1 6 NEW YORK STATE MUSEUM 



stage of alteration to serpentine. The two minerals are easily dis- 

 tinguished and the serpentine can be seen developing around the 

 margin and along the cracks of the diopside, gradually spreading 

 until the latter mineral is entirely replaced by the former. (Plate 

 i, figures i and 2.) 1 Had the process of alteration gone to com- 

 pletion, in every instance, it would, of course, have destroyed all trace 

 of the original mineral, but, even in this event, experience in other 

 localities would have suggested that the serpentine had resulted 

 from the alteration of some older silicate. 



The obvious result of the process, so briefly outlined, is to change 

 a diopside limestone into a serpentine limestone, and this is what 

 has happened in the present case. 



The composition of diopside may be represented by the formula 

 CaMgSi 2 O 6 and of serpentine by H 4 Mg 3 Si 2 O 9 and it is clear 

 that in the alteration of the former to the latter a decided chemical 

 change takes place, involving the elimination of CaO and SiO 2 and 

 the addition of OH. This change may be represented as follows: 



Diopside Serpentine 



3 CaMgSi 2 6 + 2H 2 + 3 C0 2 == H 4 Mg 3 Si 2 O 9 + 3CaCO 3 + 4SiO 2 . 

 The equation is not to be taken literally as showing the precise 

 chemistry of the process, but serves to indicate the general nature 

 of the change and, particularly, that the process involves hydration, 

 in which respect it is similar to many processes by which anhydrous 

 silicates undergo alteration. Indeed, hydration is one of the most 

 frequent effects of such mineral alteration. 



If the above equation could be accepted as an exact statement 

 of the alteration process, it would be possible, by using the specific 

 gravities of the minerals, to determine the relative volumes of the 

 diopside and the serpentine. It is evident that when one mineral 

 alters to another, the second mineral may occupy more or less space 

 than the original mineral, depending upon the relative amounts of 

 material added or removed and the relative densities of the two 

 species. Or, it is possible that these different factors may balance 

 each other so that the secondary mineral occupies the same volume 

 as the primary mineral. 



Hydration is very commonly attended by increase of volume and 

 decrease of density and, in the case of serpentine, this increase of 

 volume is large. Indeed, when a rock is largely composed of a 

 mineral, such as olivine, which alters to serpentine, the consequent 



1 The writer is indebted to Mr W. E. Cockfield, fellow in geology, Prince- 

 ton University, for the accompanying photomicrographs. 



