546 WALDEMAR LINDGREN 
feldspar grain. In schists one garnet crystal may replace hundreds 
of small individuals of biotite, quartz, and feldspar. 
Examples of replacement.—The following instances are men- 
tioned, in order to express more precisely what processes are believed 
to be active in certain cases. 
In the simple case when pyrite replaces calcite the solutions need 
to carry iron sulphide, alkaline sulphides, and carbon dioxide. As 
each particle of calcite is dissolved as bicarbonate, the solution 
becomes supersaturated for FeS, and an equal volume of this 
compound is precipitated. 
In the more complicated case of replacement of orthoclase by 
pyrite the entering solution would have the same composition as 
indicated above. As each particle of orthoclase is decomposed into 
potassium carbonate, colloid silica, and colloid aluminum silicate, 
all of which are carried away, a corresponding volume of FeS, is 
deposited. 
When pyrite replaces chlorite the entering solution may contain 
only hydrogen sulphide, iron carbonate, and carbon dioxide. As 
each particle of chlorite is decomposed into magnesium bicarbonate, 
ferrous bicarbonate, colloid silica, and colloid aluminum silicate, a 
reaction takes place between the ferrous carbonate and the hydrogen 
sulphide, resulting in the precipitation of an equal volume of iron 
sulphide. The guest mineral contains much more iron than the 
host mineral and the additional amount needed must be supplied 
by the entering solution. 
Every petrographer knows how frequently soda-lime feldspar 
or anorthite is replaced by calcite. It is usually explained as a 
simple case of the lime in the feldspar combining with carbon 
dioxide. A calculation based on respective specific gravities of 
2.75 and 2.71 will show that one cubic centimeter of anorthite 
yields 2.20 grams alumina and silica, which must be carried away. 
In order that the resulting calcite may fill the space made vacant 
by the anorthite the solutions must supply an additional amount 
of 0.97 grams CaO besides the necessary CO,, considering that one 
cubic centimeter of calcite contains 1.52 grams CaO and 1.19 
grams CO. 
