VOLUME CHANGES IN METAMORPHISM 547 
The conversion of olivine (forsterite) to iron-free serpentine is 
usually expressed by the formula: 
2Mg.,SiO,+2H,0+CO.=H, Mg,Si,0,+MgCO,. 
Replacement by equal volumes demands that one cubic centi- 
meter of forsterite (Sp.G. 3.21) must be decomposed and its space 
filled by serpentine (Sp.G. 2.50). The former measure contains 
1.83 grams MgO and 1.38 grams Si0,, while one cubic centimeter 
of pure serpentine contains 1.08 grams MgO, 1.10 grams SiO., and 
0.33 grams HO. It is therefore necessary that 0.33 grams water 
should be added and that 0.75 grams MgO and o.28 grams SiO, 
should be carried away. ‘This indicates that the foregoing formula 
does not express the process correctly. 
The well-known alteration of orthoclase to sericite is usually 
considered to be governed by one of the following formulas: 
3K AISi,O;+H,0+CO,=K HAI,Si,O.++K.CO,+6Si0, 
3KAl Si,O;+-H,0 = KHAI,Si,O,.+K.SiO,+ SiO, 
One cubic centimeter of orthoclase (Sp.G. 2.60) contains 1.68 
grams SiO., 0.48 grams AI,O;, and 0.44 grams K,O. One cubic 
centimeter of sericite holds 1.25 grams SiO., 1.06 grams AI,O,, 
0.33 grams K,O, and 0.12 grams H.0. The replacement by equal 
volume, therefore, demands removal in solution of 0.43 grams SiO, 
and o.11 grams K.O, but also an addition of 0.58 grams Al,O, and 
o.12 grams H,O. This indicates that the formulas cited above are 
certainly incorrect. The exact formula expressive of what has 
happened is probably very complicated. A crude approximation 
would be represented by the following equation: 
3(K.0- ALO,- 6 SiO,)-+4H,O-+3 Al,O,;+CO,; 
a @ ELOs KO” 2 Al\Os-16/Si0:)-£6 SiO._K,CO, 
It is thus seen that sericitization involves a great addition of 
alumina. From where is this derived? It must be remembered 
that sericitization is usually accompanied by many other meta- 
somatic processes involving replacement of aluminous silicates by 
pyrite, calcite, and chlorite. All these processes involve the 
liberation of much alumina which is ready to enter into the sericite 
