Earth Study 831 



Method The pupils should have before them as many varieties of 

 quartz as possible; at least they should have rock crystal, amethyst, rose 

 and smoky quartz and flint. 



Observations i. What is the shape of quartz crystals? Are the sides 

 all of the same size? Has the pyramid-shaped end the same number of 

 plane surfaces as the sides? 



2. What is the luster of quartz? Is this luster the same in all the 

 different colored kinds of quartz.' 



3. Can you scratch quartz with the point of a knife? Can you scratch 

 glass with a corner or piece of the quartz ? Can you cut glass with quartz ." 



4. Describe the following kinds of quartz and their uses: amethyst, 

 agate, flint. 



5. How many varieties of quartz do you know? What has quartz to 

 do with the petrified forests of Arizona? 



Forms of feldspar crystals. 



FELDSPAR 



Teacher's Story 



We most commonly see feldspar as the pinkish portion of granite. This 

 does not mean that feldspar is always pink, for it may be the lime-soda form 

 known as labradorite, which is dark gray, brown or greenish brown, or 

 white ; or it may be the soda-lime feldspar called oligoclase, which is grayish 

 green, grayish white, or white; but 

 the most common feldspar of all is 

 the potash feldspar orthoclase 

 which may be white, nearly trans- 

 parent, or pinkish. Crthoclase is 

 different from other feldspars in 

 that, when it splits, its plane stir- 

 faces form right angles. Feldspar 

 is next in the scale of hardness to quartz, and will with effort and per- 

 severance scratch glass but will not cut it; it can be scratched with a 

 steel point. Its luster is glassy and often somewhat pearly. 



Maine leads all other states in the production of feldspar. It is quarried 

 and crushed and ground to powder, as fine as flour, to make the clay from 

 which china and all kinds of pottery are made. Our clayey soils are made 

 chiefly from the potash feldspar which is weathered to fine dust. Kaolin, 

 which has been used so extensively in making the finest porcelain, is the 

 purest of all clays, and is formed of weathered feldspar; floor tiling and 

 sewer pipes are also made from ground feldspar. Moonstone is clean, soda- 

 lime feldspar, whitish in color and with a reflection something like an opal. 



LESSON CCXV 



FELDSPAR 



Leading thought Feldspar is about five times as common as quartz. 

 The crystal is obliquely brick-shaped, and when broken splits in two direc- 

 tions at right angles to each other. It is next in hardness to quartz, and 

 will scratch glass but will not cut it. 



Method If possible, have the common feldspar (orthoclase), the soda- 

 lime feldspar (oligoclase) and the lime-soda feldspar (labradorite). 



