GUIDE TO THE MIXERALOGIC COLLECTIONS 



L07 



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 short and 

 thick as in fig. 230 or elongated with vertical striations, bur 



Fig. 230 Fig. 231 



Tourmalin 



always presenting a somewhat triangular cross section (fig. 231). 

 Where doubly terminated the crystals show different modifica- 

 tions on the two extremities. Parallel or radiated crystal aggre- 

 gates are common as well as columnar and compact masses 

 (pi. 32 2 ). 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, gneiss, 

 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, a pink to red variety, is found in lepidolite. In New 

 York tourmalin is found in handsome specimens in St Lawrence 

 county; at Gouverneur and Pierrepont; also in Essex, Orange 

 and New York counties. 



Transparent varieties are sometimes cut as gems or for use in 

 certain optical apparatus. 



Staurolite HFeALSioO^ 



Staurolite is a basic iron and aluminium silicate with magnes- 

 ium (and sometimes manganese) replacing part of the ferrous 

 iron. 



