418 



MINERALOGY 



stable above 500, and the orthorhombic form the stable form 

 below that temperature. 



Leucite is a constituent of igneous rocks, especially those ter- 

 tiary or recent lavas rich in potassium and aluminium in which the 

 silica is insufficient to produce feldspars. It is associated with augite, 

 haiiynite, nepheline, olivine, apatite, and magnetite. 



In rock sections the crystals appear in polygonal or nearly circular 

 outlines, colorless and with a very low relief. Inclusions are very 

 common and symmetrically arranged in concentric belts or radiated 

 starlike. These inclusions may be glass or fine crystals of those 

 minerals which have preceded the leucite in crystallization, as 

 magnetite, apatite, or augite. Between crossed nicols the twinning 

 lamellae will appear in large crystals, showing an interference color 



, 



X#-V.i" 

 "* 



FIG. 468. Outline of a Leucite Crystal in a Section of Porphyry. 



of a very low gray of the first order, while the small rounded graii 

 may appear as isotropic. 



Leucite is easily decomposed, yielding its potassium to percolat- 

 ing waters, which may at the same time replace the potassium with 

 sodium; this replacement and hydration yields analcite, which 

 crystallizes in the same form. The leucites of the older rocks have 



