PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 72 



vary the habit greatly. The crystals best suited for measurement 

 were about a millimeter or less in length; the faces of the larger 

 crystals were too curved while the largest were an aggregate of a num- 

 ber of individuals apparently complexly twinned. Except for one 

 lot that was tabular in habit all crystals obtained were prismatic. 

 More than 50 crystals were mounted on the goniometer and provi- 

 sionally examined, but only 15 were found to be entirely suitable and 

 completely measured. The measurements showed rossite to be tri- 

 clinic with the forms c (001), h (010), a (100), m (110), and y (101). 

 Because of the absence of pyramids on all of the crystals the axial 

 ratios and the numerical value of q were not calculated. The polar 

 elements were determined to be as follows: a?o = .4969, 3/0 = . 1624, 

 ;?o = .8295, A=80° 39', /li=59° 31', f=85° 38'. The axial angles are 

 a=m° 18', ^ = 97° 24', y=89° 34'. 



CALCULATION OF THE ELEMENTS 



The crystals were measured on the two-circle goniometer, the pris- 

 matic habit rendering it easy to adjust them in polar position. The 

 prominence of the brachypinacoid and the good brachypinacoidal 

 cleavage gave satisfactory results for v° The forms whose measure- 

 ments could be used in the calculation of the elements were the follow- 

 ing: a (100), m (110), l (010), and c (001). No pyramids could be 

 found on any of the crystals, and repeated recrystallizations under 

 various conditions failed to produce any habit with the slightest 

 pyramidal development. The axial ratios and the numerical value 

 for q were therefore not determined. All measurements were made 

 upon the artificially recrystallized mineral. The suitable measure- 

 ments of the four forms upon which the elements are based are 

 given beloAv : 



Table 2. — Measurement of a {100) rossite 



Crystal No. 



Reflection 



Size of face 







2 



3 



3 - - 



Excellent 



do 



Good 



Medium 



do 



do 



/ 



86 22 



86 49 



87 8 



86 10 



87 4 



84 33 

 86 53 

 86 58 



85 40 



86 15 

 85 46 

 85 49 

 85 10 

 83 3 



4 



4 



do 



do 



Narrow 



do 



5 - - 



Excellent 



Good 



do 



do 



do 



do 



do 



do 



Medium 



do 



do 



Broad 



6 



6 



8 



8 



10 



10 



12... 



Medium 



do 



do 



do 



13 



Excellent _ 



do 



Average 







85 38 









