614 MINERALOGY 



Albite; triclinic ; n = 1.533; -y - a = .010. 



Plates, grains, lath-shaped or irregular; Twinning 

 lamellae very common ; Extinction inclined ; Cleav- 

 age, 001 and 010, usually well developed; Relief 

 very flat ; Interference colors first order gray to yel- 

 low; Optically positive (+) ; p. 411. 



Serpentine; Orthorhombic; n= 1.54; \ a = .013. 



Scales or fibrous aggregates ; A secondary product ; 

 Surface smooth ; Colorless to green ; Interference 

 colors first order gray to orange ; Optically positive 

 (+); P- 498. 



2. Index of refraction greater than that of Canada balsam. 

 a. Not pleochroic. 



Plagioclases. Triclinic; n = 1.53-1. 58 ; < y-a = .010-.013. 



Plates, grains, lath-shaped or irregular in outline; 

 Polysynthetic twinning lamellae very common; 

 Relief flat, surface smooth ; Colorless, sometimes 

 clouded ; Interference colors first order grays to 

 orange; Optically (). For extinction and dis- 

 tinction of the species see p. 411. 



Talc ; n = 1.572 ; -y - a = .050. 



Scaly or fibrous aggregates; A secondary product; 

 Relief marked, surface slightly rough; Colorless; 

 Interference colors bright second or third orders; 

 Optically negative ( ); p. 500. 



Muscovite; Monoclinic ; n = 1.587; Y a = .038. 



Scales, flakes, or shreds ; Cleavage .001 characteristic, 

 and well developed ; Tabular sections show no cleavage 

 and have a very low relief, while the elongated sections 

 showing cleavage have a marked relief and nearly par- 

 allel extinction ; Interference colors bright second order ; 

 Axial angle large; Optically negative ( ); p. 489. 



Sillimanite; Orthorhombic; n = 1.667; \ - a = .022. 



Fibrous or needle-like aggregates, often with a trans- 

 verse parting; Colorless; Relief marked, surface 

 rough ; Parallel extinction ; Interference color bright 

 upper first or lower second orders ; Optically posi- 

 tive (+); p. 461. 



Olivine; Orthorhombic; n = 1.67; "Y a = .035. 



Short prismatic, grains or irregular in outline; Cleav- 

 age 001 and 010 usually well developed; Irregular 



