MICA 241 



peroxide of iron, 6'04; potash, 8'22 ; magnesia, with oxido of manganese, 2'11; 

 fluoric acid, 1*09; water, 6-98. From an American authority we quote as follows: 



'The increased demand for gas-stoves has naturally introduced to a wider notice the 

 really valuable and useful mineral known under the name of mica. Every schoolboy 

 can pick out the mica in a piece of granite, but it is not so well known that the ' glass ' 

 forming the front of many kinds of gas-stoves is also mica. The minerals which form 

 the group of micas divide readily into two divisions; those which are silicates of 

 alumina and an alkali, and those which are silicates of magnesia. They are all 

 notable for their lustre, and for their distinct cleavage, which permits of their being 

 separated into thin sheets. In granite, the plates are rarely seen of a useful size, 

 although in the coarser descriptions of that rock plates are occasionally found a foot 

 and more in width ; but, in limestone formations, it is often found in masses of con- 

 siderable size, plates having been met with in Siberia several feet in diameter. The 

 micas chiefly met with in commerce are Muscovite and Lepidolite (or lithia mica) of 

 the first division, and Phlogopite (rhombic mica) and Biotite of the latter division, or 

 magnesian micas. Of these, the most extensively used in the arts are Muscovite and 

 Phlogopite. The former is mainly a silicate of alumina and potash, with traces of 

 iron, fluorine, chromium, &c., which impart colour to the otherwise grey or silver- 

 white plates of mica. The crystals of Muscovite are usually six-sided ; the colour 

 varying from black through grey to green, chromium being invariably present in the 

 crystal of the last-mentioned tint. This variety of mica is proof against acids, is very 

 refractory, the thin edge only fusing before an ordinary blowpipe, while the laminae 

 are very tough and flexible. Phlogopite, or rhombic mica, as it is sometimes called, 

 is mostly found in limestone, and is composed mainly of silica, alumina, and mag- 

 nesia, with traces of iron, potash or soda, and fluorine. Its colour varies from brown, 

 through brownish yellow, to grey. If it is previously reduced to fine powder, it is 

 attacked by hot sulphuric acid, but, lil Muscovite, although it whitens in the blow- 

 pipe-flame and fuses on the thin edges, it is virtually refractory to anything short of 

 an intense heat. These extraordinary properties, combined with toughness and 

 elasticity, and the peculiar facility with which it splits into thin sheets, some of which 

 approach closely to transparency, led naturally to the use of mica for windows, and 

 especially to its employment in lanterns. For many years it has been used in Russia 

 for windows, and in some parts is still to be found, though it is of course rapidly 

 giving way to the more transparent glass. So common, however, was its employment 

 for this purpose at one time in Russia, that it was frequently called ' Muscovy glass.' 

 It is found in Siberia, Sweden, and Moravia, which also supply the Lepidolite, or 

 lithia mica. In America it is found in various parts, as North Carolina, New Jersey, 

 and Canada. In some coarse granite rocks of the first-named State the mica is found 

 in considerable abundance, and there are unmistakable evidences that it was worked 

 many years ago. The commercial value of mica varies through a wide scale : the 

 large, sound, and clear sheets being naturally the highest priced, fetching as much as 

 40s. a pound. In the United States, where large quantities are used for what is called 

 ' stove glass,' that is, for the fronts of gas and other stoves, the utilisation of mica 

 has been carried further than in this country. The small and waste-stuff is there 

 made into a coarse powder and sprinkled over tar in roof-making ; finely ground, it is 

 used as a lubricant, and is sometimes used in packing deed-boxes and safes to render 

 them fireproof. The finer sheets are used for such purposes as the dials of compasses, 

 for the letters of fancy signs, and the very finest and thinnest pieces are sometimes 

 employed in lieu of enamel for covering photographs ; but one of the principal uses to 

 which the better qualities are put is the construction of shades for lamps, the nature 

 of the material rendering its decoration a comparatively easy process : chromo- 

 lithography being extensively employed in this manufacture. The preparation of the 

 mica is very easy. When first obtained it is in plates and crystals of various sizes, 

 from a quarter of an inch to even occasionally a foot in thickness, and from six inches 

 to a foot and upwards in diameter. 



' The facility with which they can be bent into various shapes, and the power they 

 possess of resisting heat, together with their transparency and naturally beautiful 

 appearance, make these thin sheets of mica of peculiar value in many situations. 

 They have been used as reflectors for some years, and a patent has recently been ob- 

 tained for an improved process of silvering plates of mica, the inventor being a 

 resident of Philadelphia. The flexible nature of the material to be silvered will 

 probably insure its introduction into the arts and industries to a greater or lesser 

 extent, and it is perhaps not impossible that it may be employed in the manufacture 

 of telescopes, although it must be confessed that its utilisation for that purpose is 

 rather improbable. In the process recently patented, sheets of mica, as free from 

 metallic deposit as can be obtained, are thoroughly washed in nitric acid, and then 

 rinsed in water. They are then placed upright in suitable vats or baths, being 



VOL. III. E 



