ALTERATIONS OF BIOTITE. 341 



By adding thirteen molecules of water instead of one, twelve molecules of 

 gibbsite instead of twelve molecules of diaspore (orthorhombic; sp. gr. 3.40) 

 will be produced. 



In equation (4), supposing the calcium carbonate to be added in 

 solution and the magnesium carbonate and the potassium carbonate to be 

 removed in solution and the silica to remain as a solid, the decrease in 

 volume is 14.86 per cent. 



In equation (5), supposing the biotite and iron oxide to be solids, the 

 calcium carbonate to be added in solution, the magnesium and potassium 

 carbonates to remain in solution, but the epidote, silica, and diaspore to 

 remain as solids, the decrease in volume is 18.45 per cent. If oibbsite 

 instead of diaspore be produced the decrease in volume will not be so much. 



If it be sup]30sed that the aluminum passes into spinel (isometric; sp. 

 gr. 3.8) instead of diaspore or gibbsite, and spinel is known to form in con- 

 nection with biotite, the number of molecules of magnesium carbonate 

 would be reduced by six in equation (4); that is, to 54. 6MgAl 2 4 would 

 replace the 12A10(OH). No water would need to be added, and five 

 moleoules of water would be produced. Finally, only twenty-nine mole- 

 cules of C0 2 would need to be added. Therefore the equation would be: 



(6) 30KHMg 2 Al 2 Si 3 O 12 +6Fe 2 O 3 +40CaCO 3 +29CO,= 



4H 5 Ca 10 Al 12 Fe 3 Si 15 O 65 +30SiO 2 +6MgAl 2 O i +54MgCO34-15K 2 CO3^5H 2 O+k. 



In this case the volume of the resultant epidote, spinel, and silica, would 

 be 14.71 per cent less than that of the biotite and 18.15 per cent less than 

 that of the biotite and hematite. 



It is a well-known fact that chlorite secondary to biotite is usually 

 accompanied by epidote and quartz. Comparing the equation (3) for the 

 formation of chlorite with equation (4) for epidote, we see why these two 

 minerals with quartz are frequently formed at the same time. For the 

 formation of chlorite from biotite additional magnesium is needed. For 

 the formation of epidote additional calcium is necessary and magnesium is 

 left over. If instead of magnesium and calcium carbonates being added, 

 as suggested in equations (3) and (4), only calcium carbonate were avail- 

 able, the excess of magnesium produced by the passage into epidote may go 

 into the chlorite, and thus epidote and chlorite be simultaneously produced. 



