OCCURRENCE AND ALTERATIONS OF SERPENTINE. 349 



SERPENTINA. 



■■■ — Serpentine occurs in substantially all the varieties of rocks in 

 which chlorite is found, but it is most abundant as a secondary constituent 

 in the igneous rocks which are very heavily magnesian, especially the 

 pyroxenites, peridotites, and similar rocks. Locally it may be so abundant 

 as a secondary constituent in rocks of this class, especially those bearing 

 olivine, as to form serpentine rocks. Serpentine develops very abundantly 

 in the sedimentary rocks which are rich in magnesian constituents, both in 

 detrital material from basic igneous rocks, and in limestones, and in various 

 transition varieties. Serpentine is a product of the zone of katamorphism, 

 including both the belt of cementation and the belt of weathering. 



As shown under the discussion of the various minerals, it is secondary 

 to actinolite, biotite, bronzite, chondrodite, clinohumite, diopside, enstatite, 

 hornblende, humite, hypersthene, muscovite, olivine, pyrope, sahlite, and 

 spinel. Of these the most important is olivine, and of second importance 

 are the pyroxenes and amphiboles. In many cases the constituents out of 

 which serpentine is formed are derived not from a single mineral, but from 

 various minerals, in which case the serpentine may replace nonmagnesian 

 minerals, as feldspar, or may form in veins. 



Alterations. — Serpentine, where long exposed to the conditions of the belt 

 of weathering, is likely to break up into various minerals, of which brucite 

 (rhombohedral ; sp. gr. 2.39), mag-nesite (rhombohedral ; sp. gr. 3.06), opal 

 (amorphous; sp. gr. 2.15), and quartz (rhombohedral; sp. gr. 2.6535) are 

 the more important. By hydration and loss of magnesia it passes into 

 webskyite (amorphous; sp. gr. 1.771). 



The reaction by which serpentine passes into magnesite, brucite, and 

 quartz may be written thus: 



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



The increase m volume is 13.02 per cent. In case opal or hydromagnesite 

 were formed the increase in volume would be somewhat greater, and the 

 reaction would involve hydration as well as carbonation. It is of course 

 possible that both brucite and magnesite may not always be formed simul- 

 taneously. If brucite and not magnesite be formed the equation is — 



(2) H 4 Mg 3 SiA+H 2 0=3Mg(OH),+2SiO,+k. 



