332 



MINERALOGY 



FIG. 409. Fluorite Twins, showing the 

 . Hexoctahedron. Weardale, England. 



Combinations of the cube, octahedron, and rhombic dodecahedron 

 occur at St. Gothard, Switzerland; at Redruth in Cornwall, 

 England; while cubes with the corners modified by the hex- 

 octahedron (421) occur at Alston Moor and Weardale, England. 



Fluorite presents a good example of a perfect octahedral cleavage ; 

 with care, perfect octahedral cleavage pieces may be split out, but 



as there are four cleavage di- 

 rections the ultimate particles 

 are tetrahedrons. In color, 

 fluorite is the most variable 

 of minerals; commonly it is 

 white, yellow, purple, or green. 

 The green specimens, termed 

 chlorophane, and occurring 

 at Trumbull, Conn., yield 

 phosphorescent light when 

 heated. The phosphorescence 

 is, however, not confined to 

 the green varieties. The 

 various colors have been at- 

 tributed to organic matter, 

 as many specimens, on heating, lose their color, and particularly 

 if heated in oil. Transparent white specimens are very valuable, 

 as, owing to the low index of refraction of fluorite, it is used 

 in the construction of physical apparatus. Some colored crystals 

 exhibit a slight birefringency caused by laminae parallel to the 

 cube or octahedral faces; colorless specimens do not show this 

 phenomenon. 



Fluorite occurs more often as a vein mineral associated with 

 sulphides or sulphates, as galena, sphalerite, and barite, and as a 

 gangue in many metalliferous deposits. It is also associated with 

 the deposits of cassiterite, for the formation of which, as a pneu- 

 matolytic agent, it is in large part responsible. Veins containing 

 fluorite may be contained in both acid and basic igneous rocks as 

 well as in the schists and sedimentary formations ; it is of common 

 occurrence lining the cavities of limestones, where it has been de- 

 posited from solution. 



Less frequently it is found as a primary mineral in granites, 

 syenites, and quartz porphyry ; in which occurrences the escape of 

 the volatile fluorides must have been prevented by the superim- 

 posed formations, as determined in the syenites of Norway. Fluor- 



