110 



NEW YORK STATE MUSEUM 



nucleus and more rarely in granular or compact masses with 

 concentric structure. The luster is vitreous and the crystals 

 are colorless or white, greenish or faintly red in color. 



Analcite is a secondary mineral occurring 

 with other zeolites in the basalt or gneiss at the 

 prominent zeolite localities previously given 

 under apophyllite. 



Natrolite Na 2 Al 2 Si 3 10 +2H 2 



Natrolite is a hydrous sodium and aluminium 

 silicate. v 



It occurs in slender, orthorhombic prisms of nearly square 

 cross section often terminated by a flat pyramid (fig. 234); these 

 are commonly grouped in radiating and 

 interlacing aggregates (pi. 34 2 ). It also occurs 

 in radiating fibrous forms and granular to 

 compact masses. The luster is vitreous; 

 the color is white, greenish or reddish, 

 the crystals are frequently colorless. 



The manner of occurrence, association and 

 localities are the same as for the other 



Zeolites. M . ca divigion Natrolite 



Muscovite (common mica, isinglass) H,KAl 3 (Si0 4 ) 3 



Muscovite is a hydrous potassium and aluminium orthosili- 

 cate. 



The crystals of muscovite are monoclinic, prismatic and tabu- 

 lar in habit, with a rhombic or hexagonal section and cleave with 

 great ease parallel to the base into extremely thin elastic plates. 

 It also occurs in disseminated scales, often grouped in globular 

 (pi. 35J or plumose (pi. 35 2 ) forms. The luster of muscovite is 

 vitreous, pearly on the cleavage planes; the color is commonly 

 gray, brown, green or yellow, sometimes violet or black. 



Muscovite is the most common of the micas and is very widely 

 distributed. It is an essential constituent of mica schist and, 

 to a less degree, of some granite and gneiss. The best developed 

 crystals occur in pegmatite dikes and veins. It is also found in 

 fragmental rocks and limestones but rarely as a secondary 

 mineral in volcanic rocks. 



