EXERCISE I. 

 THE DURABILITY OF IMPORTANT SOIL FORMING MINERALS. 



Supplies for a Laboratory Section of Twelve. Twelve each of labeled specimens (about one cubic inch in vol- 

 ume) of quartz, feldspar, mica, hornblende, and calcite. Twelve glass plates 4"x4". One iron mortar for cruih- 

 ing small pieces of the mineral specimens. Twelve reagent bottles filled with hydrochloric acid. 



DIRECTIONS. First read the following explanation of terms to be used in the study of 

 minerals. Answer such questions as accompany the explanations. Then turn to the descriptive 

 forms, pages 8, 9, and 10, and fill in a careful description of each mineral with which you are 

 supplied. 



1. (a) Name. Give the name of the mineral, (b) Color. State the color or the 

 colors of the mineral, (c) Transparency. Minerals are transparent when clear like window 

 glass; translucent when but a small amount of light passes through; opaque when no light 

 passes through. 



Name, color and transparency are necessary in identification, but give very little clue to 

 the durability of minerals. 



2. Cleavage. The tendency of minerals to break more easily in some directions 

 than in others is called cleavage. There may be one, two or three planes of cleavage as shown 

 by mica, feldspar, and calcite respectively. 



Explain how planes of cleavage may be a point of weakness in minerals. 



3. Hardness. A mineral is soft if it can be scratched with the thumb nail; hard if it 

 is difficult to scratch it with a knife blade; very hard if the specimen will scratch glass. 



When hard and soft minerals are exposed to weathering which as a rule prove to be more 

 durable? 



4. Solubility. Place a very small piece of the mineral to be studied in a test tube. 

 Add a few drops of hydrochloric acid. If the mineral is soluble it slowly disappears. The 

 dissolving of minerals is often accompanied by effervescence or bubbling. If the mineral is 

 insoluble the acid will have no perceptible effect upon it. 



Explain how solubility may determine the weakness or durability of a mineral. 



5. Porosity. A mineral is porous if a drop of water sinks rapidly into the dry speci- 

 men; compact if the drop remains on the surface for some time. 



How do you explain that a compact mineral is as a rule more durable than a porous 

 mineral? 



Conclusions as to Durability. The durability of a mineral may be quite accurately 

 judged from such properties as hardness, solubility, cleavage, and porosity. After studying 

 the properties of a given mineral and coming to some conclusion as to its durability state your 

 reasons for such conclusion. 



