360 A. J. Moses — Crystallization of Molybdenite. 



2. Molybdenite from Aldfield, Quebec. 



A specimen purchased from the Foote Mineral Co. showed 

 a barrel-shaped crystal (fig. 2), the basal plane 7x5 mm , thickness 

 4 m,n . The cleavage surface was not crumpled and was free from 

 grooves and ridges but was pitted with little etch figures of 

 not very definite shape. The pyramidal planes were striated. 



When adjusted on the two-circle goniometer by use of this 

 pitted cleavage surface, it was found that, at intervals of closely 

 60° of the vertical circle, zones were obtained which yielded 

 images of the collimator signal for two different positions, both 

 positions corresponding to a bright illumination of the entire- 

 face. That is, the striations in this crystal are due to an oscilla- 

 tion between two forms and not to gliding or translation, this 

 being further proved by the absence of grooves and ridges on 

 the cleavage. 



In each zone one of the two images corresponded to p 

 approximately 90°. The crystal was therefore readjusted until 

 these were exactly 90° and the results for the second image 

 thereafter in four zones were : 



(j) P 



0° 77°20' Dull image 



59°41' 77 21 Single image 



120 09 77 10 Brighter of two 



180 10 77 22 ^ " " " 



Average p=77° 18'. 



The variation of both (f> and p are not too great to be attrib- 

 uted to the blurred signals. For the pyramid (2021) observed 

 by Brown at Frankford, Pa., the corresponding calculated 

 ancrle is 77° 13'. 



That is, the crystal consists of a predominant pyramid (2021) 

 with striated faces, and these strise are due to an oscillation 

 between this form and the prism*. 



3. Molybdenite from Cape Breton. 



Among a number of small crystals of molybdenite in the 

 Egleston Museum labelled Cape Breton, one small crystal 4 mm 

 broad by V nm thick showed five faces of a pyramid, two of 

 which were unusually bright and intersected in a sharp edge. 

 Placed with this edge vertical in the !S r o. 2 Fuess goniometer, 

 each face yielded a single vertically distorted image and permitted 

 a rather close reading. The interfacial angle obtained was 58° 

 28-J', which corresponds to an angle with the cleavage of 77° 

 29'. The calculated angle for (2021) is 77° 13'. 



* The faces of such crystals are slightly curved and the crystals taper, pre- 

 venting an exact application of a hand goniometer. It is probable that the 

 angle cp = 72° obtained by Hidden, (this Journal, xxxii, 210, 1886) on Eenfrew 

 molybdenite in this way is due to an oscillatory combination. 



