272 DESCRIPTIONS OF MINERALS. 



especially at Vesuvius, where crystals occur from the size of 

 a pin's head to a diameter of an inch. Also found in the 

 Leucite Hills, northwest of Point of Kocks, Wyoming Ter- 

 ritory. 



The name leucite is from the Greek leukos, white. 



FELDSPAR GROUP. 

 I. Relations of the Species of Feldspar. 



The species of the Feldspar Group are related — 



A. In crystallization : (1) the forms being all oblique ; 

 (2) the angle of the fundamental rhombic prism i, in each, 

 nearly 120° ; (3) the other angles differing but little, al- 

 though part of the species are monoclinic and part tri- 

 clinic ; and (4) there being two directions of easy cleavage, 

 one, the most perfect, parallel to the basal plane 0, and the 

 other parallel to the shorter diagonal section, with the in- 

 tervening angle either 90° (as in the monoclinic species or- 

 thoclaseand hyalophane), or nearly 90° (as in the triclinic 

 species). 



B. In composition : (1) the only element in the sesquiox- 

 ide state being aluminum, and those in the protoxide state 

 either calcium, barium, sodium, or potassium, or two or 

 three of these bases together; (2) the ratio of 1 atom of R 

 to 1 of R being constant ; (3) the amount of silica in the 

 species increasing with the proportion of alkali, being that 

 of a unisilicate in the pure lime-feldspar, anorthite, that 

 of a tersilicate in the soda-feldspar, albite, or potash-feldspar, 

 orthoclase, and so directly proportioned to the alkali, that 

 the amount in any lime-and-soda feldspar may be deduced 

 by taking the lime (or calcium) as existing in the state of a 

 unisilicate, and the soda in that of a tersilicate, and adding 

 the two together. 



Anorthite has the formula CaAl 8 Si 2 . 

 Albite " " Na,A10 16 Si 6 . 



The constitution of a species containing Ca and Na 2 in 

 the ratio of 1 to 1 for the protoxide portion may be ob- 



