APATITE AND THE IRON ORES 25 



acids. In itself the mineral is less susceptible to atmospheric 

 influence than calcite, yielding much less readily to decomposing 

 agencies of a purely chemical nature. Nevertheless, Eoth^ has 

 shown that in the weathering of dolomitic limestones the mag- 

 nesia is sometimes removed by leaching, in greater proportional 

 quantities than the more soluble lime carbonate. 



Apatite, — Composition: Phosphate of lime. Hardness, 5. 



Apatite is an almost universal constituent of eruptive rocks, 

 both acid and basic, though as a rule present only in micro- 

 scopic proportions. In the granular limestones, schists, and other 

 metamorphic and vein rocks it sometimes occurs in large crys- 

 tals or massive forms in such abundance as to be of value as a 

 source of mineral phosphate for fertilizing purposes. Though 

 present in but small amounts, apatite is an important constituent, 

 since it is the only common rock constituent containing the valu- 

 able element phosphorus. 



THE lEON" OEES 



Under this head may conveniently be grouped the several 

 forms of iron oxides which occur as rock constituents, and which 

 from their opacity in even the thinnest sections, and occasionally 

 similarly in crystallographic outline, are separable with difficulty 

 by optical test alone. 



Magnetite* — Composition : FeO + FegOg == iron sesquioxide, 

 68.97%; iron protoxide, 31.03%. 



This is a wide-spread and almost universal constituent of 

 eruptive rocks, occurring in the form of scattering, small, and 

 rather inconspicuous granules, which are characterized by a 

 complete opacity and bluish lustre. "When of sufficient size to 

 be distinguished by the unaided eye, magnetite is recognized by 

 its brilliant lustre, weight, and its property of being readily 

 attracted by the magnet. Magnetite also occurs as a constituent 

 of metamorphic rocks and is sometimes found in large beds, 

 constituting a valuable ore of iron. Under continual alterna- 

 tions of heat and cold, moisture and dryness, it slowly decom- 

 poses, giving rise to hydrated sesquioxides which impart color 

 to the resultant sands and clays. 



Menaccanite (Ilmenite or Titanic Iron), — Composition: 

 (TiFe)203, a mixture in varying proportions of the oxides 



of iron and titanium. 



^ Chemiselie u. AUgemeine Geologie. 



