What is Mica?
            
Mica is a natural valuable mineral product applied to a group of a complex aluminosilicate minerals having a sheet or plate like structure with different chemical compositions and physical properties. All micas form flat six sided monoclinical crystals with a remarkable cleavage in the direction of the large surfaces, which permits them to split easily into optically flat films, as thin as one micron in thickness. When split into thin films, they remain tough and elastic even at high temperature.

What makes micas so important to command such a unique position ? Because it had been gifted with exceptional values. It possesses some of the most outstanding combination of chemical, physical, electrical, thermal and mechanical properties which are not found in any other product.
          
Processing
          
The ordinary mica crystals as they come out of a mine is in form of rough books or lumps of irregular shape, size and thickness, associated with impurities and structural imperfections. They have to undergo a long operation of cutting, sorting and processing from crude to commercial quality. The waste resulting from the production of crude to commercially useful sheet mica forms about 90 per cent Some times even the total output of mica from a mine is sold as scrap mica and it can be used as Mica Scrap or can be used for Grinding Mica Powder.
       
No machine has yet been designed to process mica mechanically. The processing technique, therefore, involves only a pair of hands, eyes and a simple knife. The operations performed consists of cobbing or cleaning the crude crystals as extracted from the mine with its associated impurities; rifting or splitting the cobbed mica into usable sheets by sickle and then by sharp knife removing major flaws and structural imperfections, such as cracks, holes, reeves, crossgrains and other physical defects to obtain the maximum usable area with minimum wastage. The trimming of mica is a skilled hand-operation in which cracks and imperfections are removed at the edges of the sheets using a knife while saving the best area of usable sheet. The knife-dressed sheet mica of irregular polygonal shapes is finally graded into different standard sizes and commercial qualities before they are offered for marketing.
       
Classification
      

For commercial and industrial purpose natural mica are mainly divided into three catagories,viz :

1. Processed Mica  2. Fabricated Mica  3. Manufactured Mica.

Processed Mica is fundamentally a natural sheet mica of an irregular size and polygonal shapes that is relatively flat and free from physical defects and structural imperfections having a minimum usable area of 4.8 sq. cm. (0.75 sq. inch), suitable to be cut, punched or stamped into specific size and shape chiefly for use by the electronic and electrical industries.

Fabricated Mica is basically a natural sheet mica cut, stamped or punched to specified size, shape and thickness for industrial and uses, such as; discs, washers, cut-films or sheets, joints, backing plates, plates, spacers, mica formers for irons, toasters, rice-cookers, etc.

Manufactured Mica means mica-based products manufactured from natural mica, such as, micanite or built-up mica, mica paper, mica heating elements, mica capacitors, mica flakes and powder, mica bricks, etc.
        
Quality grading of mica
       
The quality classification of mica, is based visual tests, depends on individual opinion. Also products of different mines vary in physical characteristics to such an extent that the development of a single standard with reasonable limits of tolerance becomes an acutely dificult task. Added to these difficulties are the facts that quality clasification has to be carried out by a large number of individual workers, and that it is highly influenced by such circumstantial factors as lighting conditions, mood of the inspector, etc. Under these circumstances any standard for quality classification of mica, such as the one described in this standard can give at best an approximate idea of a particular quality and its relative position with respect to other qualities. As such, the quality of mica depends upon the and the seller who have to work out a basis on mutual understanding and peculier requirements of the industrial applications. The application of common sense to the task of procurement is therefore always helpful especially so in the matter of choosing a right supplier as only an experienced supplier knowing the individual buyer’s specific requirements, is able to deliver to the near correct material. In fact, it is an expert’s job to process and assort mica to suit to individual buyers requirements and maintain uniform and satisfactory supplies on a regular basis.
          
Technical data of mica
     
Physically
Mica is transparent, optically flat, easily splittable into thin films along its cleavage, colourless in thin sheets, resillient and incompressible.
    
Chemically Mica is a complex hydrous silicate of aluminium containing potassium, magnesium, iron, sodium fluorine and/or lithium and also traces of several other elements, It is stable and completely inert to the action of water, acids (except hydro-fluoric and concentrated sulphuric), alkalies, conventional solvents, oils and is virtually unaffected by atmospheric action.
        
Electrically Mica has the unique combination of great dielectric strength, uniform dielectric constant and capacitance stability, low power loss (high Q factor), high electrical resistivity and low temperature co-efficient and capacitance. It is noted for its resistance to arc and corona discharge with no permanent injury, and has little or no effect when exposed to electronic radiation dosages upto 1018 nvt. Thus maintaining its a-c dielectronic material when exposed to the oxidising action of electrical discharges either on the surface or within internal gas spaces in built-up mica insulation.
       
Thermally
Mica is fireproof, infusable, incombustible and non-flammable and can resist temperatures of 600° C to 900° C (1112° F to 1652° F) depending on the type of mica. It has low heat conductivity, excellent thermal stability and may be exposed to high temperatures without noticeable effect.
        
Mechanically Mica is relatibility soft and can be hand-cut, machined or die-punched. It is flexible, elastic and tough, having high tensile strength. It can withstand great mechanical pressure perpendicular to plane but the lamination have weak cleavage and can be easily split into very thin leaves.
    

Characteristic Source  MUSCOVITE PHILOGOPITE
       
Colour - Ruby, Green, White Amber, Yellow, Silver
Density                            gm/cm³ 
                                         ib/in³ 
 1
 1
2.6-3.2
0.095-0.116
2.6-3.2
0.095-0.116
Specific Heat  1 0.21 0.21
Hardness                        Moh scale
                                         Shore test
 1
 1
2.8-3.2
80-105
2.5-3.0
70.100
Optic Axial Angle  1 55-75 5-25
Tensile strength               kgf/cm² 
                                          ibf/in² 
 2 
 2 
about 1750
about 25000
about 1000
about 15000
Shear Strength                kgf/cm² 
                                          ibf/in² 
 2 
 2 
2200-2700
31000-38000
1000-1300
14000-19000
Compression Strength   kgf/cm² 
                                          ibf/in² 
 3
 3
1900-2850
27000-32000
  ---
  ---
Modulus od Elasticity      kgf/cm² x10-³
                                          ibf/in² x10-6
 2
 2
1400-2100
20-30
1400-2100
20-30
Coefficient of expansion per C Perpendicular to cleavage plane  4 9x10-6 -36x10-6 30x10-6 -60x10-6
Calcining Temperature   C
                                           F
 3
 3 
700-800
1290-1470
900-1000
1650-1830
Maximum Operating Temperature  C
                                                            F 
 3
 3
500-600
930-1110
800-900
1470-1650
Thermal Conductivity Perpendicular to cleavage planes
                                           Gm.cal/sec/cm² /C/cm
                                           BTU/hr/ft² /° F/ft
 1 
 1
about 0.0013
about 0.31
about 0.001
about 0.24
Parallel to cleavage planes
                                           Gm.cal/sec/cm² /° C/cm
                                           BTU/hr/ft² /° F/ft 
 5
 5 
  ---
  ---
about 0.012
about 3.0
Water of Constitution %  1 4.5 3.0
Moisture Absorption  3  very low very low
Apparent Electric Strength   0.001" to kv/mm
                                                0.003" thick Volts per 0.001"
R.M.S. at 15C (60 F)  0.01" to kv/mm 
                                      0.05" thick Volts per 0.001"
 1
 1 
 1 
 1
120-200
3000-5000 
40-80
1000-2000
  ---
  ---
30-60
750-1500 
Permittivity (a 15 C (60° F)   3 6-7 5-6
Power Factor (loss Tangent) 15 C (60° F)  3 0.0001-0.0004 0.001-0.005
Volume Resistivity 25 C (77 ° F) Ohm cm  1.3 4x1013 -2x1017 1x1012 -1x1014
Acid Reaction  1  Affected by 
Hydrofluoric Acid
Affected by
Sulphuric Acid
           
Note:
The data given in this table are based on several publications, notably those given in the column titled "Source". The reference details for these publications are as follow :  
Source 1 .  "Mica & Micanite. A Critical Resume of Available Information". E.R.A. Report D/T 18 1934, 
  by  D.V. Onslow. 
   
Source 2   "Report on the Mechanical Properties of Mica" ERA Report D/T 13 1924.
   
Source 3
 "Insulating Materials for Design and Engineering Practice" by F.M. Clark Joihn Wlley
  & Sons Inc. New York, 1962. 
   
Source 4   "Some Physical Properties of Mica", Journal of National Bureau of Standards.Volume 35,
   October 1945, p.309,by P Hidnert and G. Dickson.
   
Source 5   "Thermal Conduction in Mica along the planes of Cleavage". by H.J. Goldmid and 
   A . E . Bowley. Letter in Nature, Vol. 187, 3rd September 1960, p 864.
 
Uses of natural mica sheet :  
Elecrical Apparatus : Armature Coils, Commutators, Heat Sinks, Invertors, Rectifiers,
                                        Rotating field Coils, Transformers.

Electric Heating Appliances : Cigarette lighters, Coffee percolators, Curling Irons,
                                                        Flat-Irons, Hair Dryers, Hot plates, Immersion heathers,
                                                        Rice Cookers & Boilers, Permanent wave machines,
                                                        Soldering Irons, Space heaters, Steam Iron Presses,
                                                        Toasters, Vibrators, Water Heaters, Waffle Irons,
                                                         Portable clothes dryers.

Electrical Control : Electrical and Thermal Regulators, Grid resistances Boxes,
                                    Industrial motor starters, Motor control panel, Pyrometers, Relays,
                                    Rheostat rods and terminals, Starting boxes of Trolley cars.

Electrical lighting equipment : Arc lamps, Dimmer Switches, Flashers, Large
                                                         Incandescent Lamps, Liners for Lamp Shades,
                                                         Neon-Lights.

Industrial Electric Heating Appliances : Branding Irons, Glue pots, Laundry Irons,
                                                                          Lead pots, Localized heating units,
                                                                          Miscellaneous heating elements, Thermostats.

Mechanical Uses : Compass cards, Diaphragms for hearing aids, quided missiles,
                                    Mommsen lungs Nonbreakable goggles, Oxygen breathing apparatus,
                                    Petromax chimney, Quarter wave plates for optical instruments,
                                    Shields for liquid level gauges of high pressure steam boiler,
                                    Stove and furnace inspection windows, Synthetic optical crystals.

Miscellaneous Electrical Uses : Covers for fuse plugs, Insulators, Spark plugs for heavy
                                                           duty high compression engines, Washers.

Radio, Radar, Television, etc. : Bombarders, Capacitors, Geiger-Muller Counters,
                                                          Magnetic heads of computers, Microwave Windows,
                                                          Mosaic base and targets in Iconoscopes, Solid state
                                                          applications, Transistor Shields, Tubes, Vaccum Tubes,
                                                          X-Ray Apparatus.
 
 

Uses and applications of natural sheet mica based on dielectric, optical, mechanical, electrical and thermal properties or their combination :
 
Commercial Quality Applications
V-1 Ruby Clear
V-2 Ruby Clear & S.S.
V-3 Ruby Fair Stained
V-4 Ruby Good Stained
V-5 Ruby Stained "A"
V-6 Ruby AQ
V-14 Green / Brown
V-14 1st Quality
Uses Based on dielectric properties :
High precision electronic capacitors (fixed & trimmer) for transmission of radio  waves e.g. for Radio, and Guided Missiles, etc.
Microwave transmission attenuators.
Microwave tube Windows.
Windows for geiger counter tube for measuring soft radiations detecting radio activity.
X-ray apparatus and similar devices requiring protection from heat.
Space Rockets.
Telestar transmission
Jet engine ignition systems
Target and mosaic micas used in telecasting industry in image orthocones and computer matrix.
Radiation insulation.

Uses based on optical properties :
Diaphragms in guided missiles.
Inconoscope screen in television reflectors.
Quarter-wave plates for optical instruments.
Synthetic optical crystals.
Marker dials for navigation compasses.
High pressure steam boiler gauge glasses.
Goggles.
Window covers for radiation pyrometers and thermal regulators.

Uses based on Mechanical properties :
Diaphragms for oxygen breathing equipments.
Diaphragms for detecting sound in submarine and air crafts.
Resistance & Potentiometer cards.

V-5 Ruby Stained "A"
V-7 Ruby Stained "B"
V-8 Ruby BQ
V-9 Ruby Heavy Stained
V-15 Green / Brown
V-15 2nd Quality
V-16 Green / Brown
V-16 Std. Or BQ
Uses based on Electrical Properties :
Bridges and supports for radio, television and electronic Tubes, television cathod ray tubes, hearing aid tubes,audion tubes, acron tubes, miniature tubes, incandescent lamps, etc.
Low precision and receiver condencers.
Backing plates for capacitors.
Interlayer insulation in high temperature air-cooled transformer coils.
Supports for heating filaments of high power electric lamps.
Insulation for Magnetos in ignition systems of internal combustion engines of submarines, aeroplanes, automobiles, trucks, military tanks, etc.
Wrappers, shields and washers for spark plugs.
Transistor Mounting Washers.
Strainer Cores & Skin Gates for sand castings.
Punched mica parts for Computers, Televisions, Radios, Car-ignitions and other semi-conductors and electronic components.

Uses based on optical-cum-mechanical properties :
Chimneys for gas and petromax lamps.
Diaphragms for producing, receiving and transmitting sound Waves in telephones, head phones, radio-phones, phonographs, gramophones, sound boxes, loud speakers and telephone transmitter buttons.
Window covers for combustion furnaces, anthracite lamps and oil stoves.
Incandescent lamp Fuse plugs and sockets.
Wrappers for insulating the high tension coil of radars.
Heavy electrical equipments.
Pyrometres.
Stove mica for furnace peep holes and isinglass.

V-9 Ruby Heavy Stained
V-10 Ruby Densely Stained
V-11 Black Dotted
V-12 Black Spotted
V-13 Black/Red Stained
Uses based on the thermal properties :
Various domestic electric heating applicances requiring non-inflammability and dielectric resistivity,viz. Cooking ranges, electric flat-irons, toasters, coffee perculators, kettles, ovens, rectifiers, soldering, irons, griddles, water heaters, radiaters, boilers, hair dryers, lead melting vats,glue pots, and host of other household appliances.
Commutator segment insulation and armature insulation in motors and generators that are fully enclosed to exclude dust and moisture and those that are to be used in a very hot climate at high operating temperature.
Solenoid insulation in electric motors and generators.
Insulating washers, discs and terminal plates for dynamos, commutators, transformers, electric motors, generators, lightning arresters, fuse boxes, induction coils, grid rheostats and numerous other electrical devices.