414 MINERALOGY 



lief. The interference color is first order yellow, and extinction is 

 inclined as indicated in the table on page 404. The interference 

 figure is yielded by a basal cleavage fragment, showing the emer- 

 gency of an optic axis in the plus octant, behind, and near the edge 

 of the field, with the plane of the optic axis running down to the right. 

 Anorthite is a constituent of the more basic igneous rocks, espe- 

 cially gabbros, basalts, and porphyrites, and it also appears in crys- 

 talline limestones. It is easily decomposed but less often alters to 

 kaolin, forming rather zeolites and quartz. 



Artificial crystals are easily obtained by the simple fusion of its 

 constituent oxides in the right proportion, in an open crucible. 

 Anorthite standing at the basic end of the series easily forms 

 crystals in the simple fusion; this property is lost, however, as 

 albite or orthoclase is approached at the acid end of the series. 



Celsian, BaAl 2 Si 2 8 , is a rare barium feldspar found at Jakobs- 

 berg in Sweden ; and hyalophane is a potassium barium feldspar, 

 K 2 BaAl4Si 8 O 2 4, found in a crystalline dolomite at Binnenthal, 

 Switzerland. 



SODA-LIME FELDSPARS 



The two' feldspars albite (ab) and anorthite (an) are found in 

 nature mixed in crystals in all proportions, and since they are solid 

 solutions and not double salts, there are no theoretical compositions 

 for members of the series. However, mixtures within indicated 

 limits have received distinctive names and form a complex series 

 grading from pure albite at one end to pure anorthite at the other. 



In crystalline habit they are all much alike, having two good 

 cleavages at approximately 86. They all form twins after the 

 ordinary laws, but polysynthetic twinning after the albite law with 

 striations or bands on the basal cleavage parallel to the edge c/b is 

 particularly characteristic. The variation and trend of the optical 

 properties with the change in the composition is indicated and may 

 be seen by an inspection of the table on page 404. 



The most important members of the series, with their composition 

 expressed in terms of albite (ab) and anorthite (an), are as follows: 



Albite ab + ano to ab 6 + an 



Oligoclase ab 6 + ani to ab 3 + an 



Andesine ab 3 + an to ab + an 



Labradorite ab + an to ab + an 3 



Bytownite ab + an 3 to ab + an 6 



Anorthite ab + ang to ab + an 



