280 Mr Haidinger on the Parasitic Formation of Minerals^ 



The crystals had the form of low eight-sided prisms, belong- 

 ing to the prismatic system, and contained exactly half the 

 water which is required in the mixture of the original species. 



III. Changes in Minerals containing Copper. 

 [This Section of Mr Haidinger's paper has already been 

 published by himself in this Journal, No. xiii. p. 126 — 134.] 



IV. Changes in Minerals containing Iron. 

 Through the exertions of the late travellers in Brazil, we 

 have become acquainted with octahedral crystals, often of con- 

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

 a red streak, and seem to contradict the character of the spe- 

 cies of octahedral iron-ore in the characteristic of Mohs, (Mi- 

 neralogy, Transl. vol. i. p. 439,) namely, that it should have a 

 black streak. On a more close inspection, however, the octa- 

 hedral 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 

 A. 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 regular 

 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 paral- 

 lel 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 be- 

 ing a compound of one atom of protoxide and two of perox- 

 ide of iron, expressed by Berzelius's formula Fe -f 2 Fe, while 

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

 of oxygen are 28.215 and 30.66 per cent. There is a group 

 of crystals from Vesuvius in Mr Allan's cabinet, elucidating, 

 by their coarser texture, the explanation given of the Brazilian 

 octahedrons. The rough form of an octahedron is produced 

 by very distinct flat crystals, united in various positions, of the 

 rhombohedral species, the face perpendicular to the axis of the 



