112 NEW YORK STATE MUSEUM 



Plilogopite (amber mica) (H,K,MgF) 3 Mg,Al(Si0 4 ) 3 



Plilogopite is a potassium, magnesium and aluminium fluo- 

 silicate. 



The crystals of plilogopite are similar to those of muscovite 

 and biotite but are often developed into rather longer prismatic 

 forms usually tapering slightly at either end. The color is com- 

 monly yellowish brown to brownish red, frequently a metallic 

 copper-red on the cleavage surfaces. 



It is specially characteristic of crystalline limestone and is 

 also found in serpentine. Localities are numerous throughout 

 New York, particularly in Jefferson and St Lawrence counties, 

 and in New Jersey. ^ 



Plilogopite is used largely as an insulating material in electric 

 work. 



Clinochlore (ripidolite) H s Mg 5 ALSi 3 ls 



Clinochlore is a basic magnesium and aluminium silicate. 



It occurs in monoclinic crystals closely approximating hex- 

 agonal forms in prism angle. The crystals cleave easily into 

 thin, inelastic plates resembling those of the micas. It is also 

 found in masses of coarse or fine scales and in an earthy variety. 

 In color clinochlore is commonly some shade of green, more 

 rarely yellowish, white or rose-red. 



It is frequently found in chlorite and talcose rocks and in 

 serpentine. Clinochlore partly altered to serpentine occurs 

 with magnetite at Brewster, Putnam co. N. Y. 



Prochlorite H 40 (FeMg) 23 Al 14 Si 13 90 



Prochlorite is a basic magnesium and aluminium silicate with 

 some iron and a lower percentage of silica than clinochlore. 



It is monoclinic, the crystals are commonly small with 

 strongly furrowed prismatic faces and often curiously twisted 

 into wormlike shapes. It is also found in foliated and granular 

 masses. The color varies from grass-green to blackish green 

 and the luster from feebly pearly to dull. 



It frequently results from the decomposition of mica, amphi- 

 bole, garnet, pyroxene, etc. Its association and occurrence are 

 similar to clinochlore. 



