ORDER VI. DODECAHEDRAL KOUPHONE-SPAR. 



Brittle. Hardness = 55 ... 6-0. Sp. Gr. = 2-483, 

 a semi-transparent yellowish-grey variety. 

 Compound Varieties. Massive: composition 



granular ; faces of composition irregularly streaked. 



Hare. 



OBSERVATIONS* 



L The tessular form is given here on the authority of 

 HAUY. According to the observations of Dr BREWSTEB, 

 this species possesses two axes of double refraction, and 

 must therefore be considered as one of those instances in 

 which we have to look forward to future observations, that 

 may enable us to account for this apparent exception to 

 the general law, that tessular forms are connected with 

 that structure which possesses no double refraction. 



2. The trapezoidal Kouphone-spar is composed of 



Its chemical composition is K 3 Si 4 + 6 Al Si 2 , or in 

 words, 20-89 potash, 22 76 alumina, and 56-35 silica. 

 Alone it is infusible before the blowpipe : but it fuses with 

 borax or carbonate of lime, though with difficulty, into a 

 clear globule. Reduced to powder it changes the colour 

 of the blue tincture of violets into green. 



2. This species occurs chiefly in imbedded crystals and 

 grains in lava, sometimes in compound varieties in the 

 specimens ejected by Mount Vesuvius. Besides this lo- 

 cality, it is also found at Albano and Frascati near Rome. 



2. DODECAHEDRAL KOUPHONE-SPAR. 

 Soda'ite. JAM. Syst. Vol. II. p. 52. Dodecahedral Zeo- 



VOL IT. P 



