22 THE ROCK-FORMING MINERALS 



2. Talc is a hydrated silicate of magnesia, 3MgO, 4Si0 2 , H 2 ; 

 the water varies in amount to as much as 7%. Sp. gr. = 2.56-2.8 ; 

 H = 1 . It is of a white or pale green colour, with a pearly lustre 

 and a greasy, soapy feeling to the touch. Talc is rarely found 

 crystallized ; the crystals have a false hexagonal symmetry, and it 

 is doubtful whether they should be referred to the orthorhombic 

 or monoclinic systems. Usually it occurs in flakes or foliated 

 masses, which split into thin, non-elastic leaves. Talc results from 

 the alteration of magnesian minerals. 



3. Steatite, or Soapstone, has the same composition as talc, but 

 is not foliated, and may be much harder, as much as 2.5. 



4. Serpentine is a hydrated silicate of magnesia and iron : 

 3 (MgO, FeO) 2 SiOo, 2 H 2 0. It does not crystallize, except some- 

 times in pseudomorphs. Sp. gr. = 2.5-2.65 ; £[ = 2.5-4. Its proper 

 colour is green, but it is usually mottled with red or yellow by iron 

 stains. Serpentine is generally formed from the decay of olivine, 

 less commonly from augite, or hornblende. 



Kaolinite is the hydrated silicate of alumina, A1 2 3 , 2 Si0 2 , 

 2 H 2 0. It is usually soft and plastic, but orthorhombic crystals of 

 pseudo-hexagonal symmetry may be sometimes detected with the 

 microscope. Kaolinite arises from the decomposition of the fel- 

 spars and especially of orthoclase. 



Glauconite is a hydrated silicate of alumina and iron, with small 

 quantities of lime, magnesia, potash, and soda. It is of a green 

 colour, soft and friable, and results largely from the decay of 

 augite. 



D. CALCAREOUS MINERALS 



i. Calcite, carbonate of lime, CaC0 3 . Sp. gr. = 2.72 • H = 3. 

 This mineral crystallizes in the hexagonal system, in a great vari- 

 ety of forms ; rhombohedrons and scalenohedrons are common ; 

 hexagonal prisms and pyramids less so. Cleavage is very perfect, 

 parallel to the faces of a rhombohedron, and the mineral breaks up 

 into rhombohedrons when struck a sharp blow. Calcite is rapidly 

 attacked, even by cold and weak acids, C0 2 escaping with effer- 



