Preface
Yes,
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!
Introduction
Hello, again,
fellow mineral trekkers! This month's Mineral-of-the-Month takes
us
to rural north Wilmington, a sliver of undeveloped parkland on the Delaware-Pennsylvania
border.
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.
We'll have
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
all
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/
beryllium quarry.
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
hike!
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
trees. It
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.
(Source: http://www.udel.edu/dgs/Publications/pubsonline/SP20.pdf)
 |
|
 |
| Plagioclase
occurs with quartz, mica, garnet, and beryl |
|
White Plagioclase
in Delaware's graphic granite |
| Photos
by Ken Casey |
Our Amphibolite from
metamorphosed members of the Wissahickon Formation contains
hornblende and plagioclase feldspar. In fact, a large boulder removed from
construction
near Routes 72 & 7 was taken to Newark, Delaware in 1988. The current DGS
building
was built around the boulder! Interesting architectural choice, eh?
 |
|
 |
| Pink orthoclase
crystals |
|
White plagioclase
crystals in pegmatite |
| Photos
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
both
pink and white crystals with right angle and oblique cleavages, respectively.
Quartz, mica,
amphibole, garnet, and beryl are accessory minerals.
Our intrusive gabbros contain a green plagioclase and
pyroxene.
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.
The mafic
layers contain plagioclase, pyroxene and hornblende, and are commonly boudinaged. The
felsic layers contain quartz, feldspar and less than 10% pyroxene. Original igneous
textures
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...."
(Source: http://unh.edu/esci/delaware.html)
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).
(Source: http://www.wr.udel.edu/swaphome/Publications/Final_assess/NewCastleCo/DMNH2.pdf)
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.)
For now,
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 |
Graphic granite |
DMS Members on our
hike |
| Photos
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
from localized
metamorphism, magma formed and intruded, which crystallized into this coarse-grained
pegmatite.
The final forms we see today are the angular feldspar grains and some noteworthy
geometrically-
shaped quartz inclusions, which look like Ancient Arabic cuneiform writing.
 |
|
 |
| Pink
Orthoclase rhomb |
|
White Plagioclase
& Quartz (Delaware Graphic Granite) |
| Photos
by Ken Casey |
Pink microcline
(orthoclase) rhombohedrons and the chunkier, white plagioclase crystals
inhabit our graphic granite. Clear crystalline masses of quartz and hexagonal,
silvery-white
mica books share the makeup of our local Woodlawn pegmatite. Very small
dodecahedral,
deep red garnets and pale blue-green massive and hexagonal beryls are accessories in our
granite, as well.
(Source: http://www.udel.edu/dgs/Education/woodlawn.html)
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
tracks
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
physically
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
virtual museum
under construction. So, check back with them.
Our world renowned museum is the Royal Ontario Museum in Toronto. It's
Polarised-Light
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.
Sunahara, this
resource allows us to view Plagioclase at a new
level.
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
resource
from which to make pottery and ceramics--and, yes, dentures. The purest material,
known
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
intrusive
masses of coarse grained granite, which vary in thickness for several inches up to many
feet.
These granites often become so highly feldspathic as to possess considerable economic
value,
inasmuch as the feldspar frequently becomes decomposed into Kaolin."
(Source: http://www.cagenweb.com/quarries/states/delaware.html)
Woodlawn
Quarry: A GeoAdventure in the Delaware Piedmont
http://bulletin.geoscienceworld.org/cgi/content/abstract/70/11/1425
Plagioclase Mineral Data
http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Slideshow/Show1/Show1-13.html
http://en.wikipedia.org/wiki/Plagioclase
http://www.rbmason.ca/databank/mineral/plagioclase.html
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. 
Article Contributors
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
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 Mineralogical Society.
Suggested
Reading:
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
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: kencasey98@yahoo.com.
|
|
