MINERALOGY. 



Hhomboi- 

 dal mica. 



Oryctogno- amygdaloid, which is occasionally slaty, and contains 

 y. agates. 



Sl > "Y"' Uses. The finer kinds are first boiled in oil, and 

 then cut into tables or pencils : the coarser parts, and 

 the refuse of the sawings, are melted with sulphur, and 

 then cast into coarse pencils for carpenters ; they are 

 easily distinguished by their sulphureous smell. It is 

 also used for brightening and preserving grates and 

 ovens from rust ; and, on account of its greasy quality, 

 for diminishing the friction in machines. Crucibleg 

 are made with it, which resist great degrees of heat, 

 and have more tenacity and expansibility than those 

 manufactured with the usual clay mixtures. 



MICA. GENUS VII. MICA. 



Talk-glimmer, Mohft. 



One axis. Cleavage rhomboidal. Streak green, 

 white. Hardness=1.0 1.5. Sp. gravity =: 2.7 3.0. 



This genus contains one species, viz. Rhomboidal 

 Mica. 



1. RHOMBOIDAL MICA, Jameson. Rhomboedrischer 

 Talk-glimmer, Mohs. 



Rhomboid unknown. Cleavage parallel with the 

 terminal planes of the regular six-sided prism. 



This species is subdivided into ten subspecies, viz. 

 I. Mica; 2. Finite; 3. Lepidolite; 4. Chlorite; />. Green 

 Earth ; 6. Talc ; 7. Nacrite ; 8. Potstone ; 9. Steatite ; 

 10. Figure-stone. * Clay-slate, Whet-slate, Black Chalk, 

 Alum-slate. 



FIRST SUBSPECIES. 

 MICA*, Jameson. Glimmer, Werner. 



Mica. External Characters. Its most common colours are 



grey, brown, black, and white, and very rarely peach- 

 blossom red. It occurs massive, and disseminated; 

 also in distinct concretions, which are large, coarse, 

 and small granular, and wedge-shaped prismatic. The 

 following are the secondary forms : 



f. Equiangular six-sided prism. 2. Equiangular 

 Eix-sided table. 3. Equiangular six-sided table, trun- 

 cated on four of the terminal edges. 4. Equiangular 

 six-sided table, bevelled on ths terminal planes, and 

 the edges of the bevelment truncated. 5. Rectangular 

 four-sided table. 6. Rectangular four-sided prism. 



Crystals middle-sized and small, seldom large. 

 Tables generally adhere by their terminal planes, sel- 

 dom by their lateral planes, and form druses. Some- 

 times arranged in rows, rarely in the rose-form, and 

 seldom intersecting each other. Lateral planes of the 

 tables, and the terminal planes of the prism, smooth 

 and resplendent ; terminal planes of the table longitu- 

 dinally streaked, lateral planes of the prism transverse- 

 ly streaked. Internally generally resplendent, seldom 

 shining, generally pearly, sometimes semimctallic, and 

 in the silver-white variety passing into metallic. Frac- 

 ture not discernible. Fragments tabular and splintery. 

 Translucent or transparent in thin plates, but rarely in 

 crystals of considerable thickness or length. Sectile. 

 Affords a grey-coloured dull streak. Feels fine and 

 meagre, smooth. Elastic-flexible. 



Chemical Characters. Before the blowpipe, it melts 

 into a greyish-white enamel. 



Black M tea from Siberia. 

 42.50 

 11.50 

 22.00 



sy. 



Constituent Parts. Silica, 



Alumim. 

 Oxide of Iron, 

 Oxide of Manga. 



nese, . 2.00 



Potash, . ]().00 



Magnesia, 9.00 



Loss by heating, 1.00 98.00 

 Klaprolh, E. v. s. 78. 



Gerignoslic Situation. This mineral occurs as an es- 

 sential constituent part of granite, gneiss, and mica- 

 slate, and is accidentally intermixed with other rock?, 

 both of the primitive, transition, secondary or flcetz, 

 and alluvial classes. 



Geographic Situation. The rocks in which mica oc- 

 curs, are so universally distributed, that it is not neces- 

 sary to enter into any detail of loc-ilities. 



Uses In Siberia, where window-glass is scarce, it 

 is med for windows ; also for a similar purpose in Peru, 

 and, I believe, also in New Spain, as it appears that 

 the mineral named Teculi by Ulloa, and which is used 

 for that purpose, is a variety of mica. It is also used 

 in lanterns, in place of glass, as it resists the alterna- 

 tions of heat and cold better than that substance. In 

 Russia, it is employed in different kinds of inlaid work. 

 It is sometimes intermixed with the glaze in particular 

 kinds of earthen- ware ; the heat which melts the glaze 

 has no effect on the mica; hence it appears dispersed 

 throughout the glaze, like plates or scales of silver or 

 gold, and thus gives to the surface of the ware a very 

 agreeable appearance. Some artists use it in the mak- 

 ing of artificial aventurines. 



SECOND SUBSPECIES. 



FINITE, Jameson. Finit, Werner. Micarelle, Kir- Finite. 

 wan. 



External Chnracfera.-^Colaur blackish-green, altered 

 on the surface by brown or red iron-ochre into brown- 

 ish-red. Sometimes iron-shot. Occurs massive, also 

 in distinct concretions, which are thick and thin lamel- 

 lar, collected into large and coarse granular, and crys- 

 tallized in the following figures: 



1. Equiangular six-sided prism. 2. The preceding 

 figure truncated or bevelled on all the lateral edges. 

 Owing to the number of planes, figures of this des- 

 cription have a cylindrical form. The terminal angles 

 are sometimes truncated. 



Crystals seldom middle-sized, generally small. They 

 are imbedded, and frequently intersect each other. 

 Cleavage shining; fracture glistening and glimmering, 

 lustre resinous. Fracture small-grained uneven. Frag- 

 ments blunt-angular, seldom tabular Opaque; faint- 

 ly translucent on the edges. Sectile. Easily frangible. 

 Not flexible. Feels somewhat greasy. 



Chemical Character. It is infusible before the blow- 

 pipe. 



Constituent Paris. Silica, 



Alumina-, 63.75 



Oxide of Iron 6.75100.00 



Klaprolh, Jour, des Minrs, N. 100. p. 311. 



Gengnostic and Geographic Silualions.lt is found 



imbedded in the granite of St. Michael's Mount in 



Cornwall; in porphyry in Ben Gloe and Blair-Gow- 



* Mica, from the Latin word mica, to sl.ine, given to it on account of its lustre. 



