350 A TREATISE ON METAMORPHISjVT. 



The volume of the brucite and quartz is 9.82 per cent greater than the 

 serpentine- If magnesite and not brucite be formed the equation is — 



(3) H 4 Mg 3 Si„0 9 +3CO,=3MgC0 3 +2Si0 2 +2H 2 0+k. 



The volume of the magnesite and quartz is 18.84 per cent greater than that 

 of the serpentine. 



Brauns" gives the formula for the development of webskyite as follows: 



(4) 3(H 4 E 3 Si 2 9 )— EO+12aq.=2(H 6 E 4 Si 3 13 +6aq.) 



. Where serpentine contains iron as a base, partly replacing' the magne- 

 sium, the iron is oxidized and may be hydrated, thus producing hematite 

 or limonite. 



The breaking up of serpentine occurs especially in the belt of weath- 

 ering, the transformation representing- one of the final changes in the 

 degeneration of the silicates. Alterations of serpentine in the zone of 

 anamorphism are not recorded. But the general absence of serpentine in 

 the schists and gneisses of sedimentary origin profoundly metamorphosed 

 in the zone of anamorphism is conclusive evidence that the serpentine which 

 once was in these rocks, and the associated secondary minerals, have 

 recombined to produce heavy minerals of the classes from which serpentine 

 and those other secondary minerals were originally produced. One could 

 readily form equations for such alterations by reading the equations by 

 which serpentine is formed from right to left. (See Table C, pp. 375-394.) 



TALC. 



occurrence. — Talc is practically coextensive in its occurrence with chlorite 

 and serpentine, but in its distribution is more nearly allied to serpentine 

 than to chlorite. Therefore it is found in almost every variety of rock long 

 subjected to alterations in the belt of weathering; but it is especially 

 abundant in the heavily magnesian rocks. Steatite, which is nearly pure 

 talc, is usuallj' derived from the pvroxenites or peridotites. However, talc is 

 so abundant in many schists as to give them the name talcose, or even talc- 

 schists. Also, like serpentine, it occurs abundantly in the dolomite-bearing 

 rocks and in dolomite. 



oBrauns, R. , Studien iiber den Palaeopikrit von Amelose bei Biedenkopf imd dessen Umwand- 

 lungsprodukte: Neues Jahrbuch, supp.-vol. 5, Stuttgart, 1887, p. 322. 



