GUIDE TO THE MINERALOGIC COLLECTION'S 89 



Microotine' KAlSi 3 8 



Microcline is a triclinic feldspar having the same composition 

 as orthoclase and was formerly grouped under that species. 

 The crystals are so close to those of orthoclase in angle and 

 habit that the unassisted eye is unable to distinguish between 

 the two species. Under the polarizing microscope a character- 

 istic gridiron structure is observable in a thin section of micro- 

 cline. A characteristic variety called Amazon stone has a beau- 

 tiful green color. In other respects the characteristics are essen- 

 tially the same as for orthoclase. 



Plagioclase feldspars 



The triclinic group of minerals known as the plagioclase feld- 

 spars constitute a practically continuous series from pure soda 

 alumina silicate in albite (NaAlSi 3 8 ) to pure lime alumina 

 silicate, in anorthite (CaAl 2 Si 2 O s ). The intermediate species 

 now to be discussed are mixtures of these two molecules and 

 of necessity grade into one another, so that in many cases no 

 marked division line can be drawn. If the albite molecule, 

 NaAISigOg, be represented by Ab, and the anorthite molecule, 

 CaAl 2 Si 2 8 , be represented by An, the albite-anorthite series or, 

 as they are usually called, the plagioclase feldspars, may be rep- 

 resented in composition as follows: 



Albite 



Ab 



— Ab, 



An x 



Oligoclase 



Ab 



An- Ab 3 



An, 



Andesin 



Ab 3 



An x — Ab x 



An x 



Labradorite 



Ab x 



An x — Abi 



An 3 



Bytownite (rare) 



Ab x 



An, — Ab x 



An 6 



Anorthite 



Ab x 



An G — An 





The plagioclase feldspars are characterized in general by a 

 repeated twinning parallel to the brachypinacoid which results 

 in a series of striations on the basal cleavage surface. They 

 form an important constituent of the igneous rocks, daciH 1 . 

 andesite, diorite and diabase. 



Albite (soda feldspar) 



Albite is a silicate of aluminium and sodium. 

 It occurs in triclinic crystals (fig. 206) often tabular parallel 

 to the brachypinacoid ;ui<l usually twinned parallel to the 



