480 Rogers — Dahllite (Podolite) from To?iopah, JVevada. 



Dahllite 2Ca 3 (P0 4 ) 2 .CaC0 3 4H 2 



Francolite Ca 8 (CaF) a (P0 4 ) a .CaC0 8 .H,0 



Staffelite a fibrous variety of francolite 



Collophanite a;3Ca,(POJ 9 .yCa.C0 8 .«H 9 



Dahllite, as shown by Sclialler, is probably identical with 

 podolite. The formula assigned to francolite seems anomalous, 

 as it does fit into the series given above. There are but two 

 analyses of francolite on record and they do not agree. One 

 has 9*04 per cent CaF 2 , 2*84 per cent CaC0 3 , and no water, 

 while the other has 7*68 per cent CaF 2 , 5* 10 per cent CaC0 3 

 and 1*59 per cent water. Francolite is pseudohexgonal like 

 dahllite (podolite). The axial plane is parallel to the hexagonal 

 outline, but this is probably not an essential difference. Lacroix 

 regards staffelite as a fibrous variety of francolite. Additional 

 analyses are necessary before francolite (or staffelite) can be 

 assigned definite rank as a mineral species. 



Collophanite was described by Sanclberger,* who gave the 

 formula Ca 3 (P0 4 ) 2 .H 2 after deducting 3*96 per cent CaC0 3 , 

 which he considered to be an impurity. The analysis of 

 Kottnitz gives 3Ca 3 (P0 4 ) a .CaCO a .3H 2 0, but Lacroixf gives 

 a?Ca 3 (P0 4 ) 2 .?/CaC0 3 .2H 2 0. Besides the original collophanite 

 there is only one other analysis, that of Shepard,J which after 

 deducting 4*64 per cent gypsum gives 3Ca 3 (P0 4 ) 2 .H 2 0. So the 

 composition of collophanite is not settled. There is a possi- 

 bility that collophanite is an amorphous mineral with the for- 

 mula 3Ca 3 (P0 4 ) 2 .CaC0 3 .a?H 2 0, thus bearing the same relation 

 to dahllite (podolite) that opal (Si0 2 .a?H a O) does to quartz 



(SiO,). 



Of the various carbono-phosphates only one, viz. dahllite 

 (podolite) with the formula 3Ca 3 (P0 4 ) 2 .CaC0 3 , is, in my opinion, 

 entitled to recognition as a mineral species. 



Several suggestions as to the chemical composition of phos- 

 phate rock, phosphate nodules, and similar materials have 

 recently been made. Stutzerg regards phosphate rock as made 

 up principally of amorphous calcium phosphate. Lacroix] 

 believes them to be mixtures of collophanite with smaller amounts 

 of francolite and dahllite (2Ca 3 (P0 4 ) 2 .CaC0 3 .-|H 2 0). According 

 to Cameron and Bell these materials are solid solutions of 

 CaO and P 2 5 . 



On account of the inaccuracy of the fluorine determination 

 and on account of the water, iron, aluminium, and calcium 

 carbonate present, analyses of phosphate rocks are difficult 



*Jb. Min., p. 308, 1870. 



f Mineralogie de la France, vol. iv, 56, 1911. 



% This Journal, xxxiii, 402, 1882. 



$Die Lagerstatten der Nicht-Erze, Band I, p. 267, 1911. 



| Loc. cit. 



