99 



of the two determined rhombohedra, allthough tlie form {10T2} 

 is in other cases one of the rarest of all. 



The complete form of the crystals is given in figure 1. 



On some crystals there can be seen small, very indistinct 

 negative rhombohedra but these give no reflections. 



No twins were observed. 



Etching figures are readily produced with the aid of cold 

 hydrofluoric acid or hot diluted hydrochloric acid, in both 

 cases the effect lasts at the most only a few seconds. The 



Fig. 2. Gyrolite, Nia к о m at. The etching figures on the 

 upper and lower basis are directed opposite to each other; 

 the former ones have the point of the triangle turned down- 

 wards. — The enlargment 1 : 45. 



figures are of a sharp triangular form, most commonly witli 

 concave edges as will be seen on figures 2 and 3. On the 

 first of these we see that the etching figures on the lower and 

 upper basis are directed opposite to each other. When etching 

 figures are made upon a crystal with rhombohedral faces we 

 see that the sides of the triangle produced by etching are not 

 quite parallel to the edges between cfoOOl} and «{l012} but 



