One common mineral that remains a favorite with many of us, especially elementary school
students, is Mica.  Why?  Many folks seems to enjoy it's sparkle, peelability, and bendability.  I
like it because it occurs with some of my favorite gemstones, such as Garnet and Beryl in our
own state.

     Much of our mica is found in coarse granite pegmatites, and some does make up our local
schists and gneisses. We can even find mica in our streams and sands.  It reflects sunlight,
and is easily spotted on the ground for fast retrieval.  Why not pick up a few books today?

    Also, Mica is one mineral that can occur in all three types of rock: igneous, sedimentary,
and metamorphic.

     We will cover the igneous variety this month.  So, grab your walking sticks, and let's hike!  

     The word "mica" derives from the Latin micare, meaning "to shine or to glitter".  Muscovite
Mica is named after the Muscovy region of Russia, where it was first mined as "Muscovy Glass". 
Muscovite appears in the scientific literature about 1850 (pre-IMA).

     Other names for mica from Europe are: Cat-gold, Cat-silver, Glist, Katen-silber, Katzen-silber,
Or des chats, and Glimmer.

 (Source: Mica Group Mineral Information at mindat.org)

Chemistry & Science

     Most Muscovite is silver in color, and can be peeled into thin clear sheets. It is a phyllosilicate,
which means it occurs in many layers, like leaves or pages in one crystal. Delaware mica is the
same. Found in granite pegmatites, on the Moh’s Hardness Scale, it ranges from 2-2.5.  Our
Muscovite is a light, nonferromagnesian silicate.  As most silicates are igneous in nature, our
Delaware variety is no exception. 

Shiny Delaware Mica lying in the leaves		Mica books in Delaware pegmatite
(Photos by Ken Casey)

     A phyllosilicate is "any silicate mineral having the tetrahedral silicate groups linked in sheets,
each group containing four oxygen atoms, three of which are shared with other groups so that
the ratio of silicon atoms to oxygen atoms is two to five."

  (Source: "phyllosilicate" Dictionary at infoplease.com)

  (Source: "Silicate Minerals: Phyllosilicates" at wikipedia.org) 

Computer model of Mica atomic layers		Layered Mica Crystal
(Computer model courtesy of Robert Harter, UNH)		(Drawing by Ken Casey)

     It has a chemical formula of: KAl₂(AlSi₃O₁₀)(F,OH)₂, which varies in Silicon dioxide to
Aluminum hydroxide ratio.  It is a potash mica, which is predominately potassium joined with
Aluminum.  The potassium bond charge is what holds the layers together.  Also, this variability
means that there is a possibility of Fluorine in some Delaware mica rock, hinting at it's igneous
origins.  How the layers bond together is the key to its cleavage and structural properties.

     Muscovite's layer and lattice bonds differentiate it from other silicates.  For example, "[i]f each
of the four oxygen ions bond with two silicon ions the result is a QUARTZ crystal. In the
phyllosilicates only one plane of oxygen ions bond with two silicon ions as indicated at right. This
bonding is extended in two directions to form a sheet of silicon tetrahedrons."  It is the sharing of
bonds between layers that allow for this ratio.

  (Source: http://pubpages.unh.edu/~harter/muscovite.html)

Mica atomic structure diagram (Note the large potassium ions joining two layers.) (Courtesy of Sabine Grunwald, University of Wisconsin-Madison )

     These leaf-type layers form in successions of bonded ions.  "Between the layers of K ions we
see successive layers of oxygen, silicon, oxygen, aluminum, oxygen, silicon, and oxygen. This
structure is refereed to as a "mycelle". The thickness of the mycelle being formed is about one
nanometer. On the a- and b-axes the mycelle will usually extend 10 nanometers or more."

  (Source: http://pubpages.unh.edu/~harter/muscovite.html) 

More detailed 3-Ddrawing of mica atomic structure with bonds denote by lines (Courtesy of L. G. Berry, B. Mason, R. V. Dietrich)

     Though the mycelles are complex, the aluminum silicate layer bonding by potassium ions
is simple.  At this weakest link we find opportunity to cleave a mica book with our fingers.  How
relatively strong is that?

3-D atomic model of a 2:1 mica block		Yes, we can peel sheets of mica with our fingers.
(Courtesy of Terence H. Cooper)		(Photo by Ken Casey)

     To explain, "The micas contain oxygen in octahedra as well as in tetrahedra, with both occurring
in a sheetlike arrangement. Because of the ratio of two tetrahedral sheets to each octahedral sheet,
the micas are called 2 : 1 layer minerals...In muscovite one-quarter of the silicon ions is substituted
by aluminium ions in the silicon tetrahedral layers. This imbalance in charges is satisfied by
potassium, which bonds the composite sheets together...The negative charges occurring in this
sheet are neutralized by potassium, which bonds the composite sheets together. The potassium
is positioned in the interlayer space between neighboring layers. The potassium bonding is weak,
where splitting may occur."   

  (Source: http://www.soils.wisc.edu/courses/SS325/primary.htm)

     In our clays, "[t]he basic structure of 2:1 clay minerals is two silicon tetrahedral layers and one
aluminum octahedral layer. This layer is weakly held to another 2:1 layer to make the 2:1 family of
clay minerals. An interlayer or the space between the sheets becomes an important difference
between 2:1 and 1:1 clay minerals. Smectite and Vermiculite are two kinds of 2:1 clay minerals.

  (Source: http://www.soils.umn.edu/academics/classes/soil2125/doc/s12ch1.htm)

Photo of orange-brown New Castle County, Delaware clay (Photo by Ken Casey)

 
     We can learn more about micaceous clays and soils during our lunch break.  Let's check the
University of Wisconsin's Virtual Soils Museum for more.

     Well, my peanut butter and jelly sandwich was good.  I hope your lunch was as tasty.  So,
let's get back to rock mica properties.

     We'll start by noting that micas are the most prevalent primary sheet silicate rocks, especially
in the First State. 

     Delaware Muscovite is our most common mica, also known as white mica. A good field
identification is to try peeling the thin layers away. If each layer appears clear and silvery, it is
muscovite mica.  We can see it's almost perfect basal cleavage in one direction as we peel it.

		Our state’s mica was not used commercially, but if it were, it would have been used as window glass in cast iron stoves, and as insulators in electrical equipment.  Another name for Muscovite is "isinglass", a Dutch derivation, which alludes to it's glasslike properties.  Who knows, perhaps some local antiques have Delaware Mica as one of their parts, such as a portable lantern.      Our friends at mindat.org offer a Dana reference (Dana 6:1070) to an occurrence at historic Way's Quarry in Centreville, Delaware.  Perhaps mica was mined here in the past on a small scale.      Its electrical insulating properties may be due to the gaps between the silicate layers, with potassium interfering with electron conductivity.  Ironically, potassium balanced with sodium in organic life serve as electrolytes to power the movement of muscle cells.  Perhaps one of you might invent a photovoltaic cell based on such crystallography. One could speculate a great deal, if one were into science-fiction or were a materials scientist, eh?    This 1943 Coleman lantern used mica more simply.
Coleman Lantern, mica globe (Courtesy of Terry Marsh, NCC)

     The purest muscovite from around the world is used as a substrate for the deposition of
scientific materials.  Does Delaware Muscovite fit the bill.  I do not know.  Perhaps one in our
group today might research this avenue for the future.  Happy researching!

     Our next stop is mica geology. 

   

Some Delaware Mica Geology

     Where do we find Delaware Muscovite Mica?  Well, look around.  Most of what glitters on
the ground is usually mica.  Be careful, though, sometimes it might be some broken glass.  So,
be careful picking it up, especially our younger fieldtrippers, please.

Broken auto glass on the ground		Broken mica books and sheets on the ground
(Photos by Ken Casey)

 

		When we do find a silvery piece to pick up, we could readily identify it in the field.  It may be attached to other minerals, or stand alone as free sheets or books, separated from it's pegmatite matrix by erosion.  Mica does get carried away easily by water and wind, due to its lightweight and flat characteristics, unlike most minerals.  That is why we see it strewn in sands, or down gullies.
Path and erosion gully with mica  (Photo be Ken Casey)		Mica in pegmatite and strewn about the forest floor  (Photo by Ken Casey)

 

      How can we further recognize mica?  Thin pieces look clear; whereas, thick pieces appear
more silvery.  You can bend thin pieces, due to their elasticity.  You can see sunlight right
through them--a rarity in the mineral kingdom!  And, thick pieces can be opened like a book, and
split into thinner ones by hand.

Mica sheets can bend.		One can see light through them.		One can peel the "pages".
(Photos by Ken Casey)

 

     Due to mica's easy cleavage, it may found on the surface, or buried beneath the earth in
pegmatites or metamorphosed gneiss and schist.  We'll focus on the "pegs" this trip. 

  The best place to see mica is in northern Wilmington.  Let's visit the Woodlawn Quarry
locale for a moment.  There is also the West Branch pegmatite found at Newark, Delaware,
which I believe is covered up by houses by now.  A third locale is the Mt. Cuba rest area;
this is a picnic stop for the Wilmington & Western Railroad, so we'll have to buy some tickets. 
All aboard!

     As we travel the line, let's peruse some photos of Woodlawn Quarry's mica and talk a bit
about it's geology.  (You'll have to take your own photos when we reach the picnic grove, as
the line is just recently opened after flood damage to the bridges has been repaired.  Let me
know how you make out.)  Anyway, here's some mica photos:

(Top Left): Muscovite with Garnet, Woodlawn Quarry (Photo by Arthur Koch) (Other photos): Micas from Woodlawn Quarry (All other photos in this frame by Ken Casey)

     Most of Delaware's larger mica is either locked up in coarse-grained pegmatites (as seen
at the above three locales), or eroded out into the surrounding soils at the surface.  Some
smecktites (clay soil phyllosilicates) resemble mica on a near microscopic scale.  And, true
mica appears in our schists, albeit small, yet can be viewed with the naked eye.

     Mica is also stuck to larger mineral grains that can be red, pink, white, gray, green, black,
or blue in color.  In Delaware, those grains are usually garnet, quartz, beryl, and tourmaline. 
So, mica geology is also pegmatite geology, for the most part.  We will visit pegmatites in
detail in February and March of 2008.  In fact, the theme to our March 1-2, 2008 Show is
"Pegmatite Gems and Minerals!".

     We'll just keep it easy this trip, and focus on our observations of nature.  Gear up for our
February and March excursions, as we'll hit the ground running with our vast foray into the
world of pegmatites!

Two Museums of Note

     This month's museums are the University of Waterloo's Earth Sciences Museum in Canada
and the New Jersey State Museum in Trenton.  Both have extensive natural history and mineral
collections from their respective geographic areas.  The NJSM is nearer our clubhouse at about
60 miles; whereas, Waterloo's virtual museum tour can bring us closer.

     Neighboring us in Ontario, Canada on the campus of Waterloo University is the revered Earth
Sciences Museum, which is open to the public free of charge, though donations are welcome. 
Group tours are available by request.

     The staff graciously invite us to special activities, like lectures and talks.  They also offer an
online Virtual Tour.  I particularly liked the Pegmatite tour, which includes a nice Mica specimen
from Brazil.  The museum even hosts a Gem and Mineral Show every October.

     The New Jersey State Museum offers an historic collection of the state's iron, copper, zinc,
clay, and sand mining industries.  It also houses a vast fluorescent mineral collection of Franklin
and Sterling Hill minerals.  They are also engaged in active scientific research.

     So, why not plan a visit to both in the next few weeks.  You'll be glad you did! 

  

Uses

     Delaware Muscovite Mica may have been used on a small scale for lantern glass and stove
windows.  Other than that, I suppose we just collect it for it's fun properties, and more.

		On a recent visit to Longwood Gardens, I spotted this fancy lantern.  As my family and I always like to look for something new and different, our fieldtrip that day had me looking up.  Almost immediately, I found this new looking antique hanging lamp.      Upon asking, I learned that this renovated lighting fixture in the main conservatory actually had mica as its diffuser.  Notice how the ambient glow in this exquisite bronze fixture resembles that of the mica crystal sheet above.        It was designed by Longwood's designer, Tres Fromme, and renovated by Lite Makers, Inc. of Long Island City, New York.      So, mica is still used in historic restoration today.
Longwood Gardens mica lantern (Photo by Ken Casey)

Sidelight: Even our State Mineral Sillimanite might be associated with our Mica, though not in
pegmatites.  Yes, our collections can be varied and wide by making mineral associations.

 

Links

Neighborhood Rocks: Mica from Pegmatite 

Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

http://webmineral.com/data/Muscovite.shtml

Delaware Minerals List at mindat.org

Geologic Time Scale (USGS)

 

Members' Gallery

     Here is where DMS Members can add their Delaware Muscovite Mica photos to share with us.

    

Until Next Time

     We hope you have enjoyed our historic visit to Delaware Muscovite Mica.  Please join
us next month, for another article, and we shall journey together!
     Until then, stay safe, and happy collecting.

 

 

Article Contributors

Arthur Koch, DMS Member, B. S. in Geology, Mineral Photographer

Delaware Piedmont Geology by Margaret O. Plank and William S. Schenck, DGS

 

Photo & Graphics Credits

    I would like to gratefully acknowledge the generous contributions of our fellow Delaware
Muscovite Mica enthusiasts, collectors, authors, curators, professionals, and club members who
made this work possible.  Thanks.

Arthur Koch, DMS Member, B. S. in Geology, Mineral Photographer

Robert Harter, University of New Hampshire, Department of Natural Resources