GUIDE TO THE MINBRALOGIl COLLECTIONS 



KIT 



Tourmalin (schorl) 



Tourmalin is a complex silicate of boron and aluminium with 

 appreciable amounts of either magnesium, iron or the alkali 

 metals. 



It crystallizes in the rhombohedral-hemimorphic class of the 

 hexagonal system in prismatic crystals, sometimes shori and 

 thick as in fig. 230 or elongated with vertical striatums. bu1 



Fig. 230 



Tourmalin 





always presenting a somewhal triangular cross section (fig. L':;i!. 

 Where doubly terminated the crystals show different modifica- 

 tions on the two extremil ies. Parallel or radiated crystal aggie- 

 gates are common as well as columnar and compact masses 

 (pi. 32,). The luster is vitreous to resinous; the color is com 

 monly black, brown or bluish, also blue, green, pink, or red. 

 rarely colorless or white. Some varieties are composed of an 

 internal core of red surrounded by a layer of green, others are 

 differently colored at the opposite extremities. 



Tourmalin occurs in crystalline rocks such as granite, ^m-iss. 

 mica schist, crystalline limestone, etc. The brown variety is 

 generally found in granular limestone and dolomite; a bluish 

 black kind is often associated with the tin ores; black *tour 

 malins are common in quartz, granite gneiss and mica schist; 

 rubellite, ;i pink to red variety, is found in lepidolite. In New 

 Sorb tourmalin is found in handsome specimens in St Lawrence 

 county; a1 G-ouvernenr ami Pierrepont; also in Essex, Orange 

 and New York counties. 



Transparent varieties are sometimes cn1 as gems or for use in 

 certain opt ical apparatus. 



Staurolite HFeAl.Si.O, 



Staurolite is ;i basic iron ami aluminium silicate with magnes 

 ium land sometimes manganese) replacing pari of the ferrons 

 iron. 



