150 Field Columbian Museum — Geology, Vol. III. 



M1METITE 

 EUREKA, UTAH 



FIGS. 4 AND 5. PLATE L 



On several specimens from Eureka, Utah, obtained from Maynard 

 Bixby, mimetite occurs in acicular form. In one of these specimens 

 (Mus. No. M 8384), the crystals are in the form of minute white 

 needles occurring in great abundance coating pyramidal crystals of 

 anglesite. On another specimen (Mus. No. M 8385), the crystals 

 are larger, reaching a length of 1 cm. with a thickness of .75 mm. 

 These crystals are transparent and colorless. Many of them show a 

 termination in which it is possible to recognize definite crystal planes, 

 and examination with the reflecting goniometer permits identifica- 

 tion of the unit prism m (10T0), the unit pyramid x (10T1) and the 

 basal plane c (0001). Fig. 4, PI. L, shows the characteristic devel- 

 opment. In another specimen (Mus. No, M 9383), the mimetite 

 exhibits the same habit, but the crystals are somewhat shorter and 

 have a wine-yellow color. These crystals have an average diameter 

 of .6 mm. and reach a length of 5 mm. The forms of which they 

 are composed are similar to those previously mentioned, but the 

 basal plane is more extensively developed as shown in Fig. 5, PI. L. 

 No doubly terminated crystals were found. Neither the colorless 

 nor the yellow crystals exhibit noticeable absorption or pleochro- 

 ism in polarized light in the direction of the vertical axis. On 

 heating, the yellow crystals change to a smoky color. 



OCTAHEDRITE 

 JEQUIT1NHONHA RIVER, BRAZIL 



FIGS. 2-4, PLATE LI 



Several crystals of octahedrite were presented to one of the authors 

 by Olaf E. Ray, Esq., an official of the Chicago Brazilian Diamond 

 Company. These crystals were obtained from washings of the dia- 

 mond-bearing sand of the Jequitinhonha River, near Diamantina, 

 Brazil. The crystals have the typical pyramidal character of octahe- 

 drite and range from 5 to 8 mm. in length. Their color is the 

 typical brownish-black of the mineral showing greenish-yellow by 

 transmitted light. Aside from striations the planes are splendent. 

 The edges are somewhat rounded from stream rolling, but otherwise 

 the crystals are well developed and give excellent signals with the 



