no. 2375. 



VIVIANITE FROM IDAHO SHANNON. 



417 



what sort of medium these could have developed is not clear. 

 It must have been either a yielding homogeneous fine grained muck 

 or an open cavity. If it were muck the sand which occupies tho 

 center of the specimen must have been packed in the interior of tho 

 original object when it was deposited. It is possible that the crystal 

 crust grew upon a shell formed around the sand on the interior of 

 the original tusk and found space to grow in the cavity formed by 

 tho solution and removal of tho tusk itself. 



Individual crystals of the vivianite reach a maximum length of 

 2 centimeters with a diameter of 1 centimeter, while aggregates of 

 crystals in nearly parallel position reach a length of 4 centimeters 

 with a thickness of 2 centimeters. 



The crystals are all alike in development, being prismatic by 

 elongation of the prism m(110), the orthopinacoid a (100), and the 

 clinopinacoid 6(010). The terminations are wedge shaped and con- 

 sist of o(T03), v(Tll), and #(301), the dominant orthodome £(301) 

 being a form not previously recorded on vivianite. The form and 

 development of the crystals are as shown in orthographic and clino- 

 graphic projections in figures \A and IB. The faces of ra(110) are 

 in all cases clear and brilliant, yielding excellent reflections of the 

 signal. The orthopinacoid a (100) is always more or less irregular, 

 while 5(010) is usually dull. The terminal planes are always 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, yielded blurred signals which did not permit accurate measure- 

 ments. The average angle obtained from several readings is as 

 follows : 



Angles of dome K$01) on vivianite. 



Since the calculated and observed angles are nearly 1 degree apart 

 and thej-eflections were of poor quality, this must be regarded as a 

 somewhat doubtful form. 



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

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

 ulent deep blue alteration product. The cleavage parallel to 6(010) 

 is very prominent, yielding flexible laminae. Under the microscope 

 the pleochroism of some grains is very striking and beautiful. 

 27177— 21— Proc.N.M. vol.59 27 



