OXIDKS 



373 



MAGNETITE 



Magnetite. A ferrous ferrite, FeFe 2 O< ; Fe 2 O 3 = 69.0, FeO 

 :!!.(); Isometric; Type, Ditesseral Central ; Common forms, 

 o I 1 1), d (110), other rare forms, (001), (311), (531), (310) ; Twin- 

 ning parallel to 111; Cleavage, octahedral imperfect; Brittle, 

 fr:idure uneven; H. = 5.5-6.5; G. = 5.17-5.18; Color, iron- 

 black.; Streak, black; Luster, metallic splendent or dull ; Natu- 

 rally magnetic. 



B.B. Fuses with difficulty (1225 C.), and in a strong O. F. 

 changes to red (Fe 2 O 3 ) and loses its magnetism. In fine powder is 

 soluble in hot concentrated HC1; but if it contains titanium or 

 magnesium, it may dissolve with difficulty. Insoluble in HNOa. 



General description. Crystals are octahedral in habit, with 

 the edges replaced by the rhombic dodecahedron ; other forms are 

 rare, as is also the rhombic dodecahedral habit. Some simple 

 rhombic dodecahedrons 

 are found at the Tilly 

 Foster mine, New York. 

 In twinning magnetite fol- 

 lows the spinel law, in 

 which the twinning plane 

 is parallel to an octahedral 

 face, and when this is re- 

 peated polysynthetically, 

 the crystals are striated 

 parallel to an octahedral 

 edge. Large deposits of magnetite are usually granular, coarse or 

 fine; or massive with a parting or laminated structure. Its 

 natural magnetism serves to distinguish it from all other black 

 minerals with the exception of franklinite, some specimens of which 

 are quite as magnetic as magnetite. 



In rock sections when well crystallized magnetite appears either 

 square or with six-sided outlines, and is always opaque; also as 

 rounded grains, irregular masses, or very fme disseminated opaque 

 specks. 



In rock magmas when there is an excess of iron over silica to form 

 metasilicates this excess usually separates as magnetite. Magnetite 

 is one of the first minerals to separate on consolidation. It, how- 

 ever, may contain as inclusions both zircons and apatite, earlier 



FIG. 435. Magnetite Crystals from the Tilly 

 Foster Mine, Brewster, New York. 



