J V LIEN, GENESIS OF ANTIGORITE AND TALC 



31 



that genesis, I believe, lies in the relationship in each case of a colloid 

 magnesium hydrosilicate (Type I), originating from decay of olivine or 

 other ferro-magnesian mineral, during weathering, to a complementary 

 hydrosilicate (Type II), containing more silica and magnesia and less 

 than about half as much water, into which the former has been afterward 

 converted in a lower region of metamoi-phism. 



The four known magnesium hydros ilicates may be thus arranged to 

 show this relationship of the two types: 



The processes involved in the development of these four minerals in 

 nature may be represented in part by the following equations, confining 

 our attention to the single mother-mineral, olivine, out of the twenty- 

 three known to pass into sepiolite and deweylite. 



For talc : 



4 MgFeSi04 + 8 H,,0 + 2 0-|-n aq. = (H,Mg.,Si,0,o + n aq.) 



Olivine Colloid sepiolite 



+ 2(H,Mg02 + naq.) + (H6FeA + naq.) + (H.,Si03-fKaq.) 

 Amorphous magnesium hydrate Colloid ferric hydrate Colloid silicic hydrate 



+ 67.40 per cent. 



Essential volume change (disregarding n aq.) 

 Then in a lower region : 



3 (H,Mg.,SiAo + « aq.) + A=2 H.,Mg3Si,0,.,-(-SiO., + 4 H,0 + n aq. 



Colloid .sepiolite Heat Talc Quartz 



In massive form the normal rock aggregate, steatite, has thus become 

 developed, a mixture of talc and quartz. 



Essential volume change^^ = — 33.96 per cent. 

 For antigorite: 



8MgFeSiO, + 21 H.,0 + 4 + « aq. = (H,,,Mg,Si30,6 + n aq.) +4( H.MgO., + « aq. ) 



Olivine Colloid deweylite Magnesium hydrate 



+ 2 (H6Fe,09 4- n aq.) -f 5 (H^SiOj + n aq. ) 

 Colloid ferric tiydrate Colloid silicic hydrate 



'■^ Without regard to n aq. 



