Synthesis of Mica Alvin Van Valkenburg and Robert G. Pike
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Journal of Research of the National Bureau of Standards Vol. 48, No. 5, May 1952 Research Paper 2323
Synthesis of Mica
Alvin Van Valkenburg and Robert G . Pike
A synthetic fluorophlogopite mica (KMg3 A1Si30 IO F 2), in which the (OR) ions that are
normally found in normal micas were complet.ely replaced by fluorine, has been syn thesized.
This material has essentially the same physical and electrical properties as natural phlog-
opite mica, except with somewhat lower flexibility. To grow large, usable sheets of mica.
it is necessary to obtain pr"ferred crystal orientation, which depends primarily on controlled
thermal gradients, batch composition, and rate of cooling. Platinum crucibles were found
to be the best material for holding t he batch during the m elt. Data on the physical, elec-
trical, thermal, and chemical properties of the synthetic mica are given.
1. Introduction One of the first patents on a process for producing
synthetic mica by fluorination was issued in 1919 to
"lVIica" is a general term used to describe a series the German industrial company of Siemens &
of silicate minerals that are characterized physically Halske, German patent DRP No. 367,537. Sub-
by a perfect basal cleavage and yield with ease thin, sequent patents were issued to both German and
tough laminas. Commercially, the two most widely American companies, but no attempts were made to
used micas in the electrical industry are the musco- commercialize them. At the conclusion of World
vite and phlogopite types. These are important War II the successful synthesis of a fluorine mica in
because of their high dielectric strength, thin Germany in laboratory melts as large as 100 kg was
laminas, high resistance to heat, flexibility, and low reported. Active work on mica synthesis had begun
unit cost. No substitute has yet been found for in 1938 by the Siemens-Schuchert concern in Berlin
these minerals. Because of the strategic importance and was continued under the leadership of V. Middel
of the better grades of mica and the failure of domes- until late 1944 . In 1941 , laboratory research on the
tic supplies to meet our needs, the possibility of properties and crystallization of synthetic mica was
the development of a synthetic mica to replace also started at the Kaiser-Wilhelm Institute fur
natural mica in wartime has been of great interest. Silikatforschung and continued through most of
Claims for the synthesis of mica date back to the war. The German work demonstrated that a
about 1880. A summary of the work of early synthetic fluorine phlogopite mica with a composi-
investigators is given in F. W . Clarke's "Data of tion of K4Mg12Al4Si1204oFs could be readily
geochemistry", U. S. Geological Survey Bulletin crystallized from a melt. Although the Germans
770, 1924. J. H. L. Vogt [1]1 analyzed slags from were successful in crystallizing a fluorine mica, appar-
copper smelting in Sweden and reported a mag- ently development never reached the state of fabrica-
nesium mica, which may have been a phlogopite. tion of the mica into shapes for commercial articles .
Fouque and Levey [2] synthesized a mica trachyte Data on the electrical properties of synthetic mica
by heating a powdered granitic glass with water obtained at the conclusion of World War II indicated
under pressure ; scales of mica were reported. C. only that synthetic fluorine phlogopite had electric
Doelter [3], P. Hautefeville [4], L. P. de Saint-Gilles, constants equal to or better than that of natural
and K. Chrustschoft [5] used fluorine compounds in phlogopite.
fusing various silicates, both natural and artificial, When it became known that it was possible to
and obtained micas. Later, D . P. Grigor'ev [6] synthesize a fluorine phlogopite mica, the Depart-
in his systematic studies on the synthesis of micas ment of the Navy, through the Bureau of Ships and
demonstrated the importance of fluorine in the struc- the Office of Naval Research, and the Department of
ture of micas. the Army, through the Signal Corps, planned a joint
A synthetic fluorine phlogopite mica is not a synthetic-mica program. Research contracts were
unique product, as many natural phlogopite micas negotiated with the U. S. Bureau of Mines at Norris,
contain fluorine as a partial replacement for hydroxyl. Tenn. , the Colorado School of Mines at Golden,
This substitution can be readily accomplished be- Colo., and the National Bureau of Standards at
cause the ionic radius of fluorine, 1.33 A, is ap- Washington, D. C. These three groups have been
proximately the same as that of the (OH) ion, lAO working on the various problems involved in the
A. Their respective volumes are fluorine 9.86 and synthesis of mica, exchanging ideas, and at joint
(OH) 11.48A3. F -I and (OH) -I each have 2K and meetings discussing technical data.
8L electrons, but F -I has one nucleus with a charge The synthetic-mica research program at the
of + 9, and (OH)-I possesses two nuclear charges, National Bureau of Standards is part of a broad
+8 and + I, respectively. The unique feature of program of fundamental research on fluorine-typ e
the synthetic fluorine phlogopite is that fluorine artificial minerals. The general purpose of the mica
ions completely replace the hydroxyl ions. program is to determine the laws governing the
t F igures in brackets indicate the literature references at the end of th is paper. growth of this mineral and to compare the physical,
360,
electrical, and chemical properties of the synthetic This composition, which approximate the ideal
mica with those of its natural analogue. Explora- phlogopite formula KMg3AISi 30 IOF 2 wi th the ex-
tory experiments were initiated in N ovember 1946 ception of an excess of F 2 , yielded the be t crystals.
and continu ed with Bureau appropriations, as In preparing the batch materials prior to the meILing
personnel and funds allowed, until early in 1947, operation, care must b e taken to eliminate water
when funds of the Office of T echnical Services, vapor and CO2 from the raw ingredients, as the pre-
D epartment of Commerce, p ermitted greater activity sence of these in the batch may cause premature
on the proj ect. On June 30, 1947, funds from this breakdown of the fluorinating compound, releasing
source were discontinued. Thereafter, this work was fluorine probably as silicon fluoride at temperatures
supported b y the Office of Naval Resear ch. well below the m elting point of the batch. The
fluorine thus r eleased escapes from the crucible.
2. Experimental Work A batch was prepared by calcining gibbsite
As a starting point in the mica-synthesis program, (Ab03.3H 20 ), magnesium carbonate (MgC0 3), and
the first experiments were patterned after the silicic acid (Si0 2 ) at a temperature of 1,000° C for a
German work as reported in the Fia t publications. period of several hours, usually an overnight opera-
A mica batch , wi th a composition corresponding to tion. After calcination, the fluorine compound
th e German formula published in the Office of (K 2 8iF 6 ) was added a nd the batch placed immedi-
Military Government Fiat R eport 746, was prepared ately in the furnace for m elting and crystalliza-
The b atch was adjusted to give a final composition tion. The batch materials were of "C. P ." grade .
of: Microcline, which is a natural-o ccurring feld spar,
has a composition of KAISi30 s and is th erefore a pos-
Percentage
by weight ible fluorophlogopi te batch material. By adding 2
mole of MgO and 1 mole of MgF t to 1 mole of
Ah03_ __ ___ __ _ _ 11. 6 micro cline, one should h a ve the composition of a
32. 6
~B~ ==_- -: . : - : : : ~ : =:: : : = 30. 7 fluorophlogopite. Unfortunately, micro cline or its
chemical analogue, orthoclase, is rarely found in th e
K 2SIF 6 ___ -------------- 25. 1
pme state. Sodium, which is usually present in
amounts up to 3 per cent in microcline, apparently
A 1-g sample of batch material was placed in a pre vents good cr ystaHiza.tion of fluorophlogopite.
sealed platinum-foil envelope and h eated to 1,450 Several attempts were m ade to produce a mica from
°c for 10 minutes. The charge was then cooled at a micro cline feldspar with the addition of MgO and
a controlled rate of 3 d eg/min to 1,300° C, after MgF 2 • These were mostly failures, a th e resulting
which the furnace was cooled rapidly to room melts contained abundant amounts of glass and
temp erature. The sample had completely crys tal- orthosilicates. Occasionally small mica flakes wcre
lized , forming small interlocking mica cr ystals up to observed under the microsco pe.
3 mm in diameter. Individual crystals cleaved
readily into thin flexible flakes . Impurities in the Effor ts were made to determine the tempcrature
form of irregular white patches or cloudy surfaces of cr ystallization and thc primar y phase in melts
occurred in thin layers parallel to the cleavage having the composition of the normal fluorophlogo-
directions. This preliminary experimen t d emon- pite. Both th e soak-quench technique and differ-
strated that synthetic fluophlogopite mica, with a ential thermal analysis were used . Preliminary
composition similar to the natural phlogopite mica, soak-quench experim ents, using O.3-g samples, were
could b e r eadily cr ystallized from a melt. Addi- found to be unsatisfactor y presumably because of
tional experiments indicated that the major problems excessi ve volatilization losses in such small samples.
involved in growing mica were: composition of Experiments with samples of abou t 2.5 g gave much
batch; control of losses by volatilization to prevent better reproducibility. Charges in sealed platinum
m elts from changing composition; composition of envelopes wer e h eated to at least 1,400 ° C , well
crucible; attainment of uniform and controlled above the liquidus of this composition, cooled at
thermal gradients within m elts to give a preferred rates of about 1 deg/min to th e de ired temperature
orientation to growing crys tals; prev ention of exces- and quenched in water . B ecau se the centers of the
sive seed crystal formation at the beginning of melts cooled too slowly to freeze to glass, th e condi-
crys talliza tion. tions of the charges immediately ad jacent to the
platinum envelopes were used as the criteria .
2.1. Composition and Melting Point Dsing this m ethod , the m elting temperature was d e-
Phase determinations were made on a batch con- termined to b e about 1,345° C, and the primary
sisting of: phase was found to be fluorophlogopite. When
compositions were carefully controlled , water vapor
and carbon dioxide remo ved from the ba tch by pre-
P ercentage calcining, and the charge protected from excessive
by weight
volatilization, the cr ystallization of fluorophlogo-
Ah03___ _____ - _- _- - - - - - -- 11. 79 pite took place on cooling without the intermediate
~rB~
27. 99
crystallization of any other solid phase.
K SiF==: =: : : : : : =: =: : : ==: :
34. 74
2 6 ______________ __ _ -- 25.48 The procedure and apparatus used for obtaining
differential h eating and cooling curves are the sam e
361HEATING
600 800 1000 1200 1400
TEMPERATURE, ·C
FI GU RE l. - Diff erential thermal curve of synthetic
it uorop hlogopite.
as those d escribed by Newman and Wells [7]. Dif-
ferential thermal analyses wer e made on a ample of
dear, homogeneous, synthetic, fluorophlogopite
flakes, which were carefully selected to avoid im- FIGURE 2. Small platinum crucible as removed from furnace
pllI'ities. After the first run, only one thermal effect showing results of self -sealing. (Natural size.) ,
was noted on h ea ting and one on cooling, e vidently
associated with m elting and crystallization. As in (n Mg zSi0 4 .MgF2), forsterite (Mg2Si0 4), sellaite
the case of most silicates, the effects were not abrupt (MgF2), spinel (MgA1 20 4), and glass were formed
and the temperatures of the beginning of th e endo- with the mica crystals. Similar impurities ha ve
thermal effect on h eating and of th e exothermal b een found by other investigators [8] . Several ap-
effect on cooling were greatly influenced by such proximate weight loss determinations were made on
factors as size of charge, rate of temperature change, 80-g samples, and losses of volatiles up to 8 percent
etc. The curve obtained from the first run of three were determined . A method was found to control
()btained on the same sample is shown in figure 1. volatiliza tion by using platinum crucibles, and the
The exothermic hump on h eating, b eginning at about losses were r educed to less than ~ percent. The
1,025°, is unexplained. It was absent in later runs method consisted of forming a lip on the crucible
()n the same sample. The exothermic effect on top , proj ecting at right angles to t h e crucible walls.
cooling, b eginning immediately after reversal of the A platinum cover was placed over the top of the·
furnace, is characteristic of the method and OCCllI'S crucible so that the lip and cover were flu sh with
before temperature change and heat flow have be- each other. When volatilization of the melt began,.
come uniform. The large endothermic effect on condensation or deposition of material high in Si0 2.
heating, starting at about 1,325 °, is associated with took place at the interface between the lip and cover,.
melting, and the large exothermic effect on cooling, sealing the crucible. On removing the crucible at.
beginning at about 1,280 °, is associated with crys- room temperatllI'e, the upper side walls had crumpled
tallization. These h eat effects may vary in tem- inward due to the formation of a partial vacuum
perature ()ver a ra nge of 30 deg, due to differences (fig . 2). Using the sealed crucible technique, all but.
in rates of temperature change and uncontrollable glass and MgF 2 impllI'ities were eliminated from the·
experimental conditions. mica crystals. These were present in amounts esti-
mated to b e less than 1 percent b y \Tolume in most.
2.2. Control of Volatility prepara tions.
In early crystallization experiments in which car- 2.3. Composition of Crucible
bon or platinum crucibles were used without ade- An ideal crucible might b e considered as one·
quate means of controlling volatilization, such having the following qualifications : (a) Does no t .
impurities as minerals of the fluorohumite group r eact with the melt; (b) withstands high tempera-·
362--------------------------------------------------------~e
tures for long periods of time; (c) is easily molded
into various forms; (d) can be reused or is inexpensive
enough to be discarded.
One of th e major problems in growing synthetic
mica crystals is to find a crucible that is not attacked
by th e fluorine m elt. Middle an~ ~ssociates [9] ~sed
crucibles composed largely of slhca and alumma,
but these crucibles were dissolved to such an extent
by the melt that corrections in the batch composition
had to be madc. This method was found to be quite
inadeq uate, for it was difficult to estimate the
amount of cruciblc material dissolved .
In early experiments at the Bureau many ceramic
bodies were tested for use as crucibles, but all were
unsatisfactory as the mica melt readily attacked
them. Carbon crucibles made from carbon elec-
trodes showed promise as possible containers. The
carbon did not react with the melt, and the resulting
crystals could b e easily removed from the crucible.
The chief disad \Tantages of these crucibles were that
finally divided carbon particles were disseminated
throughout the mel t, darkening the mica crystals
and affecting adversely the electrical properties of
the mica. To prevent the carbon crucibles from
oxidizing at elevated temperatures, the crucibles
had to be protected by placing them in a ceramic
crucible packed with carbon or silicon carbid e, which
interfered with esta blishing proper thermal gra-
dients within th e crucible. Another undesirable
feature of carbon crucibles was the relatively high FI GURE 3. Cylindrical crucible with conical base.
porosity of the carbon bodies and the l?resence of Platinum walls being welded together ou a steel mandrel.
bonding agents. None of the carbon crucIbles tested
were impervious to the volatile gases generated by platinum liner was fitted snugly into a ccramic
t h e fluorine melts. With the presence of bonding crucible of the correct shap e. Joints in the foil were
agents such as clay, the crucible would be attacked weld ed with a blow torch on a steel mandrel (fig. 3),
by the melt, leaving voids and avenues of escape and care was taken to make the completed cruciblE}
for gases. leak-proof.
Crucibles made of silicon carbide or lined with
silicon carbide showed several undesirable features. 2.4. Thermal Gradients
In either case it was impossible to obtain a smooth
surface, as the silicon carbid e occurred a.s large The disposition and steepness of temperature
individual crystals. These crystals acted as seeding gradients in the crystallizing melt are of critical im-
centers for the crystallizing melt, destroying th e portance in the control of size and orientation of the
possibility of obtaining good crystal orien tation. mica sheets. Obviously, these are affected by the·
Slight decomposition of the silicon carbide took characteristics of the furnace and the shape and size·
place at the contact with th e melt, releasing carbon of th e crystallizing crucible.
particles, which colored the mica crystal grey. As preliminary experiments indicated a melting
temperature of fluorophlogopite near 1,350° C,
Platinum crucibles were used in all the later crys- furnaces were necessary that could produce a tem-
tallization experiments, as they did not appear to perature within the crucible well above the melting
react with the fluorine melt. However there was a temperatuTe, say 1,500° C, in order to assure com-
tendency for the crystals to adhere to the platinum, plete removal of possible nucleation centers of
which not only made it necessary to peel the plati- unmelted material. To meet this requirement, two
num from th e crystalline cake but also oriented the furnaces were designed and built: (1) a platinum-wire
mica crystals parallel to the platinum walls. resistance furnace for crucibles with a capacity of
In order to economize, the inner wall only of the about 70 g (fig. 4), and (2) a Global' resistance
crucible was made of platinum. This consisted of furnace for crucibles with a capacity of about 8 kg
foil 0.006 in. thiclc Because platinum has little (fig. 5) .
rigidity at temperatures above about 1,400° C, th e The smaller furnace contained a cylindrical refrac-·
liner was supported by various means, depending tory core with an inside diameter of 2~ in., on which
on the size of th e container. For small melts of was wound the main heating element consisting of
abo ut 70 g, th e foil container was supported in a bed 20-gage 80-percent platinum- 20-percent rhodium
of granular refractory material contained in an outer, wire. The winding covered 9X in. of the core with
cylindrical ceramic crucible. For larger melts, the 55 turns of the wire. A booster element made of the,
363FIGURE 4. Platinum resistance furnaces f or crystallization and
phase determination work.
same gage and composition wire was wound on a
cylindrical core of 3-in. diameter. This element was
3-in. long, arranged coaxially with the inner core and
covered the upper portion of the main element. Each
winding was connected to a separate auto-trans- FI GU RE 5. L arge Globar Crystallizing furnace with elevator
former. The primaries of both transformers were lifting apparatus underneath.
connected to an on-off proportional program con- Variable speed motor at lower left lowers and raises elevator.
troller. Thus, the desired temperature gradient in
the furnace could be maintained while the temp era-
ture in the furnace could be lowered at a predeter-
mined rate. Experience showed that cooling rates of
as low as 0.2 deg/hr could be obtained from 1,450°
to 1,200 ° C.
The larger furnace was heated by means of
Global' elements. For use with a crucible with cir-
cular section, the muffl e was cylindrical with a 7-in.
bore and a h eight of 20 in. The Global' elements were
arranged in a horizontal square grid, four elements to
a set, with three sets arranged vertically. The upp er
two sets were separated from the lower set by a
horizontal baffle made of insulating r efractory. In
most runs, only the upp er two sets wer e in use.
These were controlled by an automatic potentiometer
on an on-off control of about 15 percent of the total
power, and through a platinum, platinum-rhodium
th ermocouple placed in the h eating chamber. Tem-
peratures could be controlled to within ± 10 d eg C
at 1,400° C. With the slowest cooling rates used ,
0.2 deg/hr the furnace control was not sufficiently
sensitive, therefore, fluctuations far exceeded the
cooling rate r equired.
In using crucibles with elongated horizontal cross
.,
section placed in a rectangular muffle, the Global'
elements wer e disposed parallel and close to the broad F IGU RE 6. Drawing of large elliptical crucible showing sup-
sides of th e crucible and were removed from the porting ceramic container and radi ation fin at base.
narrow sides.
Two types of platinum-foil crucibles wer e used in figure 6. The 11 0 melts made in the smaller furnace
th e smaller furnace: (1) a form with cylindrical walls provided useful information on volatilization losses,
and a conical bottom, and (2) a form with an elon- orientation control, etc. , for the design of experi-
gated horizontal cross section and wedge-shape ments in the larger furnaces. Various arrangements
bottom, similar to but smaller than that shown in of cooling fins attached to the bases of both shapes
364of crucible were tried in order to vary the thermal
gradients (figs. 6 and 7).
The rate of cooling and the steepness of the vertical
temp erature gradient wer e varied in order to deter-
mine th e effect on size of crystals and vertical orienta-
tion. The rate of cooling was varied from 15 to 0.2
deg O/h1', and th e gradient from top to bottom of the
crucible was varied between 15° and 90° O. In
general , it was found that the slower the rate of cool-
ing and th e steeper the temperature gradien t, the
larger the crystals and the better the vertical orienta-
tion. Cooling rates faster than 1 deg O/hr and
gr adien ts less and 50° 0 provided small, randomly FIGURE 7. Platinum crucible (center) showing radiation fin
oriented crystals with inclusions of gas cavities welded to cone portion of crucible.
(fig . 8) and with associated impurities of glass and (Left) Ceramic retaining crucible; (right) steel mandrel used to shape
MgF 2 • platinum crucible.
~
FIGURE 8.- Thin sections oj synthetic phlogopite taken at right-angles to the cleavage, showing cavities in various attitudes (X40) •.
997319- 52- -2 36511. Mica crystals showing a prefe1'1'ed parallel
orientation to broad sides of crucible.
One·half snrface of large melt (Xl.3).
the surface against the platinum container exerted a
marked effect on the orientation of the mica crystals.
In one experiment made in the small furnace in
platinum, the temp erature gradient was deliberately
reversed, and cooling took place from the top down .
With the very slow rate of cooling u sed, 0.2 deg O/hr,
a single horizontal crystal of mica formed over the
entire surface of the melt, and crystallization con-
tinued until it reached a thickness of about 3 mm.
Beyond this thickness, new crystals Were formed pre-
dominantly parallel to the temperature gradient.
In most well-crystallized melts, the crystals next to
the platinum walls show a marked tendency to be
parallel to the contact.
Attempts were made to obtain large crystals of
mica by seeding the melt with a single mica flake at
various temperatures before freezing took place. In
all cases, the flakes either melted or had no effect on
FIG URE 10. Nlica crystals showing a preferred parallel orientation. Pieces of corundum with known crystal
orientation parallel to broad sides of small crucible. orientation were introduced into melts to act as
(A) Plan sect ion ; (B) cross section . NaLural size. seeding centers to give a preferred orientation to the
mica nuclei. There was no indication that corundum
In order to assure the growth of large crystals, it is acted in orienting the mica crystals. A. Dietzel [10]
necessary to reduce the number of nuclei available claimed that a magnetic field imposed upon a crystal-
for new crystal formation. For this purpose, the lizing melt did have a crystal orientation effect.
bottoms of the crucibles were either reduced to thp According to his data, the synthetic mica is slightly
apex of a cone or to the trough at the base of a wedge. paramagnetic, and weak fields of about 50 to 100
In the large crucible of the shape shown in figure 7, gauss are sufficient to produce an orientation effect.
good development of vertical orientation was ob- In two experiments using a series-wound horseshoe
tained, but the lack of horizontal orientation pro- magnet that produced about 70 gauss, no magnetic
duced nonparallel crystals that were limited in effect could be observed on a synthetic mica flake :
breadth because of intersections with other growing The technique of orienting mica crystals by means of
crystals (fig. 9). a magnetic field was thought to be impracticable, and
To obtain parallel lateral orientation and to pre- further experiments were abandoned.
vent the intersecting growth of crystals, crucibles The Kyropoulos [11] technique of growing crystals
having the shape shown in figure 6 were used. As was attempted with a mica melt. The essential
indicated from the small melt shown in figure 10 and feature of this technique is to form a single crystal
the large melt shown in figure 11, a fair degree of by ~lowly withdrawing a seed from a melt. The
lateral orientation was tbus achieved. All of the surface of the melt is kept near but slightly above the
crystallization experiments made in platinum, how- freezing point, and crystallization takes place at the
ever, showed that both the surface of the melt and interface of the melt and the crystal. A platinum
366"1
plate 0.002 in . thick wa dipped into the melt and humidity maintained b y a saturated solution of
then slowly withdrawn. Platinum has good thermal CaS04.2H 20 . The results are expressod in water
conductivity, and it was expected that a seed crystal sorbed in m illigrams per cubic centimeter at Lhe end
would form at th e tip of the plate, which would b e of 1 and 2 hrs. For compari son, a natural phlogopite
below the freezing point of the m elt. Trouble was from Argentina and three glasses have been included.
experien ced in keeping the m elt just above freezing
temperature. When the plate was dipp ed , eith er the
surface of the m elt would freeze completely or no ' ;Vate r so rb ed in-
crystals would form at all. This technique did not
appear to offer good results unless better control of 1 hoUl" 2 hOll r
th e thermal relation b etween the surface of the m elt
and the plate could be obtained .
mg/ cm 3 mg/cm 3
Synthetic phlogopite __________ 46 57
3. Properties of Synthetic Mica Argentina phlogop ite __________ 56 74
P y rex glass __________________ 16 20
The properties of synthetic fluorophlogopite are Vycor glass __________________ 11 13
essentiall y the sam e as its natural analogue. In thin No. 015 gla ss ________________ 88 191
flakes, the mica is clear and transparent. As in the
natural micas, the cleavage is the most character-
istic property of the synthetic mica. Th e cleavage Thermally, the phlogopite micas have a higher
occurs parallel to the (001 ) crystallographic plane, breakdown temperature than do th e other micas,
and in thin laminas, the flakes ar e elastic bu t less so and Lhi holds true for Lh e syLh eLie fluorophlogopite.
than natural phlogopi te. The mica appears to have Several clear mica flak:es were subj ected Lo a temper-
a hardness equal to that of natural phlogopite. lLs ature of 1,000° C for a period of 3 days. The sur-
specific gravity is about 3.00, as m easured on a face of the mica had a cloudy appearance, and it
Berman Torsion B alance. The specific gravity is was noticed that mosaic struct ures had formed (fig.
troublesome to obtain, as the edges of the mica flakes 12). Microscopicall y, the th ermally treated mate-
are often frayed and will give erroneous results. rial had approximately th e sam e optical constants
The optical constants of the synthetic mica wer e as did the untreated mica, and an X-ray diffraction
m easured, using petrographic microscope techniqu es. pattern gave essentially the sam e pattern as for the
The mica is n egative in character, with the optical untreated materials.
plane being parallel or nearly parallel to b(010). Under the direction of B . L . B ean of this Bureau,
The optic angle 2V is about 9°, with m easured indices a ch emical analysis was mad e on selected clear flakes
of refraction of /3 = l.545, 1' = l.54 7, and a cal culated of synthetic mica, containing no visible impurities.
to be l.519 .
The dielectric constant and dissipation factor of
six specimen were m eas~ed at th e Bureau 's Induct-
ance and Capacitance Section. The measurements
were made at 25 ° C and at three frequ en cies: 100,
1,000, and 100,000 cps. The values of dielectric con-
stant at 1,000 cps vary from 5.0 to 7.0, with an
average value of 6.3. As the specimens were small,
}6 to %in . in their longest direction, errors in meas-
Ul'ement of dielectric constant of as much as 10
percent were to be expected . The spread in values
of some 40 percent indicates a certain amount of
variability in the dielec tric constant of the material.
The dielectric constant decreased slightly (about 2
or 3%) as the frequency was changed from 100 to
. 100,000 cps. Values of the dissipation factor varied
from 9 to 300 X 10- 4 when m easured on different
days. Th e cause for these changes with time is not
known, but all of the changes were in the direction
of smaller values. The average value at 1,000 cps
was about 60 X 10- 4 • The value of the dissipation
fac tor seemed to decrease sligh tly with frequency
between 100 and 100,000 cps.
The hygroscopic character of the mica or the
ability of mica to ab orb and retain water was
determined by Donald Hubbard of the Bureau's
Glass Section. In general, the method consisted of FIGURE 12. N[osaic strtLctuTe f ormed on surface of mica crys-
weighing the water sorbed upon exposing approxi- tals af ter heat treatment.
mately l.5 g of powdered mica that passed a 150 A scratch line from right to left has broken OJf many segments, leaving a
mesh standard sieve to the high (approximately 98 %) clear area underneath (X20).
367The results in weight percent are: were eliminated, but MgF2 and glass remained as
impurities. The glass had a mean index of about
1.51. The impurities occurred in small amounts
Sio 2 _ __ _ ________ _ ______ _ 41. 87 estimated to be less than 1 percent by volume, and
AhOa - - _- - - - - - - - - - - - - - - - 12. 97 these were randomly scattered throughout the crys-
~gO
F - ------------------
_____ ________________ _ 28. 27 tals. Mica crystals as large as 4 in. 2 in area and free
8. 52
K 2 0 _____ ______________ _ 10. 94 from impurities were obtained from melts in the
~a20 ____ ______________ _ O. 12 large globar furnace. Structural defects in the form
of gas bubbles were observed in those melts that
TotaL __ ___ _ __ _ __ _ 102. 69 were cooled quickly (greater than 1 deg C/hr).
Equivalent,O = F ___ _____ 3.51
- - - - -1 These bubbles, or cavities, appeared to occur in
TotaL_ _ _ ___ __ _ __ _ 99. 18 planes paralleling the mica cleavage planes. It was
observed that melts cooled at slower speeds con-
tained less structural defects and fewer impurities .
X-ray diffraction patterns were made of selected
mica flakes, and these were compared with a natural
phlogopite from Canada obtained from the U. S.
National Museum. The patterns were made on a
X-ray Geiger counter, using copper radiation. The
synthetic and the natural patterns are essentially
the same with minor differences in intensities and a
small difference in d-spacing, as can be observed at
the higher angles of 28 (fig. 13). As mica has a
highly developed cleavage, the problem of getting a
Tandom orientation of the powder grains is almost
impossible, hence the intensities are not truly repre-
sentative of the mica. The small difference in d-
spacing may be due to the presence of iron in the
natural sample and to the fact that the synthetic
mica does not contain any (OH) ions.
Impurities were observed in all the crystallization
experiments, and their presence apparently depended
in part upon the loss of volatile constituents. In
early crystallization experiments, using carbon and
"lmcovered crucibles, where volatilization losses were
high (about 8 to 10 % by weight) impurities of
iorsterite, norbergite, chondrodite, and glass were
present in appreciable quantities. They occurred as
millry, or cloudy, patches in the plane of the mica
cleavage. The orthosilicates were often inclosed in
glass, or they occurred in branching structures (fig. FIGURE 14. Mica flake parallel to cleavage.
14). In later experiments, when volatilization was Clear mica in lower portion of photograph. R emaining area shows dendritic
controlled by sealing the crucible, the orthosilicates structures (X 30).
FIGURE 13. X-ray powder diffraction patterns of natural phlogopite (1) and synthetic phlogopite (2).
3684 . Summary [8] 'vV. Eitel, The sy n t hesis of fluorin e mi ca of the phlogopite
group, Fiat Final R epor t No. 747 (1946). 1
[9] Ben nett S. Ellefson , Cru cibles for s ynthetic mica dcvelop-
A syn th etic fiuorophlogopite mica having the ap- ment, Fiat Final Report No . 1050 (J947).
proximate formula of K ,Mg12Al,Si120 (oF s can b e crys- [10] Paul M . T yler, Synt hetic mi ca r esearch , Fiat Final R c-
tallized from a m elt at atmospheric pressures. The p or t No . 746 (1946).
[11] S. Kyropouios, Z. anorg. chem. 154, 308 (1926); Z. Physik
ingredients th at pToduced the b est crystals consisted 63, 849 (1930).
of:
Additional References on Mica Synthesis
P er centage Paul M . Tyler, Synt hetic mica research, Fiat Final R eport
by weig ht No . 746 (1946).
W. Eitel, The synthesis of flu orine mica of the phlogopite
Si0 2 ___________ _ _________ _ group, Fiat Final Report No. 747 (1946).
34. 74 W. Eitel, Crystallochemical and microscopic investigation of
AhOa_________ - ___ - - - - - - - - 11. 79 synthetic phlogopites, F iat Final R eport No . 748 (1946).
MgO ____________________ _ 27. 99 W. E itel , Regular intergrowt h of synthetic phlogopite with
K 2SiF 6- - - - - - - - - - - - - - - - - - - - 25. 48 hydrous mica, Fiat Final R eport No . 749 (1946).
Rustum, Roy, D ecomposition a nd resy nth esis of the micas,
J . Am . Ceram. Soc. 32 , No. 6 (1949).
This composition m elts at about 1,345° 0 , and Tokiti Noda, Translation of seven J apanese papers on the
sy n thesis of flu ori ne micas, Tachi no, t ran slator . Section
when it is cooled slowly in th e order of magnitude of TIS Geo logical Smvey Bran ch , I ntellige nce Division Of-
0.3 deg C /lu', good mica cry tals can b e obtained. To fice of the Enginee r Ge neral H eadq uarters, Far East Com-
grow large crystals it is n ecessar y to obtain a pre- mand (1945).
ferred orien ta tion of the individual crystals, other- Che mical co mpos ition and optical p ropert ics of ynth etic
m ica, and microscopic stud ies, J . Soc. Chcm . Ind . (Japan)
wise growth is interrupted b y inter section of one 46, No . 8, 760 to 762 (1943) .
crystal with another. The disposition of the th ermal - , Composition of f used substan ces from flu osilicates and
gradient in a mica m elt is the most important factor mica cry tallizat ion t emperat ure (R eport No. 2); J . Soc.
governing the orien tation of a growing crystal , and Chem. (Ind . (J apan) 46, No . 10, 1082 to 1085 (1943) .
- , Co mposition of fused s ub tances from flu osilicates and
it was found that mica crystals grow with th eir mica crysta ll ir.ation tcmpcrat ure (Repor t No.1 ); J . Soc.
cleavage planes parallel to th e direction of the gra- Chern. Ind . (Japan) 46, No.9, 921 to 923 (1943). 4
dient. Th e majority of the mica experiments were - Compositio n of fused s ubstances from fluos ili cates alld mica
performed in closed platinum crucibles, using electric crystallizat ion tem pera t ure (Report No.3) J . Soc . Chem.
I nd . (Japa n) 47, No .4, 320 to 322 (1944).
resistance furn aces. - , Synthesis of bOTonic phlogopite, J . Soc. Chem. In d. (J apan)
In general , th e phys ical, ch emical, and electric 47, No . 6, 499 to 502 (1944).
properties of the synthetic fiuorophlogopite, are - , Co mpositio n of fu sed su bstances fr o m flu osilica tes and
essentially the sam e as those of a natural phlogopi te, mica crystallization tcmperature (Reports No. 4 and,;~5 )
J . Soc. Ch em . Ind . (J apa n) 47, No. 7, 623 to 627 (1944).
with the excep tio ns that the syn th etic mica tends to - , Composit ion of fu sed s ubs tances from fluosilicates and
he a little more brittle and do es no t conta in (OR) mica crystallizatio n temperature (R e por ts 6 and 7) J . Soc.
ions. Thermally , it h as a high er breakdown temp- Chem. I nd . (Japan ) 48, No. 1, 14 to 15 (1945).
erature, and for short p eriods of time can withs tand D . P . Grigo r'e v, The preparation of ar t ifi cial magn esium mica,
Zc ntr . lVlineraJ. Gcol., p . 219 to 223 (1934A ).
temperatures of 1,200° 0 without no ticeable changes. D . P. Gr igor'cv, The role of flu orine, chl ori ne, and tungsten
oxide compo un ds in t he artifi cial format ion of magnesium
5. References micas, Mem . soc. russe. m in eral. 64, 347 to 354 (1935).
D . P . Grigor'e v, t he crystallization of amp hi bolc and mica
from artificial silicate melts, Zen tr . Mi neral. Geol., p. 117
[1] J . H . L. Vogt, Berg- u . htittenmiinn . Zcitung 47, 197 to 123 (1935A).
(1888) . D. P . Grigor 'ev, Synthesis and study of phlogopite, Compt.
[2] F . A. Fouque and A. M . Levey, Compt. ren d . 113, 283 rend . Acad. Sci. URSS 43, 63 to 65 (1944).
(1891 ) . J . 'vV. Grun er, Ammonium mica synt hesized from vermicu-
[3] C. Doelter, Co mpt. re nd . 2, 178 (1888); 1, 1 (1897). lite, Am. Mineral. 24, 428 to 433 (1939).
[4] P. Hautefeville, Compt. rcnd . 104, 508 (1887). J. W . Gru ner, Formation and stability of muscovi te in acid
[5] L. P. de Saint-Gilles a nd K . Chrustschoft, Mineralog. solutions at elevated temperat m es, Am . Mineral. 24, 624
petrog. u Mitt . 9, 55 (1887). t o 628 (1939).
[6] D . P . Grigor'ev , the preparation of artificial magnesium R . R eichmann and V. Middle, Product ion of sy nt hetic mica,
mica, Ze ntr. Mi neral. Geo!., p . 219 to 223 (1934A.); A Report on t he synth etic mica at Siemens-Schuckert,
t he crystallization of a m phi bole and mica from arti- iss ued in 1942.
ficial silicate melts, Ze n tr. Minerai. Geol., p . 117 to V. Middel, Experime nts on the production of synthetic m ica
123 (1935A); t he role of fl uori ne, chlorine, and t un gsten in large experi mental apparatus, Office of the Pu blicat ion
oxidc com pounds in the artificial formation of mag- Board, D epartment of Commerce, PB32546 (1947).
nes iu m mi cas, Mem . soc. russe. min eral. 66, 118 to
123 (1937); Synthe is and st udy of phlogopite, Compt.
re nd . Sci. URSS 43 , 63 t.o 65 (1944).
[7] E. Newman and L. W . Well , Effect of so me added mate-
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