428 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



appearance as though deposited between thin plates of bony material 

 which had exfoliated somewhat under the influence of weathering. 

 Inside the cone there is a second layer of vivianite crystals which 

 have grown inward from the platy layer. The latter are some- 

 what smaller than those of the exterior layer. The central cavity 

 of the specimen is in part filled with sandy material, grains of which 

 are embedded in the crystals of the interior 

 crust. This sandy material has the appearance 

 of residual granite sand. 



The crystals of the exterior crust are well 

 formed and are aggregated into groups, the 

 individuals of which deviate a few degrees from 

 strict parallelism. These are developed in a 

 manner showing that they grew freely without 

 interference, and there are no grains of sand or 

 other substance attached to the faces nor do 

 the faces contain the impressions of any such 

 grains. Some of the recesses between the crys- 

 tals are partly filled with waxy limonite which 

 is filled with contraction cracks and which is 

 readily brushed away from the vivianite crystals 

 leaving them clear and bright. 



Individual crystals of the vivianite reach a 

 maximum length of 2 cm. with a diameter of 1 

 cm., while aggregates of crystals in nearly 

 parallel position reach a length of 4 cm. with a 

 thickness of 2 cm. 



The crystals are all alike in development, 

 being prismatic by elongation of the prism 

 ?n(110), the orthopinacoid a (100) and the clino- 

 pinacoid 6(010). The terminations are chisel 

 shaped and consist of o(T03), v(Tll), and A'(301), 

 the dominant orthodome 7l(301) being a form 

 not previously recorded on vivianite. The 

 habit of the crystals is shown in Figure 140. 

 The faces of m(110) are in all cases clear and 

 brilliant, yielding excellent reflections of the 

 signal, while the orthopinacoid a(100) is irreg- 

 ular and 6(010) is usually dull. The terminal planes are all more or 

 less etched and dulled, sometimes to such an extent that no light 

 is reflected at all. The new dome Z(301), while present as a broad 

 face, yields blurred signals which do not permit accurate measure- 

 ments. The average rho angle from several measurements was 67° 

 45', while the calculated rho for this form is 68° 37'. 



In color the vivianite is light greenish blue, although this is ob- 

 scured by the presence between and around the crystals of a pul- 



Fig. 140.— Vivianite. Pris- 

 matic CRYSTAL FROM 

 fossil tusk, Clear- 

 water County 



