mineralogical friends! This month, our
to protected parkland in New Castle County for any easy downhill hike.
Since we've been
inactive over the summer, this short jaunt will give our muscles a
chance to acclimate to
hiking and climbing, once again.
We've got a busy schedule. The weather promises to be
bright and sunny today, so
boast garnet as a draw for rockhounds. Delaware is no different,
the facility to collect them. Most Delaware garnet occurs in public
from it’s host rock over time.
We can, however, take our pick of geology
hikes to view them in situ. Or, as the Delaware
Geological Survey has organized itineraries for us, called
from the Survey's suggestions, coupled with our club's and this author's field
Our article will touch upon some of the
mineral assemblages in our state, which contain
garnet. In addition, we will take a virtual tour of one area, north of
Wilmington, which exhibits
the most prolific of Delaware’s garnet landscapes.
Our local red
garnet is both easy to locate and identify in the field, so it makes a
choice to open a new season of fieldtrips. As many states brag
(and deservedly so) upon their
own garnet reserves, I believe that disseminating Delaware's
contribution to the "garnet club" is
worthy of pursuing here. And, we get to breathe the fresh country
air of the Diamond State, while
seeking out our own garnet glory. Please do join us!
with a simple trail off of the entrance to the parks, such
as to our
Brandywine Creek State Park, north of Wilmington,
we can quickly find specimens to observe.
grab your walking sticks, and let's hike!
Easy to find garnet in host
(Photo by Ken Casey)
What's in a name?
Garnet = Pomegranate? A curious
reference. According to wikipedia.org, “[t]he name "garnet"
possibly a reference to the Punica granatum ("pomegranate"),
comes from the
a plant with red seeds similar in shape, size, and color to some garnet
Much of Delaware garnet is a variety called
“Almandine”. “The name Almandine is a corruption of
a region in
where these stones were cut in ancient times.”
“Almandine, also known incorrectly as
almandite, is a species of mineral belonging to
Group. The name is a corruption of alabandicus, which is the name
Pliny the Elder
to a stone
found or worked at Alabanda, a town in Caria in
Greek mythology and historical accounts
document its use as a gemstone and abrasive. In such
literature, garnet is steeped with cultural references, from whence we
derive its modern name.
The name “garnet” was grandfather prior to
1959 by the IMA.
Just for the sake of demonstration here (and
I’ll bring the
pomegranates for our snack.
Delaware's garnet is mainly Almandite.
Almandite is an iron-rich
nesosilicate of chemical formula
. It shares a
solid solution series with
formula: Mg3Al2(SiO4)3. That
means that at some time during its formation, Iron can replace
Magnesium, or vice-versa.
Almandite is the harder of the two. On the
Moh’s Hardness Scale, it ranges from 6.5-7.5. It’s relative
hardness makes it ideal for commonly found abrasives, as are used on a
simple sheet of everyday
modern garnet sandpaper
is manufactured by crushing
larger garnet crystals, let
forget the garnets that make their way into
sand via erosional processes. Before sandpaper,
garnet sands were used. Without being abrasive, might I suggest
that we can still use
sediments and rocks as tools to learn about paleo-environments, as "[s]edimentary
the record of the earth's ancient surface
environments, and of life."
In the photos below, garnets are likely to be
found in the river
or run sediments, but not in
the Devonian rock above
here at Red Hill. As the
Appalachian mountain chain is a mish-
currently weathering outcrops, two significant anciently-captured
environments can lay side
by side for us to see: crystalline mineral beds and sedimentary fossil
of Appalachian mountains and plateaus overlooking Susquehanna
River at Red Hill, Hyner, PA (Photos by Ken Casey)
Delaware has a similar surficial geology
boundary: crystalline Piedmont (to the west) and
some Chesapeake Group's fossil-bearing marine sediments (to the
east), albeit of differing
(yet analagous) eras. Its crystalline Lower Piedmont is of most
interest to us here.
Garnets are useful indicators to geologists
to determine both the metamorphic facies and
paragenesis of rocks. It’s slow rate of elemental diffusion, relative to
other indicator minerals,
aids scientists in the determination of the temperature-time history of
the host rock.
What can metamorphic garnet tell us?
Generally, its composition and texture tell all. Chemical
composition alludes to its original rock type. Mineral
composition of its assemblage denotes temperature and pressure
conditions of its formation. And, texture observations
speak to its degree of change.
Garnet-laden drill core samples
(Photo courtesy of Oliver Holm,
For example, one can
thermobarometry and microthermometry data from individual
mineral grains in an array of samples to determine the extent of
metamorphic events. As most
garnets studied are metamorphic, we know we can find this type at sites
of orogenic changes,
or mountain-building events. So, let's study our local foothills
Our Piedmont province was originally formed
by the Taconic Orogeny, during the Middle
Ordivician. Several intervals of mountain-building occurred since
then, until the most recent
Appalachian (or Alleghanian) Orogeny about 300 mya in the
Geologic Time Scale (USGS)
Therefore, in this highly active area of
geology change, garnets form, are eroded into
sediment, an have formed again in various events over millions of years.
geology plays host to much scientific study. (Garnetology,
So, knowing our garnets can help us to
identify and understand our local mineral assemblages.
University of Delaware Garnet virtual tour
Gneiss is the primary
rock type which hosts garnets. From microscopic
to macroscopics the size of a dime, these metamorphic dark-red silicates
may be found in our area.
(Since geology knows very few political boundaries, the formations in which
garnet occurs at the surface
stretch into nearby Pennsylvania and surrounding states, as well.)
|Almandite Garnet crystal
Photomicrograph, South Carolina's Appalachian Piedmont
||Almandite Garnets the size of dimes in
Brandywine Creek State Park
Harmon Maher ; Photo by Ken
The smallest grained gneiss
is best viewed under a microscope from a fresh core sample. For
example, they are within documented survey materials collected by the
largest may be perused upon by us on our hike to small outcrops and
streambeds in our local parks.
Our eastern Piedmont
runs northeast-southwest, the entire length of the middle-Atlantic
coastal area, intersecting at nine states and the District of Columbia.
So, of course, similar garnet occurrences can be found in other states.
Our region is marked with outcrops, exposing a modest wealth of garnet
viewing areas. We will concentrate on our area’s
Generalized Piedmont Map; Piedmont is Tan
(Courtesy of Karl Musser, Cartographer, wikipedia.org
According to DGS publication
Delaware Piedmont Geology by Margaret O. Plank and William S.
Schenck, “[m]ost Piedmont garnets are a dark-red, iron-rich variety
called almandine. They usually occur as 12-sided crystals that vary in
size from crystals so small they can be seen only under a microscope to
crystals of an inch or more across. Garnets are considered semi-precious
stones, but in the highly deformed rocks of the Piedmont they are
usually fractured and not suitable for jewelery. Garnet is also used as
More specifically, Delaware
garnets can vary in grain size and degree of metamorphism, to which
one could study for a long time, hence this article. For example,
Delaware “[g]neiss is a course-
grained rock commonly having imperfect, but prominent light-dark
layering. In the Delaware Piedmont
the light layers are composed of feldspars and quartz and the dark
layers of mica, garnet, sillimanite,
amphiboles, and pyroxenes. Gneisses are formed by the high-grade
metamorphism of either igneous
or sedimentary rocks."
A rare igneous garnet is
found in the pegmatite of the
Woodlawn Quarry. The locale may be
in the red area of the
Generalized Geologic Map of Delaware, below.
Why not try this one:
Quarry: A GeoAdventure in the Delaware Piedmont
Generalized Geologic Map of Delaware, courtesy of the Delaware
Prepared by: Nenad Spoljaric and Robert Jordan, Revised by: Thomas E.
|Physiographic Map of
Pennsylvania, Pennsylvania Geological Survey
Yes, right in our clubhouse’s
own backyard, garnets taunt us for our attention. In nearby
Brandywine Springs Park, tiny 1-2mm pink-purple garnet grains may
be found in gneiss. No
is allowed here, but some may be observed, if you know what
to look for.
garnets at BSP
||Close-up of tiny garnets at BSP
|Photos by Ken Casey
As BSP is adjacent to our clubhouse at
Historic Greenbank Mill,
and drilling studies have been
conducted near our main intersection at Price’s Corner, we can learn
from work by the DGS here.
For example, a 66-foot core sample yielded a lithology of biotite
gneiss. As a major mineral,
microscopic garnet was reported at a 1% mode. A facsimile of the
report is below:
The fabric and textures noted in the
rock core of sample 38 contain garnets as: “Garnet: Tiny xenoblastic
University of Delaware
Delaware Geological Survey
Open File Report No. 38, June 1995
Data Report on Rock Cores from Red Mill Road, Harmony Road,
Prices Corner, and Newport, Delaware
DGS ID: Cc13-17 SAMPLE NO.: 24896 QUAD: WIS
FIELD NO.: S-8-1 DATE ENTERED: 3/3/93
LOCATION: Prices Corner - core, 66' (58' ASL)
ROCK UNIT: Wilmington Complex ORIENTED SEC.:
STAINED: K-feldspar: Y Plagioclase: Calcite: Cordierite:
LITHOLOGY: Biotite gneiss
MAJOR MINERALS MODE (%) ACCESSORY MINERALS
quartz 38.0 zircon, apatite
plagioclase 38.0 monzanite halos in biotite,
biotite 22.0 colorless to yellow
opaques 1.0 RETROGRADE MINERALS
sillimanite mats x
Pale green mineral with opaques
NUMBER OF POINTS COUNTED: 400
The rock in this core is a fine-grained dark biotite gneiss with
biotite grains aligned vertically.
FABRIC AND TEXTURES
Plagioclase: Equant xenoblastic grains, partial twinning,
round inclusions of quartz and plagioclase
Quartz: Undulatory extinction; large subgrain boundaries with
Biotite: Pleochroism is light brown to dark brown; laths have a
preferred orientation and are aligned to define the foliation
Garnet: Tiny xenoblastic to subidioblastic garnets grow over
other grain boundaries; some with small inclusions of opaques;
one garnet is elongated in the foliation
Opaques: Irregular shapes; two different opaques; in
reflected light, one is dark and the other is silver
to subidioblastic garnets grow over other grain boundaries; some with
small inclusions of opaques; one
garnet is elongated in the foliation”
If you like the smaller end of the garnet
grain range, then these are the specimens for you.
If, on the other hand, you prefer to view larger garnets in the field,
we’ll pay a brief visit to the defunct
Woodlawn Quarry, also in northern Delaware. Now a wildlife refuge with
hiking trails, one can still find the
occasional surface rock. No collecting is generally allowed here, so
please enjoy nature. Perhaps now is
a good time to take our morning break, while some of us photograph a
specimen or two. Let’s focus upon
a description before clicking our shutters.
|Close-up of 3mm Garnet
crystal in graphic granite, Woodlawn Quarry
||Garnet in Graphic Granite (Feldspar, Quartz,
Muscovite), Woodlawn Quarry
|Photos by Ken Casey
As a general description of garnets here,
Woodlawn Quarry GeoAdventure states that,“[g]arnet
occurs here as tiny dark red crystals with 12 sides, called a
dodecahedron. The crystals are rare and small
and it is necessary to look carefully to find crystals. The garnets are
hard, have a glassy luster and no distinct
cleavage. When broken they look like dark red glass.”
Garnet crystals here occur in granite
pegmatites, along with feldspar, mica, quartz, and beryl. That's
the garnet we'll see here; so, take your last photos and field notes,
Mount up, everybody. It’s time for the
climax of our virtual fieldtrip: our visit to Brandywine Creek State
Here is where the largest garnets may be found. We will soon feast our
eyes on a few of these lithic beauties.
We can actually hike from Woodlawn downhill to BCSP.
Then, voila, we’re there. Fast, healthy, and little
emissions. Let’s pause for a quick drink for hydration, before we take
in the park’s garnetous splendor.
|BCSP Entrance sign on
||Thompson's Bridge near
||Trailhead bridge at BCSP
|Photos by Ken Casey
Note the slow, yet steady, moving Brandywine
Creek. It is home for wildlife, such as ducks, geese, fish,
and a variety of water plants. Feeding into the creek are smaller
streams, called “runs”. These runs are where
we will find our garnet-laden rocks. So please, tread lightly, as we
traverse the woods, through the sticker
bushes up to the smaller waterway’s edge.
|Delaware Greenway sign,
||Rocky Run video with
|Photos and video by Ken Casey
Delaware Greenways Project expands,
we’ll be able to hike directly to other geologic locations
rather efficiently on future MOTM fieldtrips. Thanks to the State of
Delaware and its partners, our educational
and recreational experiences will be enhanced.
Now, to the park-at-hand. Follow me.
I’ll guide us with the
DGS's GeoAdventure directions, so that we
may tread lightly, and leave the squirrels and woodpeckers to their
business. Don't worry, we'll see garnets!
Our hike has brought us downhill along
SR-92S, then we took a breather in the parking lot, near the park
entrance sign. As we face the trailhead, look up the hill to our left.
We will see trees and shrubs
growing over moss and fungus covered boulders. Many of these
rounded remnants are filled with
discernable garnets. Let's take a closer look.
|Rock outcrop, west of
Rocky Run, BCSP
boulder at BCSP entrance field
||Another garnet-filled boulder at BCSP
|Photos by Ken Casey
Next, we'll hike about 100 yards to “[c]ross the boulder field, turn left, and walk toward Rocky Run.
for a wall of rock bordering the northwest side of Rocky Run (D,
Figure 2). Wissahickon rocks
form the wall and the
streambed while the rounded boulders of
gneisses clog the
stream, litter the southeast banks and lie scattered
in the flood plain. The layering in the Wissahickon
wall rock is
irregular and defined by
stringers of garnet, biotite and sillimanite in
a mass of quartz and
feldspar. The garnets are dark red, either oval or
round, and may be as large as three quarters of an
inch in diameter. The
stringers, and any folds that are present, are best seen by standing in
and looking upstream. The contact between the Wissahickon and
Wilmington rocks is hidden beneath
the flood plain.
|Rocky Run at BCSP
||Boulder field at Rocky
||Close-up of garnets in matrix
|Photos by Ken Casey
"To see the contact, you need to follow the stream to the confluence
of Hurricane Run and Rocky Run
and stay on the northeast side of Rocky
Figure 2). The exposed contact is difficult to recognize
and probably interesting only to geology students at the high school or
college level. It is exposed in a ten
foot area along the northeast side
of Rocky Run where dark, fine grained
interlayered with light colored Wissahickon gneisses. The
Wissahickon rocks appear to have been melted
and recrystallized to form
granites with thin layers of garnets. The biotite and sillimanite that
occur in the
Wissahickon gneisses are replaced by tiny garnets. This
reaction in which garnet replaces biotite and
sillimanite occurs only at
very high temperatures. The
layers vary in thickness between
3 inches and 2 feet, and are dark
solid, massive rocks.”
Let's zoom in on our gneiss garnets.
|Large red garnets in
Rocky Run rock, BCSP
|Photos by Ken Casey
Now that we have sampled views of some of the largest
garnets in Delaware, let's have lunch. I'll trade
you an extra pomegranate for half of your peanut butter and jelly
sandwich. We can discuss other garnet
experiences over dessert, if you like.
Of course, our club visits collecting sites
for garnet all around our area. Please do inquire upon how to
join us as members, and can benefit from
club fieldtrips. Our trips our
setup and led by our own Bob Asreen,
Geologist, and Vice-President of Fieldtrips.
(Top): Gem Trails cover by Mark Webber
(Right): Close-up of Wissahickon Valley garnets in schist matrix
from Fairmount Park
Gem Trails of Pennsylvania and New Jersey
by Scott Stepanski and Karenne Snow, suggests that very small,
nicely faced, and deep red Almandine garnets may be found weathered out
of their host schist at Fairmount Park, Philadelphia. This author
visited there about three years ago, and found small quantity on the trail--enough
to fill a thimble or two.
I was fortunate enough to meet Ms. Snow a couple
of years ago at one of the Cape-Atlantic Rockhounds
events. She was kind enough to sign my copy of her book. I
think it brought me luck, since I've had some
good experiences collecting from locations that she and Scott Stepanski
Wissahickon Valley garnets may be collected
from Fairmount Park in Philadelphia. According to the book, garnets wash
out of the surrounding schist, to be collected on the trail.
If you wish an educational experience for
your family, ages 6-8, from one our local nature groups, try:
Delaware Nature Society Digs: Ages 6-8 + Parent
Mining and Minerals
Course #: S07-056-AS Max: 13
Date: Sunday, March 25 Time: 2:00-4:30pm
Member Price: adult $9, child $7
Nonmember Price: adult $13, child $10
Led by David Pragoff
Dig for mineral treasures including mica, quartz, and garnet at a unique
feldspar quarry near the Brandywine
River. Learn why these minerals are here and how they were mined. Bring
your rock hammer or borrow ours
as we head out on a "rock-ing" mining expedition. Van transportation
provided to the nearby site. Parent
Though this information was for a past dig, just click on the
weblink for new digs.
As a side note and segue way into a future
MOTM, I’ll mention that garnet was discovered at the now
closed Brandywine Quarry in Wilmington by Fred Hilbiber in 1924,
according to the Mineralogical Record.
It is associated with two exotic minerals,
Canbyite and Hisingerite--both found in Delaware. (see: below)
two sides to the same mineral? Stay tuned!
Volume 9, pages 1-5, 1924
CANBYITE, A NEW MINERAL
A. C. HAWKINS and EARL V. SHANNON, Rochester,
N. Y., and Washington D.C.
Some thirty-five or forty years ago, when the Brandywine
Quarry at Wilmington, Delaware, was in operation, Mr. Fred
Hilbiber, then living in that city, discovered in this
quarry the minerals here described. The quarry is located a
quarter mile northwest of the Baltimore and Ohio Railroad
bridge on Brandywine Creek. It is at present used as a
storage yard adjacent to a leather manufacturing plant.
The country rock is a gabbro, containing numerous quartz
stringers and masses due to the effects of intrusive granite,
and showing the minerals enumerated below. The minerals
associated with the canbyite are as follows:
Bornite: pure masses, iridescent, weighing altogether several
Chalcopyrite: alone, and associated with bornite and pyrite.
Chalcocite: associated with garnet and hisingerite.
Pyrrhotite: narrow stringers associated with other sulphides.
Pyrite: brilliant simple cubes, 1 to 3 mm. in diameter.
Marcasite: coating surfaces of the rock, decomposing rapidly on
Garnet: fibrous crystalline crusts associated with hisingerite.
Our MOTM format will continue to offer us
information on two places we can visit to learn more
about minerals, such as this month's "garnet". One includes an
online museum, the other offers a
book that represents its collection. Both support a real place
that you can physically visit, if you like.
Hmmm! Who has a collection
of garnets for viewing? Well, the University of Delaware Mineralogical
Museum has it’s virtual mineral tour online of
on Display. Some representative
garnets are housed
therein. Why not take a gander?
“Everything’s big in Texas“, goes the
unofficial motto. The
Museum of Natural Science, located
in Houston, Texas, has a large, world-renowned rock and mineral
collection to back up that claim. They
currently do not have an online photo gallery, but if you visit in
person, I’m told you won’t be disappointed.
The museum has just published a photo book of it’s treasured
Masterpieces of the Mineral World
by Joel A. Bartsch, Mark Mauthner, and Wendell E. Wilson. You can order
it online or via phone to view at home.
Delaware garnet generally has no actual industrial
or lapidary use. If it were found in sufficient
quality and quantity, perhaps our almandite can be employed as an abrasive
material or as a water
It does have use for scientists as an indicator
mineral for the geologic study of our area. And,
of course, collectors and nature observers, such as us, can enjoy its
beauty and scientific interest.
|Exposed garnets from
Brandywine Creek State Park, 2004
Perhaps by now, they are eroded out--who knows?
(Photo by Ken Casey)
Pyroxenes (and amphiboles), Tourmaline and Garnet (UC Berkeley)
Quarry: A GeoAdventure in the Delaware Piedmont
Delaware Minerals List
Here is where DMS Members can add their Delaware Garnet photos to share
has a treat for us--photos of some of the largest, sharpest Delaware
These are from his personal collection. And, he is the
earned his Bachelors of Science degree in Geology, and field collects
minerals. I believe he is also a part-time mineral dealer, as
more about Arthur and his garnets, visit
his photo gallery at mindat.org.
specimens are from the defunct Woodlawn Quarry, Wilmington, Delaware.
|Almandine on Muscovite,
crystal 2.4 cm x 2.0 cm
||Almandine crystal, 2.6 cm x 2.1 cm
|Almandine on Muscovite,
crystal 2.9 x 2.1 cm
||Almandine crystal, 1.5 cm x 1.3 cm
Until Next Time
We hope you have
enjoyed our historic visit to Delaware Garnet. Please join
us next month, for another article, and we shall journey together!
Until then, stay safe, and happy collecting.
I would like to gratefully acknowledge
the generous contributions of our fellow Delaware
Garnet enthusiasts, collectors, authors, curators, professionals, and
club members who
made this work possible.
Arthur Koch, DMS Member, B. S. in Geology,
Nenad Spoljaric and Robert Jordan, Thomas E.
Pickett, Delaware Geological Survey
Gem Trails cover by Mark Webber
©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
Gem Trails of Pennsylvania and New
Jersey by Scott Stepanski and Karenne Snow
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: