328 A TREATISE ON METAMORPHISM. 



potassium mineral must also be concerned in this alteration. Staurolite 

 rocks usually contain orthoclase, or at least some potash feldspar. It may 

 be supposed that these potash feldspars break up into kaolin at the same 

 time, thus furnishing the potassium necessary for the change. 

 The change of staurolite to talc may be written as follows: 



(1) 2HFeAl 6 Si 2 O 1 s+3MgCOs+15H 2 O+O=H 2 Mg 3 Si 4 O 1 s+Fe i! Os+10Al(OH)a+3COj+k. 



The decrease of the volume of the talc as compared with the staurolite is 

 44.02 per cent; but if the gibbsite (monoclinic; sj). gr. 2.35) be included 

 the increase in volume of the two is 90.96 per cent. 



In the change to chlorite the most aluminous one is chosen, amesite 

 (hexagonal plates; sp. gr. 2.71), since staurolite is so heavily aluminous 

 Moreover, it is supposed that the iron in the chlorite is to the magnesium as 

 1:3. On these suppositions the reaction may be written: 



(2) 2HreAl 5 Si 2 1 s+10H 2 0+6MgCO s =2H 8 Mg 3 FeAl 1 Si 2 6 1 8+2Al(OH) 3 +6C0 2 +k. 



The increase of volume of the chlorite and gibbsite as compared with the 

 staurolite is 103.58 per cent 



The change of staurolite to muscovite may be written: 



(3) 3HFeAl 5 Si 2 13 +K 2 C03+14H.,0+0=2H 2 KAl3Si 3 12 +Fe 3 4 +9Al(OH) 3 +C0 2 +k. 



The decrease in volume of the muscovite as compared with the staurolite 

 is 24.96 per cent; but if the magnetite (isometric; sp. gr. 5.174) and gibbsite 

 be included the increase in volume would be 68.08 per cent, and if hema- 

 tite (rhombohedral; sp gr. 5.225) or limonite (amorphous; sp. gr. 3.8) 

 form, instead of magnetite, the increase would be still greater. 



In the above equations it is entirely possible that the magnesium may 

 be added in some other form than that given, and the resultant compounds 

 be different. The same thing may be said of the potassium. It is uncer- 

 tain what becomes of the excess of aluminum. In the equations the 

 aluminum is regarded as passing into the gibbsite. However, the presence 

 of abundant gibbsite is not recorded among the alterations of staurolite, 

 although frequently corundum (rhombohedral; sp. gr. 4.025) occurs in 

 connection with it; but it is by no means certain that this corundum is one 

 of the results of the alteration of the staurolite ; indeed, it is more probable 

 that the corundum formed from gibbsite at the same time the staurolite 

 developed. 



