282 A TREATISE ON METAMORPHISM. 



They are frequently associated with hornblende and mica. Anthophyllite 

 occupies the same position in the rhombic amphiboles that bronzite does in 

 the rhombic pyroxenes, and gedrite the same position as hypersthene. As 

 already described (p. 270), the bronzites and hypersthenes alter into 

 anthophyllite. It is to be expected that gedrite in a similar manner forms 

 from hypersthene, but this particular alteration is not mentioned in the 

 standard books of reference. Also, as described (p. 310), anthophyllite 

 forms as a secondary product from olivine. 



Alterations. — Anthophyllite by hydration passes into talc ' (orthorhombic 

 or monoclinic; sp. gr. 2.75) or bastite (orthorhombic; sp. gr. 2.6). Also, 

 Lacroix states" that rarely it alters into calcite (rhombohedral; sp. gr. 

 2.7135). Supposing the magnesium is to the iron as 3:1, and that the 

 freed iron separates as hematite (rhombohedral; sp. gr. 5.225), the alteration 

 to talc may be written as follows: 



(1) 2Mg 3 FeSi 4 1 ,+2H 2 0+0=2H 2 Mg 3 Si 4 12 +Fe 2 3 f k. 



The increase in volume of the talc and hematite, as compared with the 

 anthophyllite, is 11.41 per cent. If the iron oxide be supposed to be 

 hydrated into limonite (not crystallized; sp. gr. 3.80), the increase in 

 volume would be still greater. If bastite be produced, and it be supposed 

 that the magnesium is to the iron as 3:1, the same as in the anthophyllite, 

 the equation may be written: 



(2) 3Mg3FeSi i 1 2+8H 2 0=H 16 Mg 9 Fe 3 Si 6 36 +4Si0 2 +k. 



The increase in volume of the bastite and quartz (rhombohedral; sp. gr. 

 2.6535) as compared with the anthophyllite is 34.09 per cent. If the silica 

 be supposed to be dissolved the increase in volume is 12.09 per cent. 



The particular alterations which gedrite undergoes are not described 

 in the standard text-books; therefore no attempt is made to write equations 

 for changes of this mineral. 



The alteration of anthophyllite to talc and iron oxide involves hydra- 

 tion and oxidation. The alteration of anthophyllite to bastite involves 

 hydration and desilication. Both sets of reactions are, therefore, char- 

 acteristic of the zone of katamorphism; and it is in this zone, especially in 

 the belt of weathering, that the chang'es occur. 



"Lacroix, A., Mine>alogie de la France, Paris, 1893-95, vol. 1, p. 637. 



