458 C E. Tilley — Metainorpldsm of 



lime-bearing minerals are sufficiently familiar in the metamorphism 

 of the calcareous limestones. 



The production of forsterite from dolomite involves the 

 simultaneous formation of calcite, as indeed does the formation of all 

 the minerals involved in these rocks, with the exception of a non- 

 aluminous diopside. The equations representing the production of 

 forsterite, edenite, and diopside are shown below. 



(1)2 CaMg (003)2 + SiO,, = Mg^SiO, + 2 CaCOs + 2 CO^ 

 dolomite quartz forsterite calcite 



(2) 3n + 2 (CaMg (CO3), )+ 4n -+- 3 (SiO^) + K\0, 



dolomite 



= n CaMg3 (SiOs), + 2n + 1 (CaCO^) + 4n + 3 (CO,) 

 CaMggA^ (SiOJg calcite 



edenite 



(3) CaMg (CO3), -f 2 SiO, = CaMgjSiOa), -f 2 CO., 



dolomite diopside 



The ratio R = >, A , i»/r^/^" becomes an important criterion 

 CaO + MgO 



in the formation of these minerals.^ Considering the case where 

 alumina is zero, the production of forsterite, tremolite, and diopside 

 is conditioned by the value of R, being J, f , and 1 for these minerals 

 respectively. For the aluminous amphibole- edenite R becomes 



~ where n is the ratio of tremolite molecules to Scharizer 



3n + 2 



molecules. In the presence of increasing silica, forsterite gives place 



to first tremolite, and then diopside, and this order is one of 



decreasing dedolomitization. The formation of any one of these 



minerals can be regarded as dependent on the amount of silica 



present (i.e. R) in thermal metamorphism, or the amount of dolomite 



involved within the sphere of diffusion of silica. 



The presence of alumina in the absence of silica, possibly as 

 bauxite leads to the formation of spinel as represented in the 

 equation 



CaMg (C03)2 + Al,03 = MgAl^O, + CaCOg -f CO,, 

 spinel 



When silica is present the spinel may accompany forsterite, and 

 the close association of spinel and forsterite as seen in the thin 

 slices, — spinel often present within the boundaries of forsterite, or 

 abutting forsterite grains, is evidence of co-crystallization in 

 accordance with the equation 

 3 CaMg (C03).,+SiO.,+Al2 03-Mg,SiO,-f MgAl,0,+ 3 CaC03+ 3 CO,. 



The value of R is here \. 



Apparently when R exceeds this value, spinel is less often formed, 



''■ Or in general for maijnesian limestones, R = ( ^^^ — ^ ^ — \ with the 

 modified R values for thes^ minerals. 



