we are remaining in our backyard for: Delaware
Feldspar, Part 2: Plagioclase.
No need for long travel or delays. Let's board our club bus now for our Wilmington
fieldtrip! Everyone, everywhere, please join us! Let's go!
fellow mineral trekkers! This month's Mineral-of-the-Month takes
to rural north Wilmington, a sliver of undeveloped parkland on the Delaware-Pennsylvania
Since it never
rains on our virtual fieldtrips here, we only need our backpacks,
lunches, cameras, and collecting & safety gear. So, we're on the march.
a picnic lunch and nature hike north Wilmington in the Piedmont. Enjoy!
Why Delaware Feldspar?
Again, we like our local
feldspar, because it occurs in so much of our rocks, and
within many formations in our piedmont. Also, we can see plagioclase on the surface,
due to both weathering and past mining efforts. Since nature has reclaimed nearly
of our historic mines, a nature hike is an easy way to observe evidences of these past
and present perpetual processes.
Both Orthoclase and Plagioclase were quarried here in Wilmington at the now defunct Woodlawn Quarry from 1850-1910. At
this now private wildlife refuge, run by the Woodlawn Trustees, the area was known for
both William Bancroft's flowering gardens upon reclamation of the site, and as a feldspar/
And, for educational purposes, we can get special permission to visit the site, in order
to show you the leftover mineralization in this beautiful, natural setting.
So, mount up; we're going on a
Entrance to quarry trailhead (Photo by Ken Casey)
Delaware's plagioclase occurs
mainly in coarse pegmatites, as part our piedmont. New
Castle County is the primary locale for outcrops, most hidden by overburden and
also acts as a component in our amphibolites, gabbros, and gneisses.
In Delaware, our Plagioclase either occurs as milky white
crystals in our granite
pegmatites. In our amphibolites and gneisses, it appears as smaller, clear crystals.
And, Red Clay Valley ancient basalt flows have metamorphosed into our amphibolites.
occurs with quartz, mica, garnet, and beryl
in Delaware's graphic granite
by Ken Casey
Our Amphibolite from
metamorphosed members of the Wissahickon Formation contains
hornblende and plagioclase feldspar. In fact, a large boulder removed from
near Routes 72 & 7 was taken to Newark, Delaware in 1988. The current DGS
was built around the boulder! Interesting architectural choice, eh?
crystals in pegmatite
by Ken Casey
It should be noted that our
granites do usually contain both Plagioclase and Orthoclase
feldspars in the same rock material. Identification can be made by the presence of
pink and white crystals with right angle and oblique cleavages, respectively.
amphibole, garnet, and beryl are accessory minerals.
Our intrusive gabbros contain a green plagioclase and
Concerning our gneisses, that of Rockford Park in the
Wilmington Complex at Tower
Hill is [f]ine-grained mafic and felsic gneiss, interlayered at the decimeter scale.
layers contain plagioclase, pyroxene and hornblende, and are commonly boudinaged. The
felsic layers contain quartz, feldspar and less than 10% pyroxene. Original igneous
are obscured by a penetrative foliation and granulite metamorphism. The body of Rockford
Park Gneiss at the highpoint is surrounded by the Brandywine Blue Gneiss, which is overall
more felsic. Foliation dips moderately to steeply northwest...."
In the contact-arc related Wissahickon formation, which
extends into nearby Pennsylvania,
our feldspar is a component in its mineralizations, as well.
"The Wissahickon Formation consists of
predominantly sedimentary rocks (sandstones,
mudstones and siltstones) and minor igneous lava flows that have been subjected to high
temperatures and pressures known as metamorphism. Within the formation are coarse-grained
igneous bodies known as pegmatites, composed of mica, feldspar and quartz; and
metamorphosed iron-rich basaltic and serpentine bodies known as amphibolites and
serpentinites respectively (Plank et al. 2000).
Members of the Plagioclase Albite-Anorthite Group occur in
other geologic settings
nearby. Other interesting pegmatite minerals include: Muscovite, Garnet, Beryl, and
Quartz. (Remember these, as we shall visit them on future MOTM fieldtrips.)
check them out briefly at The
Minerals of Igneous Rocks.
You might want to familiarize yourself at our lunch break with
our Field Guide for this trip:
Woodlawn Quarry: A GeoAdventure
in the Delaware Piedmont.
Fresh from our club's fieldtrip and
nature hike to Ramsey Run and the Woodlawn Quarry,
we have a preview of our experience today. Deją-vu?
|One quarry dugout
||DMS Members on our
by Ken Casey
How were these formed?
The slowly-cooled magma, which crystallized into our graphic granite,
did so within the country rock of the Wissahickon Formation. Due to heat generated
metamorphism, magma formed and intruded, which crystallized into this coarse-grained
The final forms we see today are the angular feldspar grains and some noteworthy
shaped quartz inclusions, which look like Ancient Arabic cuneiform writing.
& Quartz (Delaware Graphic Granite)
by Ken Casey
(orthoclase) rhombohedrons and the chunkier, white plagioclase crystals
inhabit our graphic granite. Clear crystalline masses of quartz and hexagonal,
mica books share the makeup of our local Woodlawn pegmatite. Very small
deep red garnets and pale blue-green massive and hexagonal beryls are accessories in our
granite, as well.
During our club's recent visit to the quarry, we noted that
all of the major minerals listed
in the bulk of scientific literature may be observed there. Since we just made fresh
there, I know we will find these readily on our virtual hike today! Let's look!
Our MOTM format now includes
information on two places you can visit to learn more
about feldspar. Both include an online museum and a real place that you can
visit, if you like.
Penn State University's Earth & Mineral Sciences Museum and
Art Gallery, located at
University Park, Pennsylvania, offers both an online virtual museum and a museum building,
which you can visit. Admission is free to both. At the moment, though, the
under construction. So, check back with them.
Our world renowned museum is the Royal Ontario Museum in Toronto. It's
Microscopy Laboratory experts host an online rock and mineral Thin Section Petrology
Data & Image Bank. Created and run by Robert B. J. Mason and Kay S.
resource allows us to view Plagioclase at a new
As we finish our lunch, let's talk about the reason why they dug holes in the earth over a
century ago to mine out the feldspar.
Since our local geology
is primed for late 19th and early 20th century mining practices,
our abundant surficial feldspar and resultant clays suggested to our predecessors a
from which to make pottery and ceramics--and, yes, dentures. The purest material,
as "dental spar" served many a dentist over the decades.
According to Thomas J. Scharf, author of History of
Delaware, 1609-1888, (as quoted by
Peggy B. Perazzo), Associated with the softer slaty micaceous rocks are probably
masses of coarse grained granite, which vary in thickness for several inches up to many
These granites often become so highly feldspathic as to possess considerable economic
inasmuch as the feldspar frequently becomes decomposed into Kaolin."
Quarry: A GeoAdventure in the Delaware Piedmont
Plagioclase Mineral Data
Delaware Minerals List at mindat.org
Feldspar Group at mindat.org
Here is where DMS
Members can add their Delaware Feldspar photos to share with us.
Until Next Time
We hope you have enjoyed our
historic visit to Delaware Feldspar. 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
Feldspar enthusiasts, collectors, authors, curators, professionals, and club members who
made this work possible. Thanks.
©2007 All contributions to this article are covered under the
copyright protection of this article Reproduction of this article must be
obtained by express
and by separate and several copyright protection(s), and are to be used for the sole
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
with the appropriate citation and credit.
written permission of the author, Kenneth B. Casey, for his contributions, authoring,
graphics. Use of all other credited materials requires permission of each
Links and general contact information are included in the credits above, and throughout
The advice offered herein are only suggestions; it is the reader's charge to use the
contained herein responsibly. DMS is not responsible for misuse or accidents caused
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 Mineralogical Society.
Delaware Piedmont Geology
including a guide to the rocks of Red Clay Valley
by Margaret O. Plank and William S. Schenck
the Author: Ken is current webmaster of the Delaware
Mineralogical Society. 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 Delaware. And,
he is currently a member of the Delaware Mineralogical Society and the Franklin-Ogdensburg
Mineralogical Society. E-mail: firstname.lastname@example.org.