488 MINERALOGY 



B.B. Intumesces and fuses at 2 to a white enamel ; yields 

 water in the closed tube. Gelatinizes with HC1 ; the solution freed 

 of silica and aluminium yields a white precipitate with ammonium 

 carbonate (calcium). 



General description. Simple individual crystals are rare, 

 usually elongated and joined with their vertical axis parallel or in 

 radiated and divergent groups, resembling pectolite and natrolite. 

 The free ends of the long acicular crystals are usually terminated 

 by the base and not by the pyramid, as in natrolite. It also occurs 

 nodular, concretionary, or massive ; comptonite is a massive variety 

 filling amygdaloid cavities of the lavas on Monte Somma, Vesu- 

 vius. A radiated variety found filling cavities in a rock at Grand 

 Marais, Lake Superior, is known as lintonite. It is often polished 

 and used for buttons and small ornaments, showing concentric 

 rings of green, red, and white. Much of this material is gathered 

 on the beach of Lake Superior, as water- worn pebbles. 



Thomsonite occurs associated with other zeolites at the various 

 points in Nova Scotia ; at Kaden, Bohemia ; Kilpatrick, Scotland ; 

 Schneeberg, Saxony. It is rare at the zeolite localities in New Jer- 

 sey, but recently identified at Paterson. It is associated with other 

 zeolites at Magnet Cove, Arkansas, and Table Mountain, near 

 Golden, Colorado. 



MICAS 



Muscovite H 2 KAl 3 (Si0 4 )3. 



Paragonite H 2 NaAl 3 (Si0 4 )3. 



Biotite HK(Mg . Fe) 2 Al 2 (Si0 4 ) 3 . 



Phlogopite . H 2 KMg 3 Al(SiO 4 ) 3 . 



Lepiddomelane A phlogopite rich in iron. 



Lepidolite H 2 KAl 3 (Si0 4 ) 3 + R' 3 AlF 2 Si 3 8 . 



Zinnwaldite Is a lepidolite with magnesia and 



iron. 



The above list includes the most important species of micas, but 

 between these are varieties representing substitution products and 

 mixtures of the two molecules of ortho- and tri-silicates, in various 

 proportions, as their compositions vary considerably. 



Chemically the micas are mostly orthosilicates, substitution 

 products derived from the general formula Al 4 (Si0 4 ) 3 , in which the 

 substitution replaces one or more of the aluminium atoms. In mus- 

 covite one of the Al atoms is replaced by H 2 K, yielding the formula 



