Phosphorite 



marked by thin layers of darker-than-ordi- 

 nary phosphorite or by films of manganese 

 oxide hke those that form the outer surface 

 of the nodules. 



71 



The bulk of the phosphorite has the opti- 

 cal. X-ray, and chemical properties of col- 

 lophanite, a nearly isotropic microcrystalline 

 mineral. In minor amounts is another min- 



(a) 



/ 



« ♦. » 1 » .•■,.'t 



(b) -^ 



^ — -— ^-* 



>mm 



(c) 



Figure 65. Polished sections of phosphorite slabs. Photographs by M. F. Karim (x 0.8). (a) Conglomeratic phos- 

 phorite dredged from Santa Monica Bay by Charles Gunnerson (lat. 33°53', long. 118°31', 190 feet). Near the middle 

 is the fibrous cross section of a mammal bone, (b ) Layered phosphorite from ridge north of Santa Barbara Island 

 (AHF 3015, lat. 33°39', long. 119°4', 1903 feet). Note irregularity of laminations which show that growth was inter- 

 mittent and characterized by a surface as irregular as that of the present top of the nodule. In contrast, the bottom is 

 typically flat, (c) Conglomeratic phosphorite from Lomita Quarry on north flank of Palos Verdes Hills. Note the pres- 

 ence of a flat slab at the bottom which may have served as a nucleus for deposition of the later phosphorite. 



