496 MINERALOGY 



B.B. Intumesces and fuses at 2.5 to a blebby glass, yielding a 

 lithium flame, especially when mixed with the potassium bisul- 

 phate flux. Only slightly attacked by acids. 



General description. Crystals are small tabular or scaly ag- 

 gregates or massive. Chemically it approaches the formula of a 

 metasilicate, or it may be a mixture of the ortho- and trisilicate as 

 the formula given would indicate, in which R' represents lithium 

 and small amounts of rubidium and caesium; as with all micas, 

 fluorine is present. 



Polylithionite is a lepidolite from Greenland, in which the silica 

 is in the proportion of a metasilicate. Zinnwaldite is a lithia mica, 

 found in the tin veins of Bohemia, containing considerable iron. 



Lepidolite occurs in pegmatites or granitic veins associated with 

 tourmaline, spodumene, amblygonite, beryl, cassiterite, feldspars, 

 biotite, and quartz, often intergrown with the biotite forming the 

 margins of the plates. 



In the United States it occurs at Hebron and Paris, Maine, and 

 at various points in Massachusetts and Connecticut ; at the rubel- 

 lite locality in San Diego County, California. 



In weathering it hydrates and probably forms cookeite, which is 

 associated with it at Paris and Hebron, Maine, and at Chester- 

 field, Massachusetts. 



CLINTONITE GROUP 



The clintonites or brittle micas are foliated micaceous minerals 

 in which the alkalies of the true micas are wanting and the mag- 

 nesia is in large part replaced by iron and calcium ; manganese may 

 also enter the molecule. They are all basic and more complex in 

 their nature and chemical composition. Their formula may be 

 expressed R 3 R"R'"O 2 (SiO 4 ) or R 3 R"R'"0 2 (Si 3 O 8 ) or a mixture of 

 the two, in which R'" is either aluminium or ferric iron, R" is Fe, 

 Ca, Mn, or Mg, and R' is H, OH, or F. They are all easily altered 

 by hydration, and it is difficult to determine whether the specimen 

 represents the true and unaltered composition or not, another 

 difficulty in the way of expressing a formula. 



Margarite, H 2 CaAl 4 Si 2 Oi2, a calcium mica, associated with 

 corundum, from which it is often derived as a secondary product. 

 It has a hardness of 3.4-4.5, G. = 2.99 ; Color, pink, gray, or yel- 

 low ; Luster, pearly. 



B.B. Fuses on the thin edges, yields water in the closed tube. 

 Crystals are rare. It is found associated with staurolite and 



