476 



MINERALOGY 



FIG. 499. Tourmaline. Jefferson 

 County, New York. 



with a mixture of flowers of sulphur and powdered red lead, the 

 positively charged end of the crystal becomes yellow as it attracts 



the sulphur, and the negatively 

 charged end becomes red as it at- 

 tracts the red lead. 



In rock sections it appears elon- 

 gated, branched, and needle-like; 

 when cut perpendicular to the verti- 

 cal axis, roughly triangular. In 

 color, white and pale shades, to 

 brown and green, especially in rock- 

 forrning tourmalines. Relief is well 

 marked and absorption very strong, 

 particularly in the dark varieties. 

 Interference colors, low second or- 

 der ; the basal section shows a dark 

 cross only, in thin sections. Opti- 

 cally ( - ). 



The black tourmaline occurs as a primary component in some 

 granites ; other tourmalines are mostly formed by fumarole action, 

 caused by the contact of igneous intrusions with limestones or 

 sedimentary rocks. Boron is the commonest of the pneumatolytic 

 agents. Tourmaline is therefore a common mineral in pegmatites, 

 crystalline schists, and granular limestones, and is usually associated 

 with cassiterite, topaz, fluorite, beryl, lepidolite, the various micas, 

 and quartz. 



In the United States the noted localities for varicolored specimens 

 are Haddam, Connecticut; Paris, Maine; Mesa Grande, Cali- 

 fornia, where it is found in pegmatites associated with lepidolite, 

 spodumene, and beryl. 



The brown variety occurs at Gouverneur, New York ; Franklin, 

 New Jersey ; Unionville, Pennsylvania, in limestones associated 

 with scapolite, spinels, or tremolite. Also in many of the Maine 

 pegmatites; near San Diego, California, in large black crystals 

 six inches across, in a feldspar. 



Tourmaline alters to chlorite, muscovite, or biotite, with which 

 minerals it is generally associated. 



Synthetically tourmaline has never been produced in the labora- 

 tory. In nature it must be formed at rather a low temperature, or 

 under pressure, as upon fusion it breaks down, forming minerals 

 with less complex molecules, as olivine and spinel. 



