Edward T. Hardman — On HcBmatite in Tyrone. 399 



Kane,^ — who says that it has been found in abundance — the resem- 

 blance is very close. A specimen I picked up at an okl pit there 

 was extremely like it, containing also a sandy nucleus ; but after 

 many searches I was unable to find any others. 



This brings us to one of the most interesting points about the ore, 

 namely, the mode of its formation. I believe it may be considered 

 a kind of pseudomorphic product, the result of the chemical action 

 of water carrying various substances in solution, on nodules of clay- 

 ironstone, the former occupants of these shale-beds ; and in order to 

 arrive at the present state of the mineral, we must understand three 

 distinct processes to have taken place. The clay-ironstone nodules, 

 of which numbers are found in the shales of the Carboniferous rocks 

 of the district, were first formed in the usual way by segregation of 

 the highly ferruginous particles of the clays, etc., in which they occur. 

 Then, in the course of time, water passing through the diversely 

 constituted rocks overhead, and therefore taking up alkaline silicates, 

 carbonic acid, bicarbonate or hydrate of lime, sulphate of lime, and 

 of course carrying oxygen, would gradually, by a long series of 

 rather complex decompositions, completely or in part alter the clay- 

 ironstone. 



The carbonate of iron might undergo alteration by several re- 

 actions, the simplest of which would be by the direct effect of 

 oxygen, thus : — 



4FeC03 -f 2H2O -f O2 = 2Fe2H204 -f 400^ 

 Carbonate of Iron. Water. Oxygen. Gothite. Carbonic Acid. 



And the carbonic acid thus set free would no doubt re-act on a portion 

 of the proto-silicate of iron also contained in clay-ironstone, pro- 

 ducing a fresh supply of carbonate of iron, to be again decomposed, 

 as in the above equation : — 



Fe"2Si04 -I- 2CO3 = 2FeC03 + SiOo* 



Proto-silicate of Iron. Carbonic Acid. Carbonate of Iron. Silica (free). 



By another series of re-actions the clay or silicates of alumina 

 would be decomposed, but would be carried off in solution. The 

 removal of these substances may be accounted for in several ways. 



(1). Silicates of alumina are decomposed by sulphate of lime, 

 forming sulphate of alumina — which is soluble— and silicate of lime : ■(• 



4(Al2Si20,2H20) + 6CaS04 = 2Al2(S04)3 ++ Ca6Al4(Si04)6 + 8H2O -\- SiO^ 

 Kaolin or Clay. Sulphate Sulphate A double Silicate "Water. Silica, 



of Lime, of Alumina, of Lime and Alumina 

 on Garnet type. 



The silicate of lime would then be decomposed by carbonic acid,§ 



^ "Industrial Resources of Ireland," p. 129. 



• See Bischof's Chem. Geol., vol. ii. pp. 59 and 61. A similar re-action is given in 

 vol. i. p. 2, solid silicate of lime in water containing carbonic acid gives cai'bonate of 

 lime and free silica. The equation is my own — 



Ca2Si04 (?) -I- 2CO2 = 2CaC03 -f SiOo 



Silicate of Lime. Carbonic Acid. Carbonate of Lime. Silica. 



t Bischof's Chem. Geol., vol. ii. p. 68. 



+ This formula is merely intended to represent graphically what might take place, 

 but by no means to assert the constitution of the body formed. 



§ Also by alkaline carbonates, by which the silica as well would be removed. 



