460 



MINERALOGY 



FIG. 491." Andalusite from Lancaster, Massa- 

 chusetts. 



such combinations are found at Lisens Alp, Tyrol, embedded in a 

 chloritic schist. Transparent crystals which display very strong 



dichroism are obtained 

 in Minas Geraes, Bra- 

 zil, and are cut as gem 

 stones showing green 

 when viewed along 

 one direction and red 

 when viewed in the 

 other. 



Many specimens of 

 andalusite contain or- 

 ganic inclusions ar- 

 ranged symmetrically, 

 the outline or cross 

 section of which varies 

 with the position in 

 the crystal. Like the 

 symmetrical inclu- 

 sions in leucite, this 

 arrangement is prob- 

 ably due to a skeletal development during the growth of the 

 crystal ; such inclusions are especially characteristic of the variety 

 known as chiastolite, found in argillaceous schists and clay slates, 

 the crystals of which are slender, prismatic, almost acicular. 



Andalusite is trimorphic with sillimanite and cyanite, all being of 

 the same percentage composition chemically, but differing in their 

 physical and crystallographical properties. Of the three, silli- 

 manite is the most stable at high temperatures, as both andalusite 

 and cyanite when heated to 1400 C. pass over to sillimanite on 

 cooling. 



In rock sections andalusite appears in almost square or in elon- 

 gated outlines. Relief is marked, and the pleochroism shows only 

 in the colored varieties. Inclusions are symmetrically arranged. 

 Interference colors are yellows of the first order. 



Andalusite is the result of metamorphism and is developed in 

 some gneisses and schists, where it is associated with sillimanite, 

 cyanite, iolite, garnets, corundum, and tourmalines. Specimens 

 with typical inclusions are found at Lancaster, Massachusetts, and 

 Rochester, New Hampshire. It is a common mineral at numerous 

 points in New England. 



