MICA, DAZE, Talc, Muscovy-glass, Glimmer

MICA, DAZE, Talc, Muscovy-glass, Glimmer, or Glist; a genus of magnesian earths, known by the following characters: 1. They consist of thin flexible particles, divisible into plates or leaves, having a shining surface. 2. These leaves or scales, exposed to the fire, lose their flexibility and become brittle, separating afterwards into thinner leaves; but in a quick and strong fire they curl or crumple, which is a step towards fusion; though it is very difficult to reduce them into pure glass without addition. 3. They melt easily with borax, the microcosmic salt, and alkaline salt; and, by means of the two former salts, may be brought to a clear glass before the blow-pipe. That which contains iron, however, is more fusible than the uncoloured earths of this kind. No loose or friable mica has yet been discovered, but all of an indurated kind. Its specific gravity, according to Fabroni, is about 3000. Kirwan tells us, that the specific gravity of this substance, when it contains much iron, is from 2535 to 3000. An hundred parts of the colourless kind contain 38 of silic, 28 of argill, 20 of magnesia, and 14 of the most dephlogisticated calx of iron. Martial mica contains also 10 or 12 per cent. of a more phlogisticated calx of iron; whence its various colours are derived, and a proportionably smaller quantity of the other ingredients. The species are,

1. Mica alba, colourless or pure mica; of which there are the following varieties. 1. Muscovy glass, consisting of large parallel plates, and as transparent as glass, found in Siberia and Sweden. This differs externally from the common talc, in being more soapy to the touch. An hundred parts of it contain 50 of silic, 45 of mild magnesia, and 5 of argill or clay. Venetian talc is white, grey, yellowish, or greenish, and semitransparent. It is much more tender and brittle than mica, and so soft that it may be scratched with the nail. Its specific gravity is 2729. 2. Mica

squamosa, composed of small plates, found in Sweden and other countries of Europe. 3. Composed of fine particles like chaff. 4. Talcum officinale, crumpled mica, composed of crumpled plates.

II. Mica colorata martialis, coloured and martial glimmer. Of this there are many varieties. 1. Brown and semitransparent, found in Lapland. 2. Consisting of fine and minute scales, of a brown, deep-green, light-green, or black colour, found in different parts of Sweden. 3. Twisted or crumpled glimmer, of a light green colour, found also in Sweden. 4. Chaffy glimmer, of a black colour, found in the stone called hornberg, occurring in most of the Swedish copper mines, as at Norberg, Flodberg, &c. 5. Crystallized glimmer, with erect scales, or with hexagonal horizontal plates, found also in Sweden.

Most of these stones are supposed absolutely to resist the fire; but this is to be understood only of certain degrees of heat, and when they are mixed with certain bodies. Cronstedt observes, however, that they may with equal propriety be called vireficient; because they melt with that degree of heat in which neither quartz nor limestone are in the smallest degree altered. They are still more readily melted when either naturally or artificially combined with a martial earth. Hence some ores, though much mixed with mica, may be very readily melted; while others, in which the same substance is mixed with quartz, it may be impossible to melt; because the mica renders the quartz so compact as to prevent it from cracking. It does the same with an apyrous clay, which is the reason why the lapis ollaris resists the fire so strongly.

M. Margraaf asserts, that he has obtained Epsom salt from talc; and Mr Fabroni informs us, that in decomposing nitre by means of a micaceous substance, as soon as the acid is distilled there rises some other substance hitherto unknown at the end of the operation: he adds, that on employing aqua regia or marine acid to dissolve this substance, the yellow colour which results from the solution shows that it contains some iron. This last assertion is confirmed by M. Monnet, who found that phlogisticated alkali and solution of galls produce a bluish colour with that of mica. He adds, that its component parts are the same with those of asbestos, excepting only that the latter contains more iron.

Cronstedt informs us, that the martial mica acquires a shining yellow colour in a calcining heat, which has induced many to examine it in hopes of finding gold; though no metal can be obtained from it except iron, which may be dissolved by means of aqua regia. A late German author indeed has pretended, that he produced from mica an unknown semimetal which resembled iron mixed with zinc. He owned, however, that he made use of a flux composed of several metals, some of which probably united with the talc, and thus deceived him. The talc cubes, which are micaceous bodies of the figure of aluminous crystals, are much valued and sought after by some mineralogists. They are met with in some parts of Sweden; and when broken are found to consist of an iron ore frequently mixed with a marcescent copper ore, and are only covered with a very thin coat of mica. The broad and transparent talc named Muscovy-glass is used instead of

glass for windows; and has this advantage above common glass, that it resists the explosion of cannon. Cronstedt thinks that it might be advantageously used for covering houses. The twisted or crumpled mica, found in some places of Sweden, is manufactured into kettles and other vessels, likewise into hearths for chimneys; "and the powder which falls in the working (says our author) may be mixed with the common salt for the distillation of the muriatic acid."

According to M. Magellan, many mineral substances may have the glittering appearance of talc without really belonging to that genus. An artificial production of this kind he happened to observe in Mr Wedgwood's work. It was an unexpected result from vitriol calcined to redness, then mixed without being washed with common salt. It now underwent a second calcination under a muffle, with a heat somewhat stronger; about the tenth degree of his thermometer. The colour was of a dark purple, and the shining particles so bright as to show their glassy form, very different from true mica.