Mica

“ Chlorite schist.—A greenish, fine-grained metamorphic rock in which chlorite is the principal mineral, but in which magnetite is a quite characteristic accessory constituent.
[466]
Staurolitic garnetiferous mica schist.—A mica schist in which garnet and staurolite are so abundant as to be essential constituents.
Clay slate.—A metamorphosed mud stone or shale. In the process of metamorphism the rock has been hardened, given a slaty cleavage, and innumerable minute scales of mica have developed to produce a silky luster upon the cleavage faces. The color may be gray, green, purple, or black. ”

“ The component parts of mica are silex, alumine, potash, iron, manganese, and traces of other substances. The colors of the mineral are various, according to the proportions of some of the ingredients. The laminæ are divisible into plates no thicker than 1/300,000th part of an inch. Entering into the composition of almost every rock from the oldest to the newest, it abounds chiefly in granite and schist, but also occurs in sandstones, and the slaty shales of the coal formation. ”

“ In the ejected blocks of crystalline limestone of Monte Somma, Vesuvius, the most perfectly developed crystals of biotite (figs. 1, 2) , or indeed of any of the micas, are found in abundance, associated with brilliant crystals of augite, olivine, humite, &c.
Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance. The small size of the sheets, their dark colour and want of transparency render the material of little value. ”

“ The surface of the ground, so dull and forbidding at first sight, besides being rich in plants, shines and sparkles with crystals: mica, hornblende, feldspar, quartz, tourmaline. The radiance in some places is so great as to be fairly dazzling, keen lance rays of every color flashing, sparkling in glorious abundance, joining the plants in their fine, brave beauty-work—every crystal, every flower a window opening into heaven, a mirror reflecting the Creator. ”

“ The mica slate is often remarkable for containing numerous and very large cubical crystals of pyrites, an occurrence much less frequent in this rock than in clay slate.
Veins of quartz traverse it in innumerable places, and it is also interspersed with compressed nodules of quartz, which bear no marks of attrition, but are intimately united to the mica slate, or chlorite slate, in which they occur. They are frequently characterized by a beautiful transparent brown colour, which renders them objects of much research and high price among lapidaries. ”

“ In the phyllites also white mica (muscovite or sericite) is more abundant as a rule than in slate, and its crystalline plates are larger; the abundance of mica gives these rocks a glossy sheen on the smooth planes of f1ssility. Many of the best Welsh slates are rich in small scales of white mica, which polarize brightly between crossed nicols. The Cornish slates are still more micaceous and rather coarser grained, so that they might be called mica-slates or even phyllites. ”

“ The most extensive application of mica at the present day is for electrical purposes. Being a bad conductor of electricity it is of value as an insulator, and the smooth flexible sheets are much used in the construction of armatures of dynamos and in other electrical machinery. For various purposes a manufactured material known as “micanite” or “micanite cloth” is much used; this consists of small sheets of mica cemented with shellac or other insulating cement on cloth or paper.
Muscovite and phlogopite are practically the only species used commercially, the former being the more common. ”

“ Some geologists are of opinion, that the alternate layers of mica and quartz, or mica and felspar, or lime and felspar, are so much more distinct, in certain metamorphic rocks, than the ingredients composing alternate layers in many sedimentary deposits, that the similar particles must be supposed to have exerted a molecular attraction for each other, and to have thus congregated together in layers more distinct in mineral composition than before they were crystallized. ”

“ Ann's Well, I found a rock of a loose coarse-grained texture, with an earthy fracture, composed of mica and hornblende in a state of decomposition, mixed with red felspar. It has a slaty structure, which in some places is more distinct than in others from there being a greater proportion of mica, and its disposition is, within a short space, sometimes vertical, sometimes inclined at a considerable angle, and dipping to different points of the compass; having the appearance of large masses irregularly heaped together. ”

“ And thus, as in other instances, a connexion in structure and habits is established between certain rocks of the primary and of the secondary classes. In all cases the mica is disposed in lamellæ parallel to the strata or laminæ of the rock; it is never as in gneiss partially disposed in different directions, still less as in granite indifferently placed. ”

“ The deposits of the United States are extensive but of low grade.
Mica. —Mica is an essential mineral, especially in electrical work. One of the commonest minerals of nearly all rocks, it becomes valuable only when it occurs in crystals or sheets of large size, which are of comparatively rare occurrence, being found only in certain pegmatite dikes. India, Canada, and the United States produce about 98 per cent. of the sheet mica of the world. Brazil, Argentina, and the former German East Africa are becoming important. India produces 65 per cent. of the total world production ”

“ Between the Worcestershire Beacon and the chasm called the Wych, the rocks, on the eastern side, are similar to those I have already mentioned, except in one instance on the top of the hill, where a rock is found composed of greenish-brown mica, intermixed with hornblende. Although mica is the chief ingredient, this rock has not the slaty structure which most micaceous rocks have, but the laminæ which are pretty large, are irregularly. grouped together, and cross each other in all directions. It is an insulated mass of very small extent. ”

“ A little feldspar is sometimes present in the rock, which thus passes into micaceous gneiss. Specimen 43 contains several crystals of red garnet, giving the variety garnetiferous mica schist. There is no other rock that contains such a large variety of beautiful accessory minerals as mica schist; and for the mineralogist it is one of the most attractive rocks. Few rocks are more distinctly stratified; and the stratification can usually be observed in hand-specimens. The mica in these rocks may be either muscovite or biotite, or both ”

“ Owing to the small quantity of felspar which it contains, and the abundance of the difficulty decomposable mica, the soils formed by its disintegration are generally inferior. Mica slate is also a mixture of quartz, felspar, and mica, but consisting almost entirely of the latter ingredient, and consequently presenting an extreme infertility. The position of the granite, gneiss, and mica slate soils in this country is such that very few of them are of much value; but in warm climates they not unfrequently produce abundant crops of grain. ”

“ When the clay has been derived from the weathering of such rocks, it is easy to understand that it may contain admixtures of quartz and mica, which are frequently visible to the naked eye, or at any rate under the microscope. Whereas clay forms a white, amorphous mass, the grains of quartz sand are decidedly crystalline, transparent and of vitreous lustre; the scales of mica, on the other hand, appearing as thin tabular crystals, mostly of a green or brown colour and exhibiting, when viewed at certain angles, a brilliant metallic sheen. ”

“ Luckily for their arms, this strata was but thin, and the yellow or blue clay which followed was comparatively easy work—here and there an awkward lump of quartz required the use of the pick. Suddenly they came to some glittering particles of yellow, which, with heartfelt delight they hailed as gold. It WAS MICA. Many are at first deceived by it, but it is soon distinguished by its weight, as the mica will blow away with the slightest puff. ”

“ Speaking generally, the Russian micas coming into commerce are potash micas, and mica purchased in England may be taken to be potash mica, especially if it is in large sheets.
At ordinary temperatures "mica" of the kind found in commerce is an excellent insulator. Schultze (Wied. Ann. vol. xxxvi. p. 655) comes to the conclusion that both at high and at low temperatures mica (of all kinds?) is a better insulator than white "mirror glass," the composition of which is not stated. ”

“ This transition is usually effected through the different kinds of slate; the slaty crystallines becoming more and more fine in texture, until at last they appear composed of nothing but very fine mica or chlorite; and this mass of micaceous substance becomes more and more compact and silky in texture, losing its magnesia, and containing more of the earth which forms the substance of clay, until at last it assumes the familiar appearance of roofing-slate, the noblest example of the coherent rocks. ”

“ As a matter of fact, a gneiss may exist in which there is no differentiation into layers; the whole of the constituents have been drawn out and elongated, any mica present becoming naturally conspicuous by its - 156 - flattened wisp-like forms. The banded gneisses, on the other hand, where layer-structure is obvious, consist in reality of bands of different rock-types. Sometimes all the layers are granitoid, but one band will contain only quartz and felspar, while another will contain the same minerals with an admixture, and perhaps a great predominance, of mica. ”

“ I have frequently noticed the transition which takes place between quartz rock and mica slate, a transition so gradual and so perfect that it is often impossible to assign the limits of the two. The quartz is often hyaline, while the mica is disseminated through it in scales of which the tendency, notwithstanding the crystalline state of the quartz, is parallel either to the laminæ or to the beds of the stone. These specimens when polished exhibit the characters of the most perfect avanturine, but their colours are only white or greyish. ”

“ Younger felspars, as well as mica, are developed, and there arise also sillimanite, garnet, andalusite and many others. The texture becomes more coarsely crystalline, and the segregation of the constituent minerals more definite along the lines of foliation. From the finest silky phyllites a graduation may be traced through successively coarser mica-schists, until we reach the almost granitic texture of the coarsest gneisses. ”

“ The folia are curved and highly inclined, generally dipping eastward. It is probable that this mica-schist is an old formation, connected with the granitic rocks and metamorphic schists near the coast; and that the one fragment of mica-slate, and the pebbles of quartz low down in the gypseous formation at Las Amolanas, have been derived from it. The mica-schist is succeeded by stratified porphyritic conglomerate [Q] of great thickness, dipping eastward with a high inclination: I have included this latter mountain-mass in the same anticlinal axis with the porphyritic streams [NN] ”

“ But notice, when the film of mica is thin you have sometimes not only light, but coloured light. Our work for some time to come will consist of the examination of such colours. With this view, I will take a representative crystal, one easily dealt with, because it cleaves with great facility—the crystal gypsum, or selenite, which is crystallized sulphate of lime. Between the crossed Nicols I place a thick plate of this crystal; like the mica, it restores the light, but it produces no colour. ”

“ A [31] large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain masses.
2. Mica slate lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quantity of mica in it is very different in different places, it is often difficult to make the distinction between them. It also passes by insensible degrees into the argillaceous rocks. ”

“ The crystals of feldspar have smooth faces, while quartz breaks with a rough surface as glass does. Feldspar loses its glassy lustre when exposed to the weather, and becomes dull, with the soft lustre of pearl.
Mica may be clear and glassy, and it ranges in colour from transparency through various shades of brown to black. It has the peculiarity of splitting into thin, leaf-like, flexible sheets, so it is easy to find out which particles in a piece of granite are mica. One has only to use one's pocket knife with a little care. Hornblende is a dark mineral which contains considerable iron. ”

“ And since the granite of mica, felspar, and quartz, is sometimes found to contain here and there a solitary crystal of hornblende, with which in a series of gradation those varieties in which hornblende at length prevails to the exclusion of the mica are connected, it is impossible to define the point at which granite is to end and syenite is to begin: there is no difficulty in modifying the application of the adjective term. ”

“ Pure siliceous rocks may be known by not effervescing when a drop of nitric, sulphuric or other acid is applied to them, or by the grains not being readily scratched or broken by ordinary pressure. In nature there is every intermediate gradation, from perfectly loose sand to the hardest sandstone. In MICACEOUS SANDSTONES mica is very abundant; and the thin silvery plates into which that mineral divides are often arranged in layers parallel to the planes of stratification, giving a slaty or laminated texture to the rock. ”