SPI Mica Sheets and Disks
Muscovite form of mica: Some interesting information
Generically, the mineral known as muscovite, is a potassium aluminum silicate hydroxide
fluoride and is considered to be a classical "silicate". From a geological classification
scheme, it is in the subclass of phyllosilicates and of course, is in the mica "Group".
In its less pure and lower cost forms, it is used as a heat and electrical insulator in a
number of industrial applications. For the analytical applications, we would recommend
always using the higher quality forms, which of course are more expensive, but one will
have a much better chance of reproducing results and there is always the chance that one
requires the higher quality mica in order to get any results at all.
Muscovite is fairly common and is found in igneous, metamorphic and detrital
sedimentary rocks. It has a layer-like structure of aluminum silicate
sheets not strongly bonded, and they are held together by the K+ ions.
For further reading on this topic see the following publication:
Gelatin on Mica Surfaces, J Phys. Chem. 94, 4611-4617.
What is the origin of the word "muscovite"?
The name origin is from "Muscovy-glass", a name at one time used for this
mineral species because of its use in the former Soviet Union, particularly
in the area around Moscow, windows.
Important characteristics of muscovite mica:
For many applications the ease of cleaving, or "cleavability" is directly related to
the perfection of the crystals. The higher quality muscovite cleaves the
easiest, and into the thinnest sheets and with the fewest imperfections that
could be seen at the atomic or near atomic scale with AFM or TEM
instrumentation, for example. Qualities lower than V-5 should not be used
in analytical applications.
Color considerations:
In nature, mica has the color of either white, silver, yellow, green or brown, and often times has
a vitreous to pearly luster. The cleaved sheets have good optical clarity, the higher the transparency
being associated with the higher the quality of the mica.
Some times the term "green mica" or "ruby mica" is heard. These are terms that refer to the "tint" of
the mica.
So-called green mica and ruby mica come in various tints which can be as dark as bottle green or as red as rust.
Obviously as the mica is split to thinner thicknesses, these shading become less apparent and for most users,
unimportant.
ASTM D-351 which classifies and grades qualities of mica, classifies and grades based on properties not based
on color or tint. Green mica is primarily available in South India (Madras Andrah area) and ruby mica primarily
in Northern India (Bihar) although varieties do crossover. So knowing you have a certain tint does not
unambiguously tell you the precise origin of the mica.
Generally speaking since the mica pieces sold by SPI Supplies are so thin, none really exhibit any real tint
so for the typical product of SPI Supplies, this entire page of discussion is not relavent. But upon special
request, we can supply any of our listed mica products in either green or ruby tint for a pricing premium,
due in part to the special order nature of the request but also because we would be doing a very short run
of the cutting of the mica into smaller pieces.
Mica grading:
While muscovite mica is found in a number of locations in the world, there are only a few locations that
have consistently good, high quality material of the V-4 and higher quality grades. SPI Supplies consistently
obtains its mica from only one source, which has proved to be highly reliable and consistent over the years.
We realize that there have been numerous misunderstandings over the years, caused by suppliers offering material
without disclosure of the grade. We are hoping that at least some of these past misunderstandings can be
clarified by the information contained in these pages.
Applications for lower quality grades of muscovite mica:
This form of mica is used in the manufacture of fireproofing and insulating
materials and ground up into a fine powder, it finds applications as a dry
lubricant. But for these applications, generally grades lower than V-5 on
the quality scale are used. SPI Supplies is not a provider of these lower
and less expensive forms of muscovite mica.
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