86 NEW YORK STATE MUSEUM 



Magnetite. This mineral, commonly called " magnetic iron ore," 

 is one of the oxides of iron. Often found in good crystals, either 

 regular octahedra or dodecahedra forms, but more often as irregiilar 

 grains. It is black, with a metallic luster; about as hard as steel; 

 and strongly attracted by the magnet. Generally closely resembles 

 ilmenite (see above). 



Magnetite is widespread throughout the Adirondacks, being found 

 as scattering grains in the anorthosite, syenite-granite, gabbro and 

 diabase, as well as in some of the Grenville gneisses. Locally, the 

 magnetite makes up extensive ore bodies, as at Mineville, Essex 

 county, and at Lyon Mountain, Clinton county. 



Mica. This is a family name including several important mem- 

 bers, all of which possess a very perfect cleavage in one direction 

 whereby the mineral peels off in exceedingly thin layers. Mica 

 most often appears as thin flakes scattered through rocks, but some- 

 times in good six-sided tabular to prismatic crystals. All micas are 

 silicates of aluminum, and all are relatively soft, being easily 

 scratched by a copper coin. 



Muscovite is a potash mica, transparent in thin sheets. It occurs 

 in relatively large masses in some of the pegmatite, and in small, 

 shiny flakes in certain Grenville gneisses and quartzites. 



Biotifc is an iron-magnesia mica, dark colored but translucent in 

 thin sheets. As small flakes it often occurs in many types of the 

 Adirondack rocks and in certain of the Grenville gneisses. 



Phlogopite is a magnesia mica, brown and translucent in thin 

 layers. It occurs only in certain Grenville gneisses, schists, quart- 

 zites and limestones. 



Microcline. See feldspar. 



Muscovite. See mica. 



Orthoclase. See feldspar. 



Phlogopite. See mica. 



Pyrite. Usually known as "iron pyrites" or ''fools gold." It 

 often shows good crystal forms as cubes, octahedrons, pyritohedrons, 

 or combinations of these. It is a sulphide of iron; scratched by 

 quartz but not by a knife ; of pale brass-yellow color with a metallic 

 lustre ; very brittle ; and has practically no cleavage. 



Pyrite occurs, in small amounts, as irregular grains or specks in 

 anorthosite, gabbro, diabase, and certain of the Grenville gneisses 

 and schists. 



Pyroxene. This is a family name including, in the Adirondacks, 

 augitc, (a silicate of magnesia, lime, aluminum and iron) ; hyper- 

 sthcne (a silicate of magnesia and iron) ; and coccoJite (a silicate 



