APPENDIX 68? 



Pyrrhotite (magnetic pyrites), FeuSi 2 . — The color is bronze-yellow 

 when fresh, but weathers readily to brown on the outside. It is darker 

 than pyrite. Pyrrhotite is softer than pyrite and can easily be scratched 

 with a knife (H = 4). Small fragments are attracted by a magnet. 

 Pyrrhotite is of little use in itself but, since it often bears nickel, it 

 is mined for that metal. The most valuable deposits occur in Can- 

 ada, but large quantities are found in Vermont, Pennsylvania, and 

 elsewhere. 



Zinc Minerals 



Sphalerite (blend, black jack, jack, zinc sulphide), ZnS. — The 

 color varies from yellow to brown-black: When a fragment is crushed, 

 the pieces look like resin. This resinous luster can usually be seen 

 whenever a specimen is fractured. The streak is light yellow. Sphal- 

 erite is softer than steel (H = 3.5) and occurs in crystals or in masses 

 with well-developed cleavage faces. It is an important ore of zinc 

 and is often associated with lead and silver ores. It is extensively 

 mined in Missouri and is of common occurrence in smaller quantities 

 elsewhere. 



Calcium Minerals 



Calcite (calc spar), CaC03. — The color, when pure, is white or color- 

 less, but when impurities are present the color depends upon the 

 foreign substance; yellow, green, gray, salmon, lavender, and other 

 colors are common. Calcite is much softer than glass (H = 3). It is 

 readily distinguished from other minerals by its strong rhomboidal 

 cleavage, its hardness, and its effervescence with acids. It is one of the 

 most widespread and abundant minerals. There are a number of 

 varieties, including dogtooth spar, so-called because of the shape of the 

 crystals; marble, a crystalline rock composed of large and small 

 grains of calcite; Mexican onyx, an agatelike rock formed by suc- 

 cessive layers of lime deposited from solution in a cavity. 



Dolomite (pearl spar), CaMg (003)2. — The color is usually white or 

 with a yellow tint. It is softer than steel (H = 3.5). Dolomite is 

 distinguished from calcite, which it resembles, by its curved cleavage 

 surfaces, its pearly luster, and its lack of effervescence with cold hydro- 

 chloric acid. It occurs in distinct crystals and forms thick strata of 

 limestone. It is a common vein mineral. 



Gypsum, CaS0 4 -2 H 2 0. — This mineral is colorless or white unless 

 tinted by impurities. It is softer than calcite and can be scratched 



CLELAND GEOL. — 44. 



