May in Delaware, fellow fieldtrippers! This month, our
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
sunny today, so
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!
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.
Hornblende contrasts, yet takes on sharp crystal forms, much like our
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
and blenden, to
'blind' or 'dazzle'. The
term blende is
often used to refer to a brilliant non-metallic
pitchblende, a lustrous form of
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.
as cited from the American Heritage Dictionary is: "[German :
horn (from Middle High German, from Old
High German; see
in Indo-European roots) +
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
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.
"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"
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.
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
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
Calcic Clino-Amphibole Subgroup,
and finally, the
There have been many ways to state
a generalized chemical formula for the group:
is an Inosilicate with chemical formula Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2,
in the igneous cooling process (hence the lower temperatures). There is hornblende
gneiss and hornblendite metamorphic
gabbro. And, the Bringhurst pluton
grains of hornblende intergrowths with
hornblende combinations may be found in
Delaware’s Piedmont Province, as
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
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
Frank Wigglesworth Clarke - 1889)"
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
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."
Moh’s Hardness Scale,
it is a 6.
Delaware Hornblende is mentioned in the DGS's
Catalog of Delaware Minerals
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.
AMPHIBOLE and AMPHIBOLITES
ACTINOLITE and HORNBLENDE
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 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
-- 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.
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,
rock that consists solely of black amphibole
These monomineralic rocks are exposed along the
track as float, but nearby at the Hercules pilot plant,
occur as bedrock. Type II, a true amphibolite, is
almost equal amounts of two minerals, amphibole and
feldspar. Type III has the composition of a quartz
consists of amphibole and/or black mica with up to 75 %
feldspar and quartz.
Amphibole. A large group of dark colored, iron- and
minerals. Black hornblende is the most common amphibole
Amphibolite. A dark metamorphic rock composed chiefly of
[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
Prepared by: Nenad Spoljaric and Robert Jordan, Revised by: Thomas E.
|Physiographic Map of
Pennsylvania, Pennsylvania Geological Survey
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
Our second museum
As Delaware Hornblende
Delaware Minerals List
Here is where DMS Members can add their Delaware
Hornblende photos to share
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.
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
©2008 All contributions to this article are covered
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for the sole purposes of
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The advice offered herein are only suggestions; it is the reader's
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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
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: