828 A TREATISE ON METAMORPHISM. 



and forms smithsonite (ZnCO ;! ). But in Missouri silica, partly in the 

 amorphous form, is very abundant; and there, when the zinc sulphide is 

 oxidized, the oxide of zinc largely unites with the silica, forming calamine 

 [(ZnOH) 3 Si0 3 ]. Both smithsonite aud calamine occur in both districts, but 

 calamine occurs abundantly only where silica is abundant. Similarly, 

 where in lagoons the iron is reduced to the ferrous form, it would unite 

 with the silica on a large scale, provided that compound were abundantly 

 present in a form suitable for union. Hence the hydrated ferrous silicate 

 of the Mesabi district is regarded as a product of the chemical reactions of 

 the zone of katamorphism. 



Another iron compound, besides ferrous silicate, which is associated 

 with iron-bearing carbonates is iron sulphide. It is believed that this 

 compound results from the reduction of the basic ferric sulphate. It has 

 been pointed out that, so far as the iron goes into lagoons as a sulphate, it 

 is thrown down as basic ferric sulphate [Fe^SO^g^FeoOg.xILO]. This 

 compound in the belt of cementation in the presence of organic matter 

 which strongly demands oxygen is reduced, the oxygen being abstracted 

 from the sulphur, and so far as possible from the iron, thus producing 

 bisulphide of iron. The excess of ferrous iron unites with the carbon 

 dioxide, or exceptionally in part with the silica, forming ferrous carbonate 

 or ferrous silicate. Supposing ferrous carbonate were formed, and the 

 reducing agent to be CO, the reaction may be written: 



2[Fe 2 (S0 4 ) 3 .5Fe 2 3 .xH 2 0]-|-33CO=3FeS 2 +21FeCO s +12C0 2 +2xHoO. 



In an analogous manner the reaction may be written for ferrous 

 silicate as follows: 



2[Fe 2 (S0 4 ) 3 .5Fe 2 3 .xH 2 0]+33CO+21Si0 2 =3FeS 2 +21FeSi0 3 +33C0 2 +2xH 2 0. 



So far as there is water in the silica or the ferrous silicate, this may be 

 added to both sides of the equation. The reducing agent may be supposed 

 to be C or H or some combination of the two. This would modify the 

 equation, but would not change the principle involved. 



Perhaps the formation of the compounds, ferrous silicate and bisulphide 

 of iron, can not logically come under the caption "iron-bearing carbonates;" 

 but since they occur with the iron-bearing carbonates as subordinate con- 

 stituents, and only in a few districts in sufficient masses to be regarded as 

 independent formations, they are here considered. 



