24 THE ROCK-FORMING MINERALS 



6. Apatite is a phosphate of lime, with chloride and fluoride of 

 calcium, which vary in relative amounts, 3 (Ca 3 P 2 8 ) , 2 (Ca, CI, F) . 

 Sp. gr. = 2.92-3.25 ; H = 5. It crystallizes in hexagonal prisms, 

 terminated by hexagonal pyramids, and also occurs in masses. 

 The original and unchanged mineral is transparent and colourless, 

 changing on alteration to opaque brown or green. Apatite is solu- 

 ble in acids, and in water containing carbon dioxide, or ammonia ; 

 it is thus dissolved out of the rocks and widely diffused in the soils, 

 where it forms a valuable plant food. 



7. Fluor-spar, fluoride of calcium, CaF 2 . Sp. gr. = 3.01- 

 3.25 ; H = 4. Crystallizes in the isometric system, usually in 

 cubes, and has a perfect octahedral cleavage. When pure, fluor- 

 spar is clear and colourless, but it is more commonly stained 

 blue, green, yellow, or brown. 



E. IRON MINERALS 



i. Haematite, or Specular Iron, is ferric oxide, Fe 2 3 . Sp. gr. = 

 4-5~5-3 > H = 6.5- Crystallizes in rhombohedrons, or more 

 commonly, in nodular masses, which are composed internally of 

 very flat crystals. The colour is black or steel-grey, which be- 

 comes red when the mineral is finely powdered. Haematite fre- 

 quently contains earthy and other impurities and is one of the 

 most important ores of iron. 



2. Limonite, or Brown Haematite, is hydrated ferric oxide 

 (2 Fe 2 3 , 3 H 2 0) containing more than 14 % of water. It is softer 

 than haematite and of a yellow or brown colour. Sp. gr. =3.6-4 ; 



H = 5-5-5- 



3. Magnetite is the black oxide of iron, Fe 3 4 (or FeO, Fe 2 3 ) . 

 Sp. gr. = 4.9-5.2; H = 5.5-6.5. Crystallizes in the isometric 

 system, usually in octahedrons, sometimes in dodecahedrons. 

 This mineral is strongly magnetic and is black in colour, with a 

 bluish-black metallic lustre, when viewed in reflected light. Mag- 

 netite is widely diffused in certain classes of rocks, and also occurs 

 in veins and beds, which form an important source of supply of 

 the metal. 



