(II) Rock phosphates 



THK NONMETALLIC MINERALS. 357 



several forms, some distinct and well defined, others illy defined and 

 passino- by insensible o-radations into one another, but all classed under 

 the oeneial term of phosphates. Their orio-in and general physical 

 properties are quite variable, and any attempt at classifyino- must be 

 more or less arbitrary. For our present purposes it is sufficient that 

 we treat them under the heads of mineral phosphates and rock phos- 

 phates, as has been done by Dr. Penrose.' These two classes are then 

 subdivided as below: 



( Apatites 1 ^^'''''' ^P^t^*^'^' 

 (I) Mineral phosphates ^. -j p^ j^^^.-^p I Chlor apatites. 



Amorphous nodular phosphates loose 



or cemented into conglomerates. 



Phosphatic limestones. 



r^ \ Soluble guanos. 



Guanos < ^ , ^^ 



( Leached guanos. 



Bone beds. 



Apatite. — ITnder the name of apatite is included a mineral composed 

 essentially of phosphate of lime, though nearly always carrying small 

 amounts of fluorine or chlorine, thereby giving rise to the varieties 

 ftucrr-apaHtc and chlor-apatite. The mineral crystallizes in the hex- 

 agonal system, forming well-defined six-sided elongated prisms of a 

 green, yellow, rose, or reddish color, or sometimes quite colorless. 

 (Specimens, Nos. 02128, ()212!), U.S.N.M., from Renfrew, Canada.) 

 It also occurs as a crystalline granular rock mass. (Specimens, 

 Nos. 62137, 6211:8, 65111, U.S.N.M.) The hardness is 4.5 to 5; specific 

 gravity, 3.23; luster, vitreous. Apatite in the form of minute crystals 

 is an almost universal constituent of eruptive rocks of all kinds and 

 all ages. It is also found in sedimentary and metamorphic rocks as 

 a constituent of veins of various kinds, and is a connnon accompani- 

 ment of beds of magnetic iron ore. It is only when occurring segre- 

 gated in veins and pockets, either in distinct crystals or as massive 



1 Bulletin No. 46 of the U. S. Geological Survey. 



^Fuchs (Notes Sur la Constitution des Gites Phosphate de Chaux) divides the 

 natural phosphates into three classes. In the first the phosphatic material is concen- 

 trated in sedimentary beds; in the second it is disseminated throughout eruptive 

 rocks, and in the third it constitutes entirely or partially the material filling veins 

 and pockets. That found in sedimentary teds occurs in rounded and concretionary 

 masses called nodules. In eruptive and metamorphic rocks the phosphate occurs in 

 the crystalline form of apatite, sometimes isolated or grouped in aggregates. In 

 veins the phosphate occurs massive and in pockets, crystalline, but not in distinct 

 crystals; rather as globular and radiating masses. To such the name phosphorite is 

 given. The three varieties show a like variation in solubility, the amorphous phos- 

 phates being soluble in citrate or oxalate of ammonia to the extent of 30 to 50 per 

 cent; the phosphorites to the extent of only 15 to 30 per cent, and the apatite 

 ' scarcely at all. The amorphous phosphates alone have proven of value for direct 

 application to soils, the other varieties needing previous treatment to render them 

 soluble. 



