376 E. G. Smith — Pseudomorphs of Limonite after Pyrite. 



Art. XXXV. — Pseudomorphs of Limonite after Pyrite; by 

 Erastus G. Smith. 



The common hydrated oxides of iron generally referred to 

 limonite are undoubtedly alteration products of ferrous ox- 

 ide, or decomposition products of other iron-bearing min- 

 erals. Their secondary nature is clearly demonstrable in 

 the various occurrences where crystalline form is yet retained, 

 giving clearly defined pseudomorphs of ferric hydrate after the 

 original mineral. Numerous occurrences of such pseudomorphs 

 after various minerals are recorded. 



The writer's attention has been called recently to such an 

 alteration of pyrite into ferric hydrate, which is of special inter- 

 est, as the crystalline form of the pyrite is sharply defined. A 

 large number of the crystals were obtained — the largest cube 

 being about one-fourth inch on a side. The color varies from 

 a pale yellow to a dark brown, and in the largest nodule exam- 

 ined the exterior exhibited the peculiar glossy appearance 

 commonly observed on limonite. The crystals occur through 

 the lower layers of the upper Buff division of the Trenton 

 limestone at Carpenter's Quarry, near Beloit, and partially line 

 cavities distributed through the layers, the remaining space 

 being in part . filled with a finely granular calcareous sand. 

 The limestone of these layers contains 2*23-142 per cent of 

 mixed Al 2 O s +Fe 2 3 .* All stages of metamorphism are repre- 

 sented, repeated qualitative examination of smaller crystals giv- 

 ing no reaction for sulphur, while some of the larger ones retain 

 a kernel of the bright yellow, but slightly altered, original 

 pyrite. The simple cube (i-i) is the most common form, though 

 traces of a pyritohedron were observed in some instances. 



The following analysis is of a nodule weighing 26 grams. 

 The exterior shows numbers of imperfect cubes, and the speci- 

 men is evidently merely a larger aggregate of the altered pyrite. 

 Its sp. gr. = 3*45 (temp. 14° C). (Limonite = 3-6-4; pyrite 

 4-82-5-2 Dana). 



i. ii.; 



Silica 06-25 per cent. 



Sulphur (S) 00-31 00-25 per cent. 



Ferrous oxide (FeO) 00-91 



Ferric oxide (Fe 2 3 ) 80-21 



Calcium oxide (CaO) 00*04 



Magnesium oxide (MgO) . 00-40 

 Loss on ignition 11*72 



99*84 



* Geol. Wisconsin, vol. ii, p. 298. 



