84 



RECORDS OF THE AUSTRALIAN MUSEUM. 



In this as in succeeding tables the calculated angles are those 

 given by Goldschmidt in his Krystallographische Winkeltabellen. 



Since a considerable number of angular measurements of topaz 

 crystals from Emmaville are now available, it may be of interest 

 to calculate the corresponding axial ratios, especially as Penfield 

 and Minor^ have shown that the ratios vary with the isomorphous 

 replacement of fluorine by hydroxyl. For this purpose the angles 

 were carefully revised, the best measurements selected, and means 

 taken. The forms chosen from measurements on six crystals and 

 the mean angles obtained are given in the table below ; as it was 

 judged that the prism m and the pyramid u yield the most 

 reliable data, the corresponding values for a and c are counted 

 twice in finding the mean ratios. 



2 Penfield and Minor— Amer. Journ. Sci., xlvii., 1894, p. 387. 



