- Gen. 4. Silex.
- Mica, Muscovy Talc. Kirwan, v. 1. 210.
- Mica. Haüy, v. 3. 208.
- Mica membranacea. M. laminosa, &c. Linn. ed. 13. t. 3. 58.
- Glimmer of the Germans.
Mica, μιχχὸς, or μιχρὸς, has been long noticed as a glittering substance, and often serves to countenance the idea that our streets are paved with gold and silver. It is one of the first things that takes the attention when fragments of the granite stones from Aberdeen are lying in our streets for paving or building, which has been frequent since the improvement of forming such durable stones into shape for that purpose; an improvement not much above half a century old. It soon becomes as it were familiar to the Mineralogist by its frequent occurrence; yet there is some confusion with regard to its identity, as it has been confounded with Talc: see tab. 182, 183, and 184.
The present specimen has most of the characters belonging to this very curious substance; it has signs of the integrant molecule, a triëdral prism, whose base is equilateral, and of the rhomboidal prism formed of two of these its primitive, with various lengthened rhomboids, hexagons, &c.: see figures. It sometimes has the laminæ bent. Its lustre is of the changeable kind; the upper plates showing the accumulated refracted silvery opacity between those beneath, each plate being transparent if separated*. It is flexible and elastic; the plates, when bent, will return to their places with a considerable spring (see bottom figure, a plate bent thus much will return—the straight line seen through shows its transparency): this elasticity, and its being destitute of unctuosity, distinguish it from Talc, as hitherto the varieties of one species have been placed among those of the other by several mineralogists.
Muscovy Talc of the older authors is undoubtedly Mica, so named in contradiction to Venetian Talc; which, although nearly allied, is yet a distinct species, and may be looked on as a good sample of Talc.
- * Its fine lustre we cannot imitate without hazard of the colour changing; we therefore must request our friends to consider the shining parts of the plate as a fine silvery or pearly lustre.