286 A TREATISE ON METAMORPHISM. 



(monoclinic ; sp. gr. 3.38), calcite, quartz, iron oxides, and siderite (rhombo- 

 hedral; sp. gr. 3.855). Under deep-seated conditions biotite (monoclinic; 

 sp. gr. 2.90) is frequently a product of the alteration of hornblende, and with 

 the biotite epidote may simultaneously form. Rarely serpentine is also pro- 

 duced. While these are the usual alterations of hornblende, in some cases, 

 under conditions of high temperature, hornblende alters into augite (mono- 

 clinic; sp. gr. 3.4), with the simultaneous separation of magnetite (isometric; 

 sp. gr. 5.174). Such alteration of the hornblende has been noted, according 

 to Lacroix, both in lavas and in bombs." In the lava the change is attributed 

 to the action of the magma, being analogous to resorption; but in the bombs 

 it is attributed to heat alone. The alterations of glaucophane and riebeckite 

 are not described in the standard text-books. Arfvedsonite under certain 

 conditions changes into an acmite (monoclinic; sp. gr. 3.525) free from 

 calcium.'' With the acmite occur limonite (amorphous; sp. gr. 3.80), mag- 

 netite, and sometimes lepidomelane (monoclinic; sp. gr. 3.0-3.2). The 

 change of hornblende to augite and the change of arfvedsonite to acmite 

 are the reverse of the process of uralitization. 



The alteration of tremolite to talc may be written as follows : 



(1) CaM< j3 Si 4 1 2+H 2 0+C0 2 =H 2 Mg 3 Si 4 12 +CaC0 3 -(-k. 



The increase in volume of the talc and calcite as compared with the 

 tremolite is 25.61 per cent. The decrease in volume of the talc alone as 

 compared with the tremolite is 0.83 per cent. 



The alteration of actinolite to talc, supposing the excess of iron to sepa- 

 rate as hematite (rhombohedral ; sp. gr. 5.225), and supposing that the 

 Mg : Fe : : 2 : 1, is as follows: 



(2) 6CaMg 2 FeSi 4 12 +4H 2 0+6C0 2 +30=4H 2 Mg 3 Si 4 12 + 6CaC0 3 +3Fe 2 3 +8Si0 2 +k. 



Supposing all the compounds to remain as solids the increase in volume 

 is 20.33 per cent. If magnetite be formed instead of hematite the increase 

 in volume is somewhat less; if limonite be formed, considerably more. If 

 it be supposed that all the compounds except the talc are dissolved the 

 decrease in volume is 36.51 per cent. 



The alteration of actinolite to the variety of serpentine known as 

 bastite may be written as follows: 



(3) Ca(MgFe) 3 Si 4 12 +2H 2 0+C0 2 =H 4 (MgFe) 3 Si 2 9 +CaC0 3 +2Si0 2 + k. 



"Lacroix, A., Mineralogie de la France, Paris, 1893-95, vol. 1, pp. 668-669. 

 6 Brogger, W. C, Die Mineralien der Syenitpegmatitgange der Siidnorwegischen, Augit- und 

 Nephelin-Syenite: Zeitachr. fur Kryst. und Min., vol. 16, 1890, pp. 406-407. 



