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PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 72 



prismatic habit varies somewhat, due to the relative size of the macro- 

 pinacoid and the unit prism. When the orthopinacoid is large the 

 crystals have an almost equal horizontal thickness. When the prism 

 is prominent and the macropinacoid small or lacking the crystals are 

 almost lathlike. The length of the crystals 

 also varies. Many crystals showing a combina- 

 tion of the three pinacoids are stumpy and even 

 almost equidimensional ; in the latter case they 

 appear as rhombs. The crystals rarely are long 

 needlelike blades. 



TWINNING 



Among the larger crystals twins are the rule 

 but are so intricately intergrown that it is diffi- 

 cult to know just what relations the individuals 

 have to each other. Among the smaller crystals 

 twins are rare. In one lot, however, twins hav- 

 ing the general appearance of the butterfly twins 

 of gypsum were found. In these the twinning 

 plane is the macropinacoid (100). These twins 

 are similar to Figure 2. 



PHYSICAL AND OPTICAL PROPERTIES 



Contributed by William F. Foshag 



The color of rossite varies from Martius yel- 

 low in the small crystals to pinard yellow in the 

 larger crystals. The luster varies from vitreous 

 pearly to glassy. The hardness lies between 2 

 and 3. The specific gravity, determined by float- 

 ing clear crystals in a suitable bromo form-carbon 

 tetrachloride mixture is 2.45. The fresh mineral 

 is brittle and has a good cleavage parallel to the 

 clinopinacoid. 



Eossite is a biaxial with a large axial angle. 

 The plane of the optic axes is essentially parallel 

 to the axis c with Z=(7 and roughly bisects the 

 angle between a (100) and h (010). The base 

 and the brachypinacoid show the emergence of an optic axis near the 

 edge of the field. The extinction direction measured on the basal 

 plane and from the edge 001-^010 was found to be 16° ±3. The 

 indices of refraction measured on the recrystalized mineral by the 

 immersion method are as follows: 



a= 1.710, ^=1.770, y= 1.840 



Fig. 2. — Twin crystal 

 of eossite 



