerals of tliis 

 group are so close in three different directions that one must be very 

 careful that the correct position has been chosen. This should always 

 be verified optically if at all possible, as on poor crystals it would be 

 difficult to distinguish with certainty some of the angles. Tliis sim- 

 ilarity in angular values can be well shown by the following table: 



Comparison of angles shoiving similarity. 



It is possible that the mineral vilateite/ the violet crystals of Des 

 Cloizeaux's hureaulite, belong to this group. The crystals are de- 

 scribed as monoclinic, but the angular differences from orthorhombic 

 symmetry are slight, and Lacroix states that optically the mineral 

 behaves somewhat like an orthorhombic substance. The similai'ity 

 in axial ratios to scorodite, etc., can be seen in the following table. 

 For vilateite, half the a axis, as given by Lacroix, has been taken. 



Comparison of axial ratios. 



1 Lacroix, A. Mineralogie de la France, vol. i, 1910, p. 477. 

 s Second orientation. 



