84 WATSON AND GOOCH: VIVIANITE FEOM FLORIDA 



The contact between the overburden and the phosphate bed 

 is sharply defined. The land pebble phosphate belongs to the 

 Bone Valley gravel formation, which is generally regarded as 

 Pliocene in age, although Matson^ thinks there is some evidence 

 of the Miocene age of the phosphate. The formation is be- 

 lieved to be of marine origin,^ the materials of which were prob- 

 ably deposited under shallow water conditions. 



DESCRIPTION OF THE VIVIANITE 



The mineral is distributed irregularly through the ocherous 

 matrix as single crystals and crystal aggregates. Individual 

 crystals attain a maximum length of 22 mm., are prismatic in 

 form, usually much elongated, and not infrequently flattened 

 parallel to 100, with vertical striations sometimes developed. 

 End faces were not observed, the crystals terminating usually in 

 rough and uneven surfaces. In the crystal aggregates, the un- 

 attached end of the individuals sometimes exhibits acicular 

 form. Fibrous structure, usually radiate or divergent, is some- 

 times developed. 



Perfect cleavage is developed parallel to 010. The luster is 

 pearly on cleavage faces, vitreous on other faces. Hardness 2; 

 specific gravity 2.693. Color varies from light or pale green 

 through blue-green to deep indigo blue, the pale green being pre- 

 dominant. Streak colorless to faint bluish white, which rapidly 

 changes to deep blue on exposure. The finely ground powder 

 after exposure to sunlight for more than 30 days remained deep 

 blue without any indication of changing to brown as reported by 

 some writers. Crystals vary from transparent to translucent, 

 becoming more or less opaque on prolonged exposure. 



The optical properties of the pale-green fragments of the 

 vivianite similar to those yielding the analysis in table 1, kindly 

 determined for the writers by Mr. E. S. Larsen, gave the follow- 

 ing results : Optically ( + ) ; 2V large ; dispersion not strong ; 

 X is normal to 010; Z makes an angle of 28° 30' ± 1° with c. 



3 Matson, G. C. Op. cit., p. 69. 



'' At Pembroke aggregates of silicified oyster shells have been observed by Dr. 

 Sellards and the junior writer (Gooch) in the pebble phosphate deposit. 



