78 Dr. Blum on Pseudomorphous Minerals. 



Chlorite with the form of Hornblende. — At Greiner in the 

 Tyrol there is a deposit of mica slate situated between gneiss and 

 hornblende rock ; and the hornblende is changed to a leek green 

 and dark green chlorite, some of the folia of which are an inch 

 or more broad. Rose describes similar facts, and mentions some 

 crystals of hornblende that were chlorite externally to a line or 



more in depth. In the change 2(Ca Si+Mg^ Si^ ) loses 2Ca and 



receives 9(Mg, Fe) 3A1, 12k, thus producing 3[(Mg% Fe^) Si-|- 



Al Si+2MgH2], or 3 atoms of chlorite. 



Among the pseudomorphs of metallic minerals we cite only 

 the following : — 



Brown Iron Ore with the form of Pyrites. — The change of 

 pyrites to brown iron ore appears to be a very common one in 

 nature, and is in progress at the present time. It consists in the 

 removal of the sulphur from the pyrites, (sulphuret of iron,) and 

 the substitution of oxygen and water, and is supposed to take 

 place generally by the formation first of a hydrated sulphate of 

 iron, the sulphur and iron each uniting with oxygen, and then, 

 following this, a removal of the sulphuric acid by some alkali or 

 earth in the vicinity. Instances are given of gypsum thus pro- 

 duced by this acid combining with the lime of the gangue. A 

 more direct mode, without the change to a sulphate, is as follows : 

 the water present gives hydrogen to the sulphur to form sulphu- 

 retted hydrogen, and oxygen to oxydate the iron; and with the 

 oxyd thus formed, other portions of water combine and produce 

 the hydrate of iron or brown iron ore. In this process some sul- 

 phur is set free, and thus Dr. Blum accounts for the incrustations 

 of sulphur often found with decomposing pyrites, or in cavities 

 originally occupied by this mineral. To illustrate this: — 2 atoms 

 of pyrites [Fe^ S*] with 6 of water [H'^ 0«] afford 1 atom of 



Fe^ 0% H^ 0=* ( = Fe2 H^ brown iron ore), and 3 atoms of sul- 

 phuretted hydrogen [H« S^'], besides 1 atom of sulphur which 

 is set free. 



The change producing these pseudomorphs commences with 

 the exterior, and it is quite common to find crystals with a thin 

 incrustation of brown iron ore. Cubes, pentagonal dodecahedrons 

 and compound crystals thus altered are of frequent occurrence. 



Among pseudomorphs in which there is a total change in the 

 constitution of the mineral or a complete replacement of the 



