408 



KKCOHDS OF THE AUSTKALIAN MUSEUM. 



is found in reticulated masses, forming very attractive specimens. 

 Very typical are arrow-head twins on r (130), sometimes opaque 

 white and of considerable size, at other times ti-ansparent, when 

 they are as a rule smaller. Twinning on m (110), the more usual 

 law, I have not observed, but Miigge'- and Spencer'^ mention its 

 occurrence. 



Simple crystals are not common ; one such is represented in PI. 

 Ixxvi., figs. 1,2. It is tabular on b which is striated parallel to 

 ])rism and brachydome edges ; r oscillates slightly with h. 



It yielded the following forms and angles : 



When twinned on r and long prismatic in habit the crystals 

 resemble PI. Ixxvi., fig. 3 ; if the prisms are short vertically with 

 predominant r, and the faces of the domes k and k meet in an 

 edge, above, below, and at the sides, the resemblance to an arrow- 

 head is ver}^ striking. A typical twin of this form is shown in 

 PL Ixxvii., figs. 1,2; here the notch is formed by r in oscillatory 

 combination with m, and the edges are replaced by r oscillating 

 with 6. 



The forms and angles obtained are as in the table belo\\', in 

 which we have the mean result of measurements on three 

 crystals : 



- Miigge— Neues Jalirb. Miu., ii., 1897, p. 78. 

 « Spi'iicer — Min. Mag , xiii , 1901, p. 39, f n. 



