210 PROCEEDINGS OF THE ACADEMY OF [1896. 



THE CRYSTALLIZATION OF MOLYBDENITE. 

 BY AMOS P. BROWN. 



Although molybdenite, Mo Sj, has been known to mineralogists 

 since crystallography was first studied, its crystalline form has never 

 been satisfactorily determined. It has been provisionally assigned 

 to the hexagonal and monoclinic systems by different authorities, 

 the general opinion being that it is hexagonal. The crystals that 



have thus far been examined can be ex- 

 plained on a hexagonal basis, but they 

 are not sufficiently lustrous to admit of 

 very exact measurement, and the softness 

 ofthe substance also militates against the 

 exact determination of its angles. The 

 locality at Frankford, Philadelphia, has 

 long been known to mineralogists as 

 affording well crystallized molybdenite, and I have for some years 

 been collecting material from there with a view of making a crystal- 

 lographic study of the mineral. Having in hand some crystals 

 which are sufficiently lustrous for measurement on the reflecting 

 goniometer I have examined them and obtained some positive 

 results. 



The crystals are hexagonal in habit, consisting of six sided prisms 

 and barrel shaped crystals, the best of which are not more than 

 6 mm. in diameter. They strongly recall some mica crystals and 

 seem often to show a twinning with the basal pinacoid as the composi- 

 tion face. A number of crystals were examined, the one giving the 

 best results being a nearly perfect hexagonal plate of some 5 mm. 

 diameter, which represents a broken crystal, only one termination 

 being preserved. Nearly all of the faces gave fair images but the 

 basal pinacoid was uneven, due to slight crumpling, and gave several 

 images. By observations on a number of crystals these angles could 

 be checked, however. The pyramid as a termination was not ob- 

 served, all crystals examined showing the basal termination. The 

 following crystallographic constants were observed : 



