THE TETCLINIC FELDSPARS 



XO 





S1O2 



AlA 



K2O 



NaaO 



CaO 



Microcline ...... 



65.00% 



18 00% 



17.00% 



* « * • 



.... 



Albite 



68.00 



20.00 



« * * ■ 



12.00% 



« . . • 



Oligoclase ...... 



62 00 



24.00 



* * • « 



9.00 



6.00 



Labradorite 



63.00 



30.00 



• « « « 



4.00 



13.00 



Anorthite 



43.00 



37.00 



• * • • 



* • • « 



20.00 



Considering only the last four of these, as arranged, it will 

 be noted that they become gradually poorer in the acid element 

 silica, and richer in alumina and other bases; that is, they 

 become more basic. Also that albite carries some 12% of soda 

 and no lime, that oligoclase carries 9% of soda and 5% of lime; 

 labradorite but 4% of soda and 13% of lime, while anorthite, 

 the most basic of all, has no soda, and carries 20% of lime. 

 They have hence come to be known, respectively, as soda feld- 

 spar, soda-lime feldspar, lime-soda feldspar, and lime feldspar. 

 As a matter of fact, however, these varieties all grade into one 

 another, through the replacing power of the various elements, 

 and are regarded, not as true species, but rather as isomorphous 

 admixtures, forming what is known as the alhite-anorthite series. 



Their distinction, either in hand specimens by the unaided 

 eye, or in thin sections by the miseroscope, is a matter of con- 

 siderable difficulty, and as in addition to other characteristics 

 they have in common two eminent cleavages occurring at oblique 

 angles, it has become customary to group all under the general 

 term of plagioclase, a name derived from two Greek words signi- 

 fying oblique and fracture. We can then treat of the subject 

 under the heads of (1) microcline and (2) plagioclase. 



(1) Microcline (Triclinic Potash Feldspar). — As a rock con- 

 stituent, this feldspar is in every way identical with orthoclase, 

 from which it can be distinguished only in thin sections under 

 the microscope. Its composition, manner of occurrence, and 

 associations are those of orthoclase, and need not be repeated 

 here. Anorthoclase is a triclinic soda-potash feldspar of a form 

 closely resembling that of orthoclase and for all present purposes 

 may be regarded as orthoclase in which soda replaces a con- 

 siderable proportion of the potash. 



(2) The Plagioclases. — With the exception of albite the 

 plagioclases are all prominent and essential constituents of the 



