IRON MINERALS 25 



4. Ilmenite is a titaniferous ferric oxide, (Ti, Fe) 2 3 . Sp. gr. 

 = 4.5-5.2 ; H = 5-6. When crystallized, this mineral is rhom- 

 bohedral, but is generally massive. 



5. Siderite is ferrous carbonate, FeC0 3 . Sp. gr. = 3.7-3.9; 

 H = 3.5-4-5. Crystallizes in rhombohedrons, the faces of the crys- 

 tals frequently much curved, and often the crystals are very much 

 flattened. When fresh, the mineral is grey or brown. It is but 

 slightly acted on by cold acids ; hot acids dissolve it with efferves- 

 cence. Mixed with clay, siderite forms clay iron-stone, a valua- 

 ble ore. 



6. Iron Pyrites, bisulphide of iron, FeS 2 . Sp. gr. = 4.9-5.2; 

 H = 6-6.5. Crystallizes in the isometric system, usually in cubes, 

 sometimes in dodecahedrons, and has a very characteristic brassy 

 lustre and colour, to which it owes the popular name of " fools' 

 gold." It is very hard, cannot be scratched with a knife, and 

 strikes fire, like flint, when struck with steel. The mineral is 

 soluble in nitric acid : it is widely disseminated in the rocks. 



7. Marcasite, or White Iron Pyrites, has the same composition 

 as pyrites, but crystallizes in the orthorhombic system, in modified 

 prisms, but more commonly occurs in nodular masses, with a radial 

 structure. It has the same hardness as pyrites, but is not quite 

 so heavy. Sp. gr. = 4.68-4.85. In colour it is paler than pyrites, 

 with a tendency to grey, green, or even black. It decomposes 

 very readily and after a few months' exposure, even to dry air, 

 often crumbles to a whitish powder. 



The iron minerals are seldom largely represented in any given 

 rock, with the exception of the ore beds, but iron is one of the 

 most widely diffused of substances, few rocks being altogether free 

 from it, and its various compounds play a very important role as 

 colouring-matter in the rocks. Ferrous carbonate gives no colour 

 to the rock in which it occurs, and such rocks are apt to have a blue 

 or grey tint, due to other substances, both organic and inorganic. 

 When such rocks are exposed to the action of air and moisture, 

 ferric oxide and ferric hydrates are formed, the former giving a 

 red colour and the latter various shades of yellow and brown. 



