74 MINERALOGY. 
13. Mica (compound silicate of alumina). 
The common or bi-axial mica crystallizes according to the monoclinic 
system. A second variety, which is distinguished for its optical properties, 
and which crystallizes according to the hexagonal system (jig. 69), is the 
uni-axial mica. The common bi-axialgnica is a very useful mineral. Its 
crystals are nearly regular rhombic plates, belonging to an oblique rhombic 
prism as the primary form. It exhibits the most perfect cleavage of all 
minerals, being capable of splitting into exceedingly thin leaves. It is used 
for window panes, and covers for microscopic objects, and is very useful in 
the cultivation of microscopic water plants. It occurs, in large leaves, in 
Siberia, and also in New Hampshire; and in general forms a constituent of 
all granite, embracing in its composition, silica, alumina, potassa, manganese, 
and fluoric acid. 
14. Huclase (compound silicate of alumina). : ‘ 
This exceedingly rare mineral occurs in Brazil, crystallized according to 
the monoclinic system (fig. 58). It is transparent#of a vitreous lustre, 
mountain green, passing into white and blue; brittle. 
15. Idocrase (compound silicate of alumina). 
This mineral occurs in very perfect crystals of the dimetric system 
( figs. 53, 48), with a vitreous or resinous lustre, sometimes striated on the 
surface. It is semi-transparent, brown, or of various shades of green. It 
consists of silica, alumina, oxydes of manganese and of iron, and lime. 
[t was first found in the ancient lavas of Vesuvius; more recently, in other 
places. 
16. Garnet (compound silicate of alumina). 
Garnet, of which there are many varieties, presents itself in very perfect 
crystallizations of the regular system. The form shown in pil. 34, figs. 33 
and 64, as also the rhombic dodecahedron ( fig. 61), are especially prevalent. 
The faces of the crystals possess a vitreous lustre, are pearly smooth, or partly 
striated or rough. The colors are exceedingly various, being red, ferruginous, 
brown, yellow, black, white, and green (pl. 35, fig. 6). The transparency 
is either perfect, imperfect, or wanting entirely. The most remarkable 
varieties are the noble, the Bohemian, and the oriental garnet or almandine, 
of a beautiful cherry-red, blood-red, or brownish-red color, occurring in 
crystalline rocks, in gneiss, and chlorite slate. Another kind much 
esteemed is the pyrope, of a blood-red color, occurring in loose grains in 
clay near Bilin in Bohemia. Other varieties are colophonite, grossular, &c. 
17. Beryl (silicate of alumina and glucina). 
The noble beryl, or emerald, is a bluish-green or green gem. The most 
valuable crystals are found in Peru. Those of four grains are worth from 
twenty to twenty-five dollars ; of twenty-four grains, from five hundred and 
fifty to six hundred dollars. The crystallized forms belong to the hexagonal 
system. The most frequent of these are shown in pl. 34, figs. 22, 49, 57, 67, 
68, 69. A less important variety is found in beryls of a green, blue, and yellow 
color, either transparent or opake. Opake beryls are not rare, and occur 
sometimes very perfect in granite, as shown in pl. 35, fig. 15, a sketch 
drawn from nature. 
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