THE EARTH'S ROCK FOUNDATIONS 57 



mixture, as is usually the case, the fused product has the proper- 

 ties of this mineral and is still known as an orthoclase feldspar 

 or potash feldspar. The mixtures of anorthite and albite are 

 known as plagioclases or soda-lime feldspars. A distinctive name 

 has been given to that feldspar that is a product of the fusion of 

 anorthite and albite in about equal amounts. It is called 

 labradorite. 



All the feldspars cleave readily in two directions, and the 

 cleavage faces are at right angles to each other (in the orthoclases) 

 or at slightly oblique angles in the plagioclases. The cleavage 

 faces of the plagioclases are striated with many fine parallel lines. 

 In directions other than along the cleavage planes feldspar breaks 

 with an uneven fracture. Even in small fragments found in such 

 rocks as fine-grained granite it is usually possible to see the 

 cleavage faces with the hand lens sufficiently distinctly to recog- 

 nize the mineral. 



Pure feldspars are colorless, but they are seldom pure. Ortho- 

 clase is usually tinged with red, varying from pale pink to deep 

 brick red; the color seems due to the presence of fine particles 

 of iron oxide scattered throughout the mineral. Plagioclase is 

 commonly gray, while labradorite is likely to be dark, smoky 

 gray, or even black. The colors are not dependable as absolutely 

 reliable distinguishing features, however, since plagioclase is 

 sometimes red, and orthoclase may be gray or dark. The ready 

 cleavage in two directions at right angles or nearly right angles 

 to each other, the vitreous luster on fresh fractures in other planes, 

 and the hardness are the chief features to be relied upon in field 

 determination. The feldspars have a hardness of 6, scratching 

 glass, but being in turn scratched by quartz. The feldspars are 

 probably the most widely distributed of rock-forming minerals, 

 though not occurring in such large quantities as those previously 

 mentioned. 



Chemically, the feldspars are representative of a large major- 

 ity of the minerals which, like them, are complex compounds of 

 various basic elements with some one of the series of silicic acids. 



