90 NEW YORK STATE MUSEUM 



same plane (albite law) or with the macro axis as the twinning 

 axis (pericline law). It is common in pure white granular 

 masses or in aggregations of straight or curved 

 laminae. The luster is vitreous or pearly and 

 the prevailing color white or less commonly bluish, 

 gray, red or green of light tints; an opalescence 

 or play of color is not uncommon on the cleavage 

 surface. 

 Albite is frequently found in cavities and seams 



Fi?r. 206 



Albite iu acidic rocks and is frequently a matrix for such 



minerals as tourmalin, beryl, chrysoberyl, topaz etc. Interest- 

 ing crystals of albite are found at Moriah, Essex co. N. Y. 



Oligoclase (soda lime feldspar) 



Oligoclase is a silicate of aluminium, sodium and calcium. 



It does not often occur crystallized. Cleavable masses, are 

 characterized by the fine striations, common to the plagioclases, 

 but particularly well developed in this species. The luster is 

 vitreous to pearly and the color whitish with faint tints of 

 gray, green, red or yellow. 



It occurs with orthoclase and albite in granitoid rocks and 

 in rocks of volcanic origin. Interesting crystals are found in 

 St Lawrence county , N. Y. 



Labradorite (lime soda feldspar) 



Labradorite is a silicate of aluminium, sodium and calcium. 

 It is rarely found in small triclinic crystals but is commonly 

 met with in dark gray cleavable masses which often display a 

 remarkable change of color as the light is reflected from a 

 cleavage surface. The luster is vitreous to pearly and the color 

 in general darker than that of the other plagioclases. 



Labradorite is usually associated with pyroxene and aniphi- 

 bole in many basic rocks. It is the chief feldspar found in the 

 Adirondack region of New York. 



Anorthite (lime feldspar) 



Anorthite is a silicate of aluminium and calcium. The tri- 

 clinic crystals of anorthite are usually prismatic in habit, 

 twinned, as with albite, and colorless, white or reddish yellow. 



