ADIRONDACK MAGNETIC IRON ORES I5I 



of the most recent investigations; the work of Penneld and Foote 1 

 affords in fact quite convincing evidence of its validity. According 

 to that formula it contains theoretically FeO, 46.75 per cent and 

 Ti0 2 , 53.25 per cent. There is alwaj's some Fe 2 3 present and 

 usually more or less MgO. The latter replaces the FeO, while 

 the former substitutes probably for the ilmenite molecule, since the 

 close similarity in the structure and crystal form of hematite and 

 ilmenite indicates that they are practically isomorphous. A general 

 formula for ilmenite, accordingly, is Fe Ti0 3 .n Fe 2 3 . 



'The fact that the titaniferous ores are not homogeneous aggregates 

 is sometimes apparent from a- macroscopic examination. The mag- 

 netite may be "recognized by its parting planes parallel to the octa- 

 hedron, the grains always breaking with smooth surfaces. It is 

 the most abundant constituent as a rule. In the intervals between 

 the grains are particles of brighter metallic luster that show rough 

 fracture. These are only slightly attracted by the magnet and 

 when isolated prove to be ilmenite. 



To bring out the physical relations of the minerals the method 

 of preparing polished surfaces and etching with acid can be used 

 to good advantage. 2 Some results obtained with Adirondack 

 ores are shown herewith [pi. 14]. The photographs were taken 

 directly with a camera in ordinary light, as the texture of the ores 

 is sufficiently coarse to be revealed without the use of the micro- 

 scope. It will be seen that there is a good contrast between the 

 magnetite and ilmenite, the former being dulled and pitted by the 

 solvent action of the acid, while the latter retains the brilliancy 

 imparted to it by polishing unimpaired. From the etched surfaces 

 a fairly close estimate of the richness of the ore may be formed by 

 comparing the relative areas occupied by the ore and gangue min- 

 erals, though the latter do not appear distinctly in the photograph. 



In the specimens that have been examined the magnetite and 

 ilmenite are distinguishable without difficulty. There is a clear 

 separation of the particles and no notable tendency toward inter- 

 growth or inclusion on the part of either. The boundaries are sharp. 

 Both minerals belong to the same order of crystallization, though 

 the ilmenite seems to have begun to form somewhat earlier than 



1 Xote Concerning the Composition of Ilmenite. Am. Jour. Sci. 154. 

 ser. 4. 1897. p. 108. 



2 Pieces of the ore an inch or so in diameter are cut with a diamond saw 

 or ground down to a comparatively smooth surface on a wheel such as is 

 used for preparing rock sections. The surfaces are then polished with fine 

 emery,, finishing off with putty powder on cloth. The etching is performed 

 by submersion in a 20 per cent HC1 solution for half an hour. 



