236 Ford — Neptunite Crystals from San Benito Co., Gal. 



Fig. 4. 



the writer has been able to examine a large number of the 

 neptnnite crystals from this locality and it was thought that a 

 brief description of them would be of interest. 



Occurrence. — The neptunite crystals occur associated with 

 those of benitoite, both minerals being embedded in a white 

 matrix of crystalline granular natrolite. The neptunite crys- 

 tals that are now in the Brush Collection vary in size from 

 slender prismatic crystals of only a few millimeters in length 

 to those of 2-| cm in length and as much as 7 mm in thickness. In 

 general the crystals are brilliant black in color, but wherever 



they have been fractured they show 

 by internal reflections the red-brown 

 color which is characteristic of the 

 mineral when in thin section. 



Crystallograjphic. — The crystals 

 from San Benito County are excep- 

 tionally uniform in their habit, the 

 following series of forms being found 

 on almost every crystal studied : 

 a(100Vm(110), 5(111)^(112), tf(lll), 

 ^r* (211), j? (311). The crystals are 

 prismatic in habit, quite different 

 from those first described by Flink* 

 from Greenland, but more like a later 

 type described by him'f and resem- 

 bling still more a crystal figured by 

 Wallenstrom4 The habit of devel- 

 opment of the California crystals is 

 represented in the figures which have 

 been drawn from specimens in the 

 Brush Collection. Figure 1 repre- 

 sents the commonest type in which the 

 forms present are : prism, m (110), 

 prominent, a (100) in narrow trun- 

 cations, the base, c (001) very small, 

 as also 5(111) and * (112). The 

 prominent terminal faces are the 

 negative pyramids o (111), the new 

 Figure 2 shows a slight modification 

 in the development of these forms, and figure 4 shows the same 

 forms, with the exception of g which failed, but in an unsym- 

 metrical development which is unusual on the crystals. This 

 figure was drawn by Mr. D. D. Irwin. Figure 4 is a doubly 

 terminated crystal, like figure 2, half embedded in its matrix 

 of white natrolite. The faces of the crystals studied were, as 

 a rule, bright and gave excellent reflections on the goniometer 

 with the exception of the new form g (211). In every crystal 



*Zeitschr. Kr., xxiii, 346, 1894. fMedd. om Gronl., xxiv, 120, 1902. 

 {Geol. For. Forh., xxvii, 149, 1905. 



form ^(211) and jp (311). 



