MICA , a
See also:group of widely distributed
See also:rock-forming minerals, some of which have important commercial applications . The
See also:principal members of the group are
See also:phlogopite and
See also:lepidolite (q.v.) . The name mica is probably derived from the Latin micare, to shine, to glitter; the German word glimmer has the same meaning . The
See also:mineral was probably included with selenite under Pliny's
See also:term lapis specularis . Mineralogical Characters.—The micas are characterized by a very easy cleavage in a single direction and by the high degree of flexibility,
See also:elasticity and toughness of the extremely thin cleavage flakes . They all crystallize in the
See also:system, often, however, in forms closely resembling those of the
See also:rhombohedral or orthorhombic systems . Crystals have usually the
See also:form of hexagonal or rhomb-shaped scales, plates or prisms, with
See also:plane angles of 60° and 120°, and, with the exception of the basal planes, are only rarely bounded by smooth and well-defined faces . The crystal represented in fig. r is bounded by the basal pinacoid c (oot) parallel to which is the perfect cleavage, the clinopinacoid b ((a10) parallel to the plane of symmetry, and the pyramids m (221) and o (112) . The angles between these pyramids and the basal plane are 851° and 73° respectively . The prism (
See also:Ito) at 90° from the basal plane is not
See also:developed as a crystal
See also:face, but is a plane of twinning, the two individuals of the twin being
See also:united parallel to the basal plane (fig . 2) . The different
See also:species of mica have very nearly the same forms and interfacial angles, and they not infrequently occur intergrown together in parallel position .
The best developed crystals are those of Vesuvian biotite . When a cleavage flake of mica is struck a
See also:blow with a blunt
See also:needle-point a six-rayed
See also:star of cracks or " percussion figure " is developed: the rays intersect at angles of approximately 60°, and the pair most prominently developed are parallel to the plane of symmetry of the crystal . A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt
See also:punch is gradually pressed against a
See also:sheet of mica ; this is known as the " pressure figure." These cracks coincide with planes of easy separation or of gliding in the crystal; they are especially useful in helping to determine the crystallographic
See also:orientation of a cleavage flake of mica when crystal faces are absent . Sheets of mica which have been subjected to
See also:earth-movements are frequently cracked and ridged parallel to these directions, and are then valueless for economic purposes . In their
See also:optical characters the micas exhibit considerable variations . The indices of refraction are not high, the mean
See also:index being about 1.58–i•6o, but the
See also:double refraction is very strong (0.04–0.05) and is negative in sign . The
See also:angle between the optic axes varies from 70°–50° in muscovite and lepidolite to to–o° in biotite and phlogopite; the latter are thus frequently practically uniaxial . The acute bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in convergent
See also:light when placed on the stage of a polarizing microscope . The plane of the optic axes may be either perpendicular or parallel to the plane of symmetry of the crystal, and according to its position two classes of mica are distinguished . To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlogopite, lepidomelane andmost biotites . Dark coloured micas are strongly pleochroic . Ac-curate determinations of the optical orientation, as well as the symmetry of the
See also:etching figures on the cleavage planes, seem to suggest that the micas, except muscovite, may be anorthic rather than monoclinic in
See also:crystallization .
The different kinds of mica vary from perfectly colourless and transparent—as in muscovite—through shades of yellow,
See also:green, red and
See also:brown to black and opaque—as in lepidomelane; the former have a pearly lustre and the latter a submetallic lustre on the cleavage surfaces . Sheets of mica very often show coloured rings and bands (
See also:Newton's rings), due to the interference of light at the surfaces of
See also:internal cleavage cracks . The spec.
See also:gray. varies between 2.7 and 3.1 in the different species . The hardness is 2–3; smooth cleavage surfaces can be just scratched with the
See also:nail . The micas are
See also:bad conductors of
See also:heat and
See also:electricity, and it is on these properties that many of their technical appiications depend . Inclusions of other minerals are frequently to be observed in mica . Flattened crystals of garnet, films of
See also:quartz, and needles of
See also:tourmaline are not uncommon . Cleavage sheets are frequently disfigured and rendered of little value by brown, red or black spots and stains, often with a dendritic arrangement of iron oxides . Minute acicular inclusions, probably of
See also:rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of " asterism " in some micas, particularly phlogopite: a candle-flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star . Chemical Composition.—The micas are extremely complex and variable in composition . They are silicates, usually orthosilicates, of aluminium together with alkalis (potassium, sodium, lithium, rarely rubidium and caesium), basic hydrogen, and, in some species magnesium, ferrous and ferric iron, rarely chromium,
See also:manganese and barium . Fluorine is also often an essential constituent, and titanium is sometimes
See also:present .
The composition of the several species of mica is given by the following formulae, some of which are only approximate . It will be seen that they may be divided into two groups—alkali-micas (potash-mica, &c.) and ferromagnesian micas—which correspond roughly with thedivision into light and dark micas .
ANDREAS VOKOS MIAOULIS
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