ALUMINA. 



323 



Fig. 269. 



Fig. 268. of the same village. Crystals resembling the unibi- 



naire of Haiiy, Fig. 268, but having the planes M 

 much narrower than in his figure ; and also those simi- 

 lar to Fig. 269, have been found here by Dr. Horton. 

 M ond 135° ; M on c 115° 15' ; P on c 142° 54'; c 

 on d 127° 6'. 



At Worcester in Massachusetts, idocrase is found, 

 closely resembling the specimens from Eger in Bohe- 

 mia. It is in seams and veins in a quartz rock, and 

 is associated with pyroxene and garnet. 



GARNET. 



[From the French, referring to the colour of pomegranate seeds.] 



Garnet. Cleaveland, Phillips, Shepard and Dana. — Grenat. Haiiy and Beudant, (the latter makes it a sub- 

 genus, including several species usually considered as varieties, viz. Grossulairc, Almandine, Melanite, and 

 Spessartine.) — Dodecahcdral Garnet. Jameson. — Dodecaedrischcr Granat. Mohs. 



r>s- 2"0. Description. Colour red, brown, green, black, grey and yellow. 



It occurs regularly crystallized ; also massive. Primary form a 

 rhombic dodecahedron. Fig. 270. P on P 120°. Cleavage paral- 

 lel with the dodecahedron, but imperfect. Fracture conchoidal, 

 splintery, uneven. Lustre vitreous, inclining to resinous. From 

 transparent to opaque. Hardness from 6.5 to 7.5. Specific gra- 

 vity from 3.35 to 4.24. Before the blowpipe, garnet generally 

 melts, with little or no effervescence, into a dark coloured globule, 

 having a vitreous lustre, and which is often attracted by the magnet. 

 An examination of the crystalline forms will probably afford the best means of distinguishing 

 garnet from the other minerals which it resembles. Hyacinth and leucite differ in being infu- 

 sible ; while idocrase, which it often resembles, easily melts into a shining yellow glass. 



Under the general name of garnet, are now usually included several minerals which, although 

 they differ considerably in some of their characters, are principally composed of the same 

 constituents, but in variable proportions. They agree so far, that all those which are regu- 

 larly crystallized occur in the rhombic dodecahedron and its varieties. It is quite probable, 

 however, that some of these varieties will hereafter be found to possess specific differences. 



Varieties. Common Garnet. The colour of this variety is some shade of brown. It is 

 opaque or only translucent, in which it differs from Precious garnet. It is found crystallized 

 and ill granular masses ; sometimes it has a lamellar structure. 



Precious Garnet — Almandine. Tiiis is usually some shade of red, often beautiful. It is 

 commonly translucent, often transparent. It occurs crystallized, has a conchoidal fracture, 

 and a shining vitreous lustre. 



