Mineral of the Month--December
Delaware Muscovite Mica
Potassium Aluminum Silicate Hydroxide Fluoride
Welcome, friends of
mineralogy! Our Mineral-of-the-Month has us visiting a common, yet
underappreciated phyllosilicate: Delaware Muscovite Mica.
Our mica books aren't found in the library,
but in the surface rock of Delaware. We are so
fortunate to get a break in the cold and nippy weather--it's a bright
and sunny day! Put on your
sunglasses, warm coats and hats. Let's go!
One common mineral that remains a favorite with many of us, especially
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
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?
So come on along, and let’s enjoy a brief virtual
As we continue with our Delaware Mineral
Series, our club's goal is to familiarize our friends
with the most easily recognizable minerals first. Mica is one of
our easiest to identify in the
Also, Mica is one mineral that can occur in all
three types of rock: igneous, sedimentary,
We will cover the igneous variety this month.
So, grab your walking
sticks, and let's hike!
Quarry Trail, Wilmington, Delaware
(Photo by Ken Casey)
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.
Mica Group Mineral
Information at mindat.org)
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.
Muscovite is a light, nonferromagnesian silicate. As most silicates are igneous in
Delaware variety is no exception.
Delaware Mica lying in the leaves
||Mica books in Delaware pegmatite
|(Photos by Ken
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."
Dictionary at infoplease.com)
It's name derives from
φύλλον phyllon, leaf". The formation of
silicate tetrahedra in
parallel sheets has a general subformula Si2O5, this is Silicon:Oxygen
in a 2:5 ratio. This leafy
structure shows its cleavage along the weak crystallographic axes.
"Silicate Minerals: Phyllosilicates" at wikipedia.org)
model of Mica atomic layers
||Layered Mica Crystal
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
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
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
extended in two directions to form
a sheet of silicon tetrahedrons." It is the sharing of
between layers that allow for this ratio.
|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."
|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
is simple. At this weakest link we find opportunity to cleave a
mica book with our fingers. How
relatively strong is that?
model of a 2:1 mica block
||Yes, we can peel sheets of mica
with our fingers.
of Terence H. Cooper)
||(Photo by Ken Casey)
To explain, "The micas
contain oxygen in octahedra as well as in tetrahedra, with both
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,
splitting may occur."
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
minerals. An interlayer or the space between the sheets becomes an
between 2:1 and 1:1 clay minerals. Smectite and
Vermiculite are two kinds of 2:1 clay minerals.
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
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
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
Its electrical insulating properties may be due to the gaps between the
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
speculate a great deal, if one were into science-fiction or
were a materials scientist, eh?
This 1943 Coleman lantern used mica more simply.
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
Our next stop is 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.
glass on the ground
||Broken mica books and sheets on
|(Photos by Ken
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.
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
|(Photos by Ken
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
locale for a moment. There is also the West Branch pegmatite found at
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.
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:
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
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
"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!
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
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
Virtual Tour. I particularly liked the
Pegmatite tour, which includes a nice
from Brazil. The museum even hosts a
Gem and Mineral Show every October.
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
So, why not plan a visit to both in the next few
weeks. You'll be glad you did!
Muscovite Mica may have been used on a small scale for lantern glass and
Other than that, I suppose
we just collect it for it's fun properties, and more.
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
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
Makers, Inc. of Long Island City, New York.
So, mica is still used in historic restoration today.
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
Neighborhood Rocks: Mica from Pegmatite
Quarry: A GeoAdventure in the Delaware Piedmont
Delaware Minerals List
Geologic Time Scale (USGS)
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.
Arthur Koch, DMS Member, B. S. in Geology,
would like to gratefully acknowledge the generous contributions of our
Muscovite Mica enthusiasts, collectors, authors, curators, professionals, and
club members who
made this work possible.
Arthur Koch, DMS Member, B. S. in Geology,
©2007 All contributions to this
article are covered under the copyright protection of this article
and by separate and several copyright protection(s), and are to be used
for the sole purposes of
enjoying this scholarly article. They are used gratefully with
express written permission of the
authors, save for generally-accepted scholarly quotes, short in nature,
deemed legal to reference
with the appropriate citation and credit. Reproduction
of this article must be obtained by express
written permission of the author, Kenneth B. Casey, for his
contributions, authoring, photos, and
graphics. Use of all other credited materials requires permission
of each contributor separately.
Links and general contact information are included in the credits above,
and throughout this article.
The advice offered herein are only suggestions; it is the reader's
charge to use the information
contained herein responsibly. DMS is not responsible for misuse or
accidents caused from this
article. All opinions, theories, proofs, and views expressed within this
article, and in others on this
website, do not necessarily reflect the views of the Delaware
Delaware Piedmont Geology including a guide to the rocks of
Red Clay Valley
by Margaret O. Plank and William S. Schenck
About the Author:
Ken is current webmaster of the Delaware
He has a diploma in Jewelry Repair, Fabrication &
Stonesetting from the Bowman Technical School, Lancaster,
PA, and worked as jeweler.
He has also studied geology at the University of
And, he is currently a member of the Delaware
Mineralogical Society and the Franklin-Ogdensburg
Mineralogical Society. E-mail:
Invitation to Members
Want to see your name in
print? Want to co-author, contribute, or
author a whole
Mineral of the Month
article? Well, this the forum for you!
And Members, if you have
pictures, or a story you would like to share,
please feel free to offer. We'd like to
post them for our mutual enjoyment.
Of course, you get full photo and author credit,
and a chance to reach other collectors,
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that you check your facts, give credit where it
is due, keep it wholesome for our Junior Members
watching, and keep on topic regarding
You don't even have to be
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work together to publish your story. A
handwritten short story with a Polaroid will do.
If you do fancier, a text document with a
digital photo will suit, as well.
Sharing is the groundwork from which we can get
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club's webpages can reach any person surfing the
net in the world, and even on the International
Space Station, if they have a mind to view our
are hoping for a possible tie-in to other
informative programs upon which our fellow
members might want to collaborate. Contact
any officer or board member with your
will be a surprise.
For 2008, we are waiting for your suggestions.
What minerals do
want to know more about?
Most of the
Mineral of the
selections have come from most recent club
fieldtrips and March Show Themes, and from
inspriring world locales, and suggestions by our
members, thus far. If you have a
suggestion for a future
Mineral of the
Month, please e-mail me at:
email@example.com, or tell me at our
the Fossil Exposures in the Chesapeake Bay Area" presented by Dr. Lauck
General Club Meeting:
We are meeting at
*Show Committee Meeting, April or May, 2013
*New Home/Lapidary Committee, 2013
*Board Meeting, April, 2013
Next Field Trips
DMS March Show
March 1-2, 2014 at DelTech Stanton
Our 2013 Show
Theme was: "All
That Glitters is as Good as Gold!"
March Show 2013
also commemorates our 50th Show!
It's shiny, yellow, and
is a symbol of 50 Years!Can you guess?
Brook Crick Goethite" by Joe Dunleavy