270 A. F. Rogers — Collophane. 



edition of the System of Mineralogy, 4 says : ' l Besides the 

 definite mineral phosphates, including normal apatite, 

 phosphorite [nsed here as a variety of apatite and not as 

 rock name] , etc., there are also extensive deposits of amor- 

 phous phosphates, consisting largely of bone phosphate 

 (Ca 3 P 2 8 ), of great economic importance, though not 

 being a definite chemical composition and hence not 

 strictly belonging to pure mineralogy. Here belong the 

 phosphatic nodules, coprolites, bone beds, guano, etc." 



Does the amorphous substance that makes up the bulk 

 of the phosphorites deserve recognition as a distinctive 

 mineral ? A mineral may be defined as a naturally occur- 

 ring, homogeneous, inorganic substance of definite or 

 fairly definite chemical composition and with character- 

 istic physical properties. Since the time that Dana's Sys- 

 tem of Mineralogy was written, it has come to be recog- 

 nized that all minerals do not have a definite chemical 

 composition and that some variability must be allowed. 

 Among prominent examples may be mentioned pyrrhotite, 

 chalcocite, tetrahedrite, and nephelite. The main con- 

 stituent of phosphorite like all amorphous minerals is 

 admittedly variable in composition but it is definite within 

 certain limits as can be seen from the tabulated analyses 

 on a later page. Its properties are so characteristic that 

 it may be recognized by physical tests alone. The fact 

 that the phosphorites grade* into phosphatic limestones 

 and shales is no argument against the recognition of collo- 

 phane as a distinctive mineral, for opal grades into opal 

 shale and psilomelane grades into indefinite manganese 

 dioxides. 



The amorphous equivalents of crystalline minerals, 

 with the single exception of opal, have not been given 

 recognition as distinctive minerals until Cornu's work in 

 1909. Recently the writer 5 has urged the adoption of 

 Cornu's plan, though he believes that distinctive names 

 should be used instead of some of the names proposed by 

 Cornu. If such names as opal, psilomelane, limonite, and 

 halloysite are used for amorphous substances of variable 

 composition, then a distinctive name also should be used 

 for the amorphous calcium carbonophosphate. It bears 

 the same relation to crystalline dahllite (3Ca 3 P 2 8 . 

 CaC0 3 ) that opal does to chalcedony or quartz. 



4 P. 769, 1892. 



6 Jour. Geol., 25, 515-541, 1917. 



