OCCURRENCE AND ALTERATIONS OF PHLOGOPITE. 343 



sillimanite and hypersthene do not form at the surface, but at depth. 

 Corresponding- with these physical-chemical facts, the reaction is one of 

 dehydration and reduction of volume. 



PHLOGOPITE. 



Phlogopite is potasshun-hydrogeii-magnesiuni-aluminum silicate. 



occurrence. — Phlogopite has an occurrence which is somewhat different 

 from that of biotite. It is especially characteristic of metamorphosed 

 impure carbonates, such as dolomitic marbles. In these rocks it is often 

 associated with pyroxene, amphibole, etc. 



Alterations. — The most frequent alterations of phlogopite are to hydro- 

 phlogopite (monoclinic; sp. gr. 2.303, kerrite) and chlorite (monoclinic; 

 variety of penninite; sp. gr. 2.649). It is also said to alter to talc (orthor- 

 hombic or monoclinic; sp. gr. 2.7-2.8). In these last two alterations 

 gibbsite (monoclinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic; sp. gr. 3.40) 

 must simultaneously separate. 



The reactions for these changes are as follows: 

 For hy drophlogopite : 



(1) 2H 2 KMg 3 AlSi 3 Oj 2 +7H 2 0+CO,=2(H 3 Mg 3 AlSi30 12 .3H 2 0)+K 2 C0 3 +k. 

 For chlorite: 



(2)2H 2 KMg 3 AlSi 3 12 +6Mg00 3 +7H 2 0=2[H 3 Mg 6 AlSi 3 12 .6(OH)]+K 2 C0 3 +5C0 2 +k. 



For talc and gibbsite: 



(3) 4H 2 KMg 3 AlSi 3 12 +6H 2 0+4C0 2 =3H 2 Mg 3 Si 4 12 +4Al (OH) 3 +3}IgC0 3 +K,C0 3 +H 2 0+k. 

 (4)2H 2 KMg 3 AlSi 3 12 +C0 2 +4H 2 0=H 2 Mg 3 Si 1 1 2+H 4 Mg 3 Si 2 9 +2Al(OH) 3 +K 2 C0 3 +k. 



Iii equations (3) and (4) if diaspore instead of gibbsite were produced 

 less water would be needed. 



Disregarding the carbonates, the increase in volume for hy drophlogo- 

 pite, equation (1), is 26.89 per cent; for chlorite, equation (2), is 41.02 

 per cent. The decrease for talc and gibbsite, equation (3), is 7.79 per 

 cent, and for talc and diaspore 18.27 per cent. The increase in volume of 

 the serpentine, talc, and gibbsite, equation (4), is 5.23 per cent. 



All of these reactions are those of hydration and solution. They are 

 characteristic of the zone of katamorphism. 



