ALUMINUM-SILICATE GROUP. 317 



andalusite to a large extent. Like andalusite, its development may be 

 promoted by the presence of intrusive rocks, especially granites. In such 

 cases sillimanite frequently develops nearer the intrusive masses than does 

 the andalusite, the sillimanite therefore being the mineral which forms under 

 conditions of more advanced metamorphism. It is frequently associated 

 with g-arnet and with spinel and staurolite, sometimes with iolite (cordierite). 

 Sillimanite is derived from andalusite, biotite, corundum, cyanite, diaspore, 

 and gibbsite. 



The occurrence and associates of cyanite are similar to those of silli- 

 manite; but a very frequent additional associate is corundum, and where 

 formed by the assistance of igneous rocks the cyanite is likely, on the 

 average, to be closer to the intrusive than the sillimanite, although of 

 course they ordinarily overlap. As a metamorphic mineral, cyanite is 

 derived from andalusite, corundum, diaspore, and gibbsite. 



Tremolite, actinolite, and diopside are frequent associates of andalusite, 

 sillimanite, and cyanite, especially of the last two. 



The special homes of the aluminum-silicate group of minerals are 

 the metamorphosed argillaceous sedimentary rocks. As is well known, 

 kaolin is one of the chief constituents of such rocks, and doubtless it is 

 from this mineral in large part, under deep-seated conditions, that the 

 aluminum-silicate minerals are formed. If it be supposed that these heavy 

 miuerals develop from kaolin, the process would be one of dehydration 

 and separation of silica. This silica may separate either as quartz or may 

 unite with other compounds, such as calcium and magnesium or other 

 bases, to form silicates. The breaking up of the kaolin may be repre- 

 sented by the following equation: 



( 1 ) H 4 Al 2 Si 2 9 = Al 2 Si0 5 + 2H 2 0+Si0 2 -}-k. 



Supposing the mineral produced were andalusite, the volume of the anda- 

 lusite and quartz is 25.40 per cent less than that of the kaolin. If it be 

 supposed that calcium carbonate is present at the same time, and that the 

 freed silica unites with it, the equation may be written : 



(2) H i Al 2 Si 2 9 +CaC0 3 =Al 2 Si0 5 +CaSi0 3 +2H 2 0+C0 2 +k. 



In this case the volume of the andalusite and wollastonite is 32.32 per cent 

 less than that of the kaolin and calcite. If the heavier mineral silli- 

 manite or cyanite be produced the decrease in volume is even greater. 



