ALTERATIONS OF OLIVINE. 309 



talc develops so generally from the olivines, because talc is a metasilicate 

 Ordinarily accompanying the serpentine one or more of the following- 

 minerals may be found: Tremolite (monoclinic; sp. gr. 3.0), actinolite 

 (monoclinic; sp. gr. 3.10), talc (orthorhombic or monoclinic; sp. gr. 2.75), 

 hydrotalcite (hexagonal; sp. gr. 2.04-2.09), magnesite (rhombohedral; sp. gr. 

 3.06), breunnerite (rhombohedral; sp. gr. 3-3.2), siderite (rhombohedral; 

 sp. gr. 3.83-3.88), quartz (rhombohedral; sp. gr. 2.6535), opal (amorphous; 

 sp. gr. 2.15), magnetite (isometric; sp. gr. 5.174), chromite (isometric; sp. gr. 

 4.445), hematite (rhombohedral; sp. gr. 5.225), and limonite (amorphous; 

 sp. gr. 3.80). One of the most frequent combinations of minerals with 

 serpentine is magnesite, quartz or opal, and magnetite. Frequently the 

 magnetite may partially or completely replace the hematite or limonite. 

 The formation of the serpentine is frequently accompanied by tremolite or 

 actinolite with iron oxide. It is much less frequently accompanied by talc. 

 In some instances the olivine has passed directly into magnesium carbonate 

 and hematite or limonite, but the former commonly being largely removed 

 in solution. 



Other alterations of olivine are into anthophyllite (orthorhombic; sp. 

 gr. 3.15) into actinolite, hematite, and spinel (isometric; sp. gr. 3.8), but 

 these are by no means comparable in importance to the change to serpentine. 



Beginning with the simplest alteration to serpentine, if an olivine be 

 taken in which the magnesium is to the iron as 3:1, and magnetite being 

 the only mineral which accompanies the serpentine, the reaction may be 

 written as follows: 



(1) 3Mg 3 FeSi 2 O s +6H 2 0+0=3H 4 Mg 3 Si 2 9 +Fe 3 4 +k. 



The increase in volume of the serpentine and magnetite as compared with 

 the olivine is 29.96 per cent. 



Supposing the magnesium is to the iron as 1:1 and the iron passes into 

 magnetite, the reaction is — 



(2) 3MgFeSi0 4 +2H 2 0^0=H 4 Mg 3 Si 2 9 +Fe 3 4 +Si0 2 +k. 



The increase in volume of the serpentine, magnetite, and quartz as compared 

 with the olivine is 15.19 per cent. 



If it be supposed that a third of the magnesium passes into magnesite, 

 and that silica also separates, the reaction may be written as follows: 



(3) 3Mg 3 FeSi 2 O 8 +3C0 2 +4H 2 O+0=2H 4 Mg 3 Si 2 O 9 + Fe 3 4 +3MgC0 3 -2SiO 2 +k. 



