32 THE CHICAGO ACADEMY OF SCIENCES. 



Magnetite. Fe 3 O 4 . Regular. 



This mineral occurs in considerable quantities in various 

 parts of the county. It is most in evidence in the sands of 

 the lake shore, where it has been segregated by the action 

 of the waves, which, beating upon the shore, sort the 

 materials roughly according to solubility, specific gravity 

 and size. Broad bands of black material are seen stretching 

 along the sands, sometimes continuously, sometimes in dis- 

 jointed patches. If continuous, they would form deposits of 

 economic importance, and companies have been organized for 

 their exploitation, but there are no deposits of such a size as to 

 be of any promise. The result of several experiments with 

 lake shore sand collected upon the university campus at Evans- 

 ton, gave an average of two per cent magnetite. The method 

 of separation here would be that which was used by Edison 

 upon the magnetic sand of the New Jersey coast. A horse- 

 shoe magnet drawn through the sand quickly becomes covered 

 with a brush of magnetite. The grains forming the brush are 

 so small as to require the use of a lens for distinguishing them. 

 A low-power microscope, (say forty diameters), is sufficient for 

 the most favorable study of the grains. The grains show pre- 

 vailing octahedrons with rounded edges, due to the attrition of 

 the grains upon each other. The individual planes are often 

 pitted as is so commonly the case with meteoric iron. Some- 

 times they are brilliantly tarnished, at others oxidized into 

 limonite or into red hematite. The cleavage is octahedral; the 

 fracture, conchoidal; hardness, 5.5, to 6.5; specific gravity, 5. 

 The color is iron black; the streak, black; the lustre, metallic; 

 one of the most opaque of minerals, even the thinnest sections 

 allowing no light to pass through. Powder soluble in hydro- 

 chloric acid, infusible before the blow pipe. One of the most 

 magnetic of minerals. Its magnetism is neither a function of 

 direction nor of size. Particles that pass through a one hun- 

 dred-mesh sieve, do not differ from the coarsest grains. Its 

 magnetism decreases with oxidation. 



To bring grains of magnetite into position for study under 

 the microscope, it is only necessary to place them in a drop 

 of water on an object glass, cover, and draw a needle over the 

 glass. The attractive power of the needle is enough to draw 

 the grains into position. 



Microscopic sections of metamorphic ancj igneous rocks 

 of the drift show magnetite grains as common accessory con- 



