﻿Vol. 6l.] MICROSCOPIC STRUCTURE OF SERPENTINE. 713 



increase greater. 1 We may, therefore, say that the ratio of the silica 

 to the magnesia is practically unaltered. 



Two molecules of enstatite (2MgO -f- 2Si0 2 ) become one of ser- 

 pentine by the addition of one molecule of magnesia and two of 

 water, and if the enstatite be ferriferous still more magnesia must' 

 be added; or, if the magnesia in the rock remain as before, some 

 silica must be removed in adding water. If, for simplicity, we 

 assume augite to contain equal amounts of magnesia and of lime, 

 the conversion of two molecules of augite (MgO + CaO + 2Si0 2 ) 

 into serpentine requires the addition of two molecules of magnesia 

 and the same of water and the removal of one of lime— or, in other 

 words (apart from the water), this signifies the removal of a large 

 amount of silica and lime from the rook — obviously the greater 

 change. So it is not surprising to find augite more persistent than 

 the other two minerals ; especially since, as we can see from a table 

 of analyses of diopside and sahlite, the lime generally exceeds the 

 magnesia in the approximate ratio of 17 : 13. The evident fact 

 that pressure (leading to reduction of volume) is favourable to the 

 conversion of augite into serpentine (antigorite) is significant. It 

 also follows, that to convert forsterite into talc requires tho removal 

 of more than half the magnesia and the addition of a little water, 

 but if any iron be present the loss is not quite so great. When 

 enstatite is altered into talc, either a little silica must be added 

 with the water, or a corresponding amount of magnesia removed — 

 a less serious change. The conversion of serpentine into talc means 

 either the addition of two molecules of silica and the removal of one 

 of water, or the removal of half the magnesia and much of the 

 water. In regard to this, the frequent presence of a magnesian 

 carbonate where this change has occurred may be significant. 



XIY. General Conclusions. 



These investigations, which we fear may have seemed rather 

 prolix, into the production of serpentine from each of its parent 

 minerals, and as a rock- constituent, justify, we trust, the following 

 conclusions : — 



(1) That a tint and pleochroism are accidental rather than 

 essential characteristics of the variety of the mineral serpentine 

 named antigorite. 



(2) That, if low polarization-tints be regarded as an essential 

 characteristic of antigorite, a closely-associated mineral must exist, 

 which is distinguished only by greater brightness of these. If the 

 minerals can be isolated and subjected to analysis (which we have 

 not the means of applying), they may prove to bo distinct : but 

 the way in which, as described in the preceding pages, they seem to 

 graduate one into the other, leads us to believe that they are varieties 



1 It is noteworthy that in gabbros where grains of olivine are converted into 

 serpentine the neighbouring felspar is much cracked, as if by their swelling. 

 Also, in regard to the chemical change, we may note the rather slow alteration 

 of the non-ferrifei'ous olivine in the diamantiferons breccia described above. 



