88 Mr HAIDINGER on the Parasitic Formation 



IV. Changes in Minerals containing Iron. 



Through the exertions of the late travellers in Brazil, we 

 have become acquainted with octahedral crystals, often of consi- 

 derable magnitude, of a particular ore of iron. They afford a 

 red streak, and should seem, therefore, together with other in- 

 stances of the same kind that had been observed, to form a con- 

 tradiction to the character given for the species of octahedral 

 iron-ore in the Characteristic of MOHS *, namely, that it should 

 have a black streak. On a more close inspection, however, the 

 octahedral masses are found to be composed of a great number of 

 small crystals, resembling those of the rhombohedral iron-ore, a 

 species, one of whose characters is in fact the red streak ob- 

 served. A specimen from Siberia, given to Mr ALLAN by Sir 

 ALEXANDER CRICHTON, presents the same change, excepting 

 that in this specimen the individuals of the rhombohedral iron- 

 ore are so minute, that they form a compact mass, contained 

 within smooth planes, having the situation of the faces of a re- 

 gular octahedron. As in the decomposed anhydrite, these planes 

 are not the remains of cleavage, but they existed in the octa- 

 hedral iron-ore previous to its decomposition, as fissures parallel 

 to its octahedral cleavage. The chemical change necessary for 

 transforming the mixture of octahedral iron-ore into that of 

 rhombohedral iron-ore, is a very slight one, the former being 

 a compound of one atom of protoxide and two of peroxide 



* 



of iron, expressed by BERZELIUS'S formula Fe + 2 Fe, while the 







latter is the pure peroxide, or Fe. The relative contents of oxy- 

 gen are 28.215 and 30.66 per cent. There is a group of crys- 

 tals from Vesuvius in Mr ALLAN'S cabinet, elucidating, by their 



* Treatise on Mineralogy, Transl. vol. i. p. 439. 



