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                           Mineral of the Month--May 2008

                              Delaware Hornblende

                                                       Calcium Magnesium Iron Aluminum Silicate Hydroxide





                                        "Delaware Hornblende"

                                              By Ken Casey

Why Delaware Hornblende?
What's in a name?
Chemistry & Science
Some Hornblende Geology
Two Museums of Note
Members' Gallery
Article Contributors
Photo & Graphics Credits
Suggested Reading
Invitation to Members
Past Minerals of the Month

Delaware State Flag

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


Late in the igneous cooling process...


...Delaware Hornblende occurs!

(Top, left): , Wilmington, Delaware
Photo by Ken Casey 2008

(Top, right):
Photo by Ken Casey 2008



     Welcome to May in Delaware, fellow fieldtrippers!  This month, our Mineral-of-the-Month
takes on a walkabout in New Castle County's Piedmont Province to witness our Hornblende

     Spring is here, and the pollen count is high so far, so bring a light jacket, and some allergy
relief, if you have need of it.  The weather promises to be bright and sunny today, so
Let's go!




    We have covered many of Delaware's prettier rock-forming minerals over the past two years,
so we are left with some of the more important, yet plainer, constituents of the bedrock upon
which Delaware rests.  No rest for us though, until lunch, though.

     So come on along, we have a Delaware Hornblende fieldtrip to make!

Why Delaware Hornblende?


     Delaware Hornblende is not a glamorous mineral, but a constituent of much of our igneous
bedrock. We may sometimes find it in the odd amphibolite and schist. It is not a mineral,
so to speak, but a group of complex silicates. To our eye in the field, most hornblende is
indistinguishable form other series members, except by color, from gray to green to black.

     Hornblende can be found mainly in New Castle County. We can visit at least four characteristic
specimens at the
Iron Hill Museum in Newark, Delaware. Three of the specimens are from
Sherwood Park, and one is from the Stony Batter Quarry--both in Delaware.

      Our local Hornblende contrasts, yet takes on sharp crystal forms, much like our gemmier
native rocks.  If you plain and simple on the outside, yet complex on the inside, please do join us!



What's in a name?   


     "The word hornblende is derived from the German horn and blenden, to 'blind' or 'dazzle'. The
blende is often used to refer to a brilliant non-metallic lustre, for example, zincblende and
pitchblende, a lustrous form of
uraninite."  The word "blenden" also means "to deceive".  The
deception is that hornblende looks like a metal-ore, but did not yield appreciable amount of,
say, iron, as did zincblende.


     Hornblende's etymology as cited from the American Heritage Dictionary is: "[German : Horn,
(from Middle High German, from Old High German; see ker-1 in Indo-European roots) + Blende,
; see blende.]"


     The International Mineralogical Association's Commission on New Minerals and Mineral Names
(IMA CNMMN) Subcommittee on Amphiboles set forth their general recommendations for hornblende
and amphibole nomenclature in 1978.  In 1997, the IMA further specified names based upon chemical
composition.  The name "hornblende" is not listed in the published updates.

     Of course, hornblende itself was once known as a single mineral.  We now apply this name to a
series range of minerals, based upon varying amount of elemental components.  We still employ the
name "hornblende" in common parlance.  Maybe I'm old-fashioned, but it's easier to remember.

    The "Hornblende Group" and "Ferrohornblende" (for example) are accepted.  Most other descriptions,
such as "Iron-hornblende" have been discredited, though both terms essentially say the same thing.

     It is interesting to note that "horn" and "blende" derive from German.  Why add the Greek "ferro" or
English "Iron"?  Why not, stay consistent with the German, and add "Eisen" as the iron-ic prefix.  We
would then get "Eisenhornblende".  But, would this name be redundant, as the earliest term was used
to call all similar minerals "hornblende"? 

     I am not refuting the wisdom and the hard ward that our respected colleagues put forth for the
purpose of an improved scientific nomenclature.  I am merely suggesting an alternate name, which
would be based upon linguistics.  I do, however, applaud their constistent use of scientifically
accepted prefixes, which does help.


Chemistry & Science

     Hornblende is the most common amphibole minerals.  An "amphibole" may be one of 60
minerals which comprise a group of double-chain inosilicates with a similar root chemical
formula.  They form at lower igneous temperatures in the presence of water.  Amphiboles
crystallize mainly into members of the orthorhomic and monoclinic crystal classes.

     We can further narrow our definition.  Above, we mentioned that "Hornblende" is now a
"Group" name for a solid-solution series of igneous minerals.  We currently classify it as
occuring in the
Amphibole Group, Calcic Clino-Amphibole Subgroup, and finally, the
Ferrohornblende-Magnesiohornblende Series.

     There have been many ways to state a generalized chemical formula for the group:

 (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2  (University of Delaware)

 (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(OH)2[(Si,Al)8O22]  (Geology at

     Hornblende is an Inosilicate with chemical formula Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2, and
forms late in the igneous cooling process (hence the lower temperatures). There is hornblende
gneiss and hornblendite metamorphic gabbro. And, the Bringhurst pluton has microscopic
grains of hornblende intergrowths with spinel. Other hornblende combinations may be found in
Delaware’s Piedmont Province, as well.

     Macroscopically, two long, dark crystals are most likely found in Delaware igneous rocks:
one is black tourmaline (schorl), the other is a hornblende members.  These two minerals are
not usually found in the same rocks, however.  Schorl will always be black; whereas, the
hornblende might occur as dark green or brown.

    "The crystal form tends to be long prisms with a diamond-shaped (rhomboid) cross section,
sharp ends with a 56-degree angle and the other two corners with 124-degree angles. That is
the main way to distinguish an amphibole from the other dark minerals."




The amphiboles have a molecular structure made of double chains of silica (SiO4) tetrahedra, surrounded by metal and hydroxyl ions. They form at high temperatures in igneous and metamorphic rocks that contain water. Their dry cousins, the pyroxenes, do not have hydroxyl and consist of single silica chains. Both mineral groups tend to crystallize in long prisms or needles, but amphiboles have a lozenge or diamond shaped cross-section with corner angles of 124 and 56 degrees whereas pyroxenes are nearly square in cross section at 87 and 93 degrees.

In the study of volcanism and igneous magma ascent, Hornblende reaction rims are a commonly used tool with which to evaluate.

Rocks of the Wilmington Complex commonly contain hornblende within the banded gneiss and bordering amphibolites.

It is interesting to note that, Delaware's only known cave (The Beaver Valley Cave), is nestled within hornblende-bearing rock.  Other older names are "Indian Cave" and "Wolf Rock Cave".  As early as the Revolutionary War, historical account exists of cave usage by soldiers.  And, as early as 1889, geological reports of hornblende are reported.

"Along the Wilmington
and Northern Railroad, between Dupont and Adams stations, near
Henry Dupont's place, the mica- schists strike N. 50 E., dip 55 N.,
and are deeply exposed in the cut. A short distance farther south is
another fine exposure of the black hornblende rocks, which strike N.
62 E., dip 40 N.(page 39, Report of Work Done in the Division of Chemistry and Physics, Mainly During ... - Page 39

by Frank Wigglesworth Clarke - 1889)",M1


The cave is carved out of a high outcropping of highly metamorphosed hornblende schist.

"The rock in which this cave is formed is rather unique to speleology. Its origin is presently under investigation and appears to be a combination of rock slippage, fallen boulders and/or sea action. The Delaware Geological Survey shows the cave in the Wissahickon Schist Formation, a dense micaceous schist, gneiss and migmatite and specifically in the Northeastern facies containing dense garnet granulite and biotite schist.(9). This is probably of Precambrian age composed of igneous and metamorphic rocks of the Glenarm Series. Forney reports that the cave is located in a 50 foot wide and 20 foot high outcropping of hornblende schist which is highly metamorphased.(l0). He further adds that the local rock has been identified on geological maps as mica schist, but in the Survey's ten samples a ten-power magnifying glass is required to see the crystals. The schist looks like dark fine-grained granite except that it shows bedding in places the rock has 1 cm. crystals of milky quartz, which were stained brown when the mica (an iron compound) weathered to form rust. In places on the schist are 1/3 cm. crystals of garnet, which is not surprising since garnet mining was performed just across the border in Pennsylvania. The schist has a hardness of 7 and easily scratches glass. In Richard Ward's report, the cave would be located in Type D Amphibalites.(11). Amphibalites near the gneiss contain calcic plagioclose, hornblende and traces of hypersthene. Those farther away are hypersthene-free, and the plagioclose is replaced in part by epiodate."


     On the Moh’s Hardness Scale, it is a 6.


Delaware Hornblende is mentioned in the DGS's Catalog of Delaware Minerals



Some Delaware Hornblende Geology

White Clay Creek forms a scenic valley incised in the rolling Piedmont terrain of southeastern Pennsylvania and northwestern Delaware. Some 600 million years ago, the Preserve was part of a large continental area that subsided and was covered by a shallow sea. Through time, sediment composed of sand, silt, and mud spread over the sea floor. At various intervals, volcanoes poured lava onto these deposits. Gradually the sediments hardened into sedimentary rocks. About 460 million years ago, an immense mountain-building episode folded and heated the rocks and completely changed their character. The rocks in the Preserve "cooked" at elevated temperatures and pressures for some 70 million years, long enough for the new minerals to develop. Approximately 390 million years ago, the Preserve was uplifted and cooled, which halted the metamorphism. Since then, the minerals have remained largely unchanged. The lava flows became very dark gray amphibolites. Nearly black hornblende dominates these rocks; interspersed feldspar grains tend to be medium gray to white.


A family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water.

A rock made up mostly amphibole and plagioclase feldspar. Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.

Actinolite has no aluminum; it and is needle-shaped and light green.

Blue Amphibole:
Blue amphibole contains sodium and, of course, is bluish in color.

Family of silicate minerals containing varying amounts of potassium, sodium and calcium along with aluminum, silicon and oxygen. Potassium feldspars contain considerable potassium. Plagioclase feldspars contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white, gray, or pink.

Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks.

-- Excerpts from:
USGS/NPS Geology in the Parks Website, 2002

     Essentially, Delaware's Piedmont Unit rock builds up from Precambrian to Cambrian, Ordivician, then Silurian era events.

It seems that most rocks formed over those millions of years contain hornblende, from the ancient continental core and Precambrian Baltimore Gneiss (biotite-hornblende gneiss) and the latest volcanic arc rock.  Some material from the metamorphosed Wilmington Complex has hornblende, too, as the dominant lithology is gneiss amphibolite.   Many of the other component gneisses of the WC contain hornblende, as well, except the famous Brandywine Blue Gneiss, or "Delaware Blue Rock".

The Wilmington Complex (WC) is composed of granulite-grade gneisses and plutonic igneous rocks (Ward 1959; Foland and Muessig 1978). It is located in northeastern Delaware and southeastern Pennsylvania, but most exposures occur in Delaware. These rocks represent the highest grade of metamorphism in the Central Appalachian Piedmont. The Wilmington, Delaware Complex is in contact in the north and northwest with deep marine metasediments of the Wissahickon Formation, and on the west by the arc volcanics of the James Run Formation (Higgins 1977). Field relationships imply that the northwestern contact of the WC is a fault contact, but this contact has not actually been observed. To the southeast, the WC is uncomfortably overlain by sediments of the Atlantic Coastal Plain (Dirska 1990).

The Wilmington Complex (WC) can be divided into two parts. The western portion of the complex is composed of two varieties of gneiss: the most common form is a pyroxene-quartz-plagioclase gneiss, while the other type is an interlayered felsic and mafic gneiss of variable composition (Dirska 1990). Both gneisses have been metamorphosed to the hornblende granulite subfacies of the granulite facies (Srogi 1991). The eastern portion of the WC is the Arden pluton, which is composed of both an orthopyroxene-bearing mafic rock and a felsic plutonic rock. The felsic rocks appear to occur as isolated pods floating within the gabbros. The pods may represent intrusions, or possibly roof pendants (Ward 1959), but their nature cannot be definitively determined as the contacts between the Arden pluton and surrounding gneisses are nowhere exposed. It is also possible that the contact is actually a thrust fault (La Porta n.d.).

amphibolite - Medium-grained, dark colored metamorphic rock composed of hornblende and plagioclase with minor biotite, quartz, sphene and epidote; formed by moderate pressure/medium to high temperature (amphibolite facies) metamorphism of mafic igneous rocks such as basalts.

tonalite - A coarse-grained igneous rock composed of sodium plagioclase, quartz, hornblende and/or biotite, with accessory apatite, zircon and iron oxide.

Wissahickon Formation:


Cover photo: Photograph of the large boulder

in the Delaware Geological Survey

lobby. The boulder was removed from the

“The Ridge”, a development constructed

near the intersection of Paper Mill Road

(Route 72) and Limestone Road (Route 7),

and taken to the site of the new Delaware

Geological Survey building in 1988. The

boulder was positioned on the concrete

pad, and the building was built around it.

The boulder is a metamorphic rock from

the Wissahickon Formation called an

amphibolite. It is composed of the minerals

hornblende and plagioclase feldspar.


Amphibolites and Other Amphibole-bearing Rocks

In this outcrop there are three rock types that contain

amphiboles. Type I is an extremely coarse-grained, monomineralic

rock that consists solely of black amphibole (hornblende).

These monomineralic rocks are exposed along the

track as float, but nearby at the Hercules pilot plant, they

occur as bedrock. Type II, a true amphibolite, is composed of

almost equal amounts of two minerals, amphibole and

feldspar. Type III has the composition of a quartz diorite, and

consists of amphibole and/or black mica with up to 75 %

feldspar and quartz.



Amphibole. A large group of dark colored, iron- and magnesium-bearing silicate

minerals. Black hornblende is the most common amphibole in the

Delaware Piedmont.

Amphibolite. A dark metamorphic rock composed chiefly of amphibole and


[my photo of boulder here]



There is biotite in most all of the gneises of Delaware's Piedmont.    

"The hornblende gneiss contains plagioclase, quartz,

hornblende, and biotite with/without orthopyroxene.

Accessory minerals are garnet, muscovite, clinozoisite,

perthitic orthoclase, iron-titanium oxides, sphene, and

apatite. Hornblende grains are pleochroic in shades of

brown-green and green.

The amphibolites are composed of subequal amounts

of hornblende and plagioclase with minor quartz, biotite,

clinopyroxene, and orthopyroxene (Wo o d r u ff and Plank,

The Baltimore gneiss is a "quartztzo-feldspathic gneiss, biotite gneiss, biotite hornblende gneiss, and amphibolites".

"Highly metamorphosed biotite gneisses of the Wissahickon are often impossible to distinguish from the biotite gneisses of the Baltimore Gneiss.  Features that characterized the Baltimore Gneiss are (1) intense migmatization, (2) highly variable strikes of foliation, (3) general absence of sillimanite and primary muscovite , and (4) relic granuliate facies assemblages containing orthopyroxene."

The Wissahickon in Delaware contains some metasedimentary pelitic gneisses with biotite, quartz, plagioclase (oligoclase to andesine), and various iron-titanium oxides.


                   Generalized Geologic Map of Delaware, courtesy of the Delaware Geological Survey
                       Prepared by: Nenad Spoljaric and Robert Jordan, Revised by: Thomas E. Pickett

Physiographic Map of Pennsylvania, Pennsylvania Geological Survey












Two Museums of Note

     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 Iron Hill Museum, which contains several specimens of our
Delaware Hornblende.

     Our second museum 



     As Delaware Hornblende




Delaware Minerals List at



Members' Gallery

     Here is where DMS Members can add their Delaware Hornblende photos to share with us.



Until Next Time

     We hope you have enjoyed our historic visit to Delaware Hornblende.  Please join
us next month, for another article, and we shall journey together!

Until then, stay safe, and happy collecting. hardhat2a.gif (5709 bytes)



Article Contributors

Delaware Piedmont Geology by Margaret O. Plank and William S. Schenck, DGS



Photo & Graphics Credits

    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, Mineral Photographer

Marchex, Inc., World Flag Database

Delaware Piedmont Geology by Margaret O. Plank and William S. Schenck, DGS

Nenad Spoljaric and Robert Jordan, Thomas E. Pickett, Delaware Geological Survey

Physiographic Map of Pennsylvania, Pennsylvania Geological Survey

Photomicrograph by UCLA's Petrographic Workshop

Gem Trails cover by Mark Webber

2008 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


KEN.JPG (31503 bytes)

   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 2008, we are waiting for your suggestions.  What minerals do you want to know more about?

aniagate.gif (1920 bytes)


Most of the Mineral of the Month 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:, or tell me at our next meeting.



Past Minerals of the Month
April 2008 Mineral of the Month: Delaware Biotite Mica
March 2008 Mineral of the Month: Delaware Pegmatites
February 2008 Mineral of the Month: Exotic Pegmatites
January 2008 Mineral of the Month: Delaware Quartz, Part 2
December 2007 Mineral of the Month: Delaware Muscovite Mica
November 2007 Mineral of the Month: Delaware Beryl
October 2007 Mineral of the Month: Delaware Quartz, Part 1
September 2007 Mineral of the Month: Delaware Garnet: Almandite
August 2007 Mineral of the Month: Schorl (Black Tourmaline)
July 2007 Mineral of the Month: Rubellite
June 2007 Mineral of the Month: Elbaite 
May 2007 Mineral of the Month: Delaware Feldspar, Part 2 
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 10:15:20 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