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                           Mineral of the Month--May

                              Delaware Feldspar

                                                   Sodium Calcium Aluminum Silicate

                                     Plagioclase Group:  (Na, Ca)Al1-2Si3-2O8


                                        "Delaware Feldspar, Part 2: Plagioclase"

                                              By Ken Casey

Why Delaware Feldspar?
Chemistry & Science
Woodlawn Quarry
Two Museums of Note
Members' Gallery
Article Contributors
Photo & Graphics Credits
Suggested Reading
Invitation to Members
Past Minerals of the Month

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        Delaware State Flag

Image courtesy of Marchex, Inc.
©2007, World Flag Database

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Delaware has useful Feldspars...


...of historic significance!

(Top, left): Delaware Plagioclase Feldspar
Photo by Ken Casey ©2007

(Top, right): Delaware Plagioclase Feldspar in graphic granite
Photo by Ken Casey ©2007


     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!



     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

     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.

IMGP4197.JPG (616975 bytes)      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)



Chemistry & Science

     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.


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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?

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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...."


     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.)   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.


Woodlawn Quarry


     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?

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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.

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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.


     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!

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Large graphic granite boulder with Plagioclase Smaller feldspar boulder


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Fran, Bob & Karissa study to surface geology Large Muscovite Mica books in granite


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Clear quartz, White Plagioclase, Green Beryl Forest floor strewn with feldspar mine tailings


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Karissa points to Blue Beryl in boulder Close-up of Blue Beryl


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Coarse-grained pegmatite on-site End of our nature & geology hike
Photos by Ken Casey


Two Museums of Note

     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."




Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

Plagioclase Mineral Data

Delaware Minerals List at

Feldspar Group at


Members' Gallery

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.
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Article Contributors




Photo & Graphics Credits

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.

Marchex, Inc., World Flag Database

©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


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   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:

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, hobbyists, and scientists.  We only ask that you check your facts, give credit where it is due, keep it wholesome for our Junior Members watching, and keep on topic regarding rockhounding.

You don't even have to be experienced in making a webpage.  We can 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 your story out there.

Our 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 website!

We 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 suggestions.

Our next MOTM will be a surprise.  For 2007, we are waiting for your suggestions.  What minerals do you want to know more about?

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Most of the Mineral of the Month selections have come from most recent club fieldtrips and March Show Themes, and from inspriring world locales. thus far.  If you have a suggestion for a future Mineral of the Month, please e-mail me at:, or tell me at our next meeting.



Past Minerals of the Month
April 2007 Mineral of the Month: Delaware Feldspar: Orthoclase
March 2007 Mineral of the Month: "The Colors of Fluorite"
February 2007 Mineral of the Month: Pennsylvania Fluorite
January 2007 Mineral of the Month: Sillimanite
December 2006 Mineral of the Month: Hedenbergite by Karissa Hendershot
November 2006 Mineral of the Month: Brandywine Blue Gneiss
October 2006 Mineral of the Month: Spessartite by Karissa Hendershot
September 2006 Mineral of the Month: Native Silver
August 2006 Mineral of the Month: Kryptonite
July 2006 Mineral of the Month: Azurite
June 2006 Mineral of the Month: Pyromorphite
May 2006 Mineral of the Month: Tsavorite by Karissa Hendershot
April 2006 Mineral of the Month: Variscite
March 2006 Mineral of the Month: Petrified Wood, Part II
February 2006 Mineral of the Month: Petrified Wood, Part I
January 2006 Mineral of the Month: Strontianite by Karissa Hendershot
December Mineral of the Month: Clinozoisite
November Mineral of the Month: Bismuth
October Mineral of the Month: Wulfenite by Karissa Hendershot
September Mineral of the Month: Turquoise
August Mineral of the Month: Peridot
July Mineral of the Month: Ruby
June Mineral of the Month: Antarctic Fluorite
May Mineral of the Month: Dolomite, Part 2
April Mineral of the Month: Dolomite, Part 1
March Mineral of the Month: Calcite
February Mineral of the Month: Agate
January Mineral of the Month: Fluorite
December Mineral of the Month: Pyrite
November Mineral of the Month: Stilbite  
October Mineral of the Month: Celestite   


Comments and questions:

This page last updated:  February 19, 2011 10:14:48 AM




Next Meeting

April Program, Monday, April 8, 2013:

"Destruction of the Fossil Exposures in the Chesapeake Bay Area" presented by Dr. Lauck Ward

General Club Meeting:
April 8, 2013

We are meeting at
Greenbank Mill

Special Meetings:

*Show Committee Meeting, April or May, 2013

*New Home/Lapidary Committee, 2013

*Board Meeting,  April, 2013

Next Field Trips


Past Fieldtrips

Next Show
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 Report






Fossil Forum

"Dinny, the Dino"

"Belemnites are coming"


MOTM June also commemorates our 50th Show!

It's shiny, yellow, and is a symbol of 50 Years!Can you guess?


Collecting Adventure Stories:

"Sunny Brook Crick Goethite" by Joe Dunleavy