On the Moh’s Hardness Scale, it ranges from 6.5-7.5.

Biotite-Phlogopite Series

Biotite - K(Mg,Fe)₃(Al,Fe)Si₃O₁₀(OH,F)₂
Phlogopite - KMg₃(AlSi₃O₁₀)(F,OH)₂

Both Biotite and Phlogopite are mentioned in the DGS's Catalog of Delaware Minerals

"

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)₃AlSi₃O₁₀(F, OH)₂. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.

Biotite is a sheet silicate. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bond together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" (muscovite) -- both form in some rocks, in some instances side-by-side.

Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. It has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It has a hardness of 2.5 - 3, a specific gravity of 2.7 - 3.1, and an average density of 3.09 g/cm³. It appears greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly lustre, and a grey-white streak. In its weathered yellow, sparkly form, it is a common type of “fool’s Gold” (Pyrite is the official “fool’s Gold”). When biotite is found in large chunks, they are called “books” because it resembles a book with pages of many sheets.

Biotite is found in a wide variety of igneous rocks and metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius and at Monzoni. It is an essential phenocryst in some varieties of lamprophyre. Biotite is occasionally found in large sheets, especially in pegmatite veins, as in New England, Virginia and North Carolina. Other notable occurrences include Bancroft and Sudbury, Ontario. It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature.

It is not industrially useful, but it is mined using quarrying and underground mining (depending on the depth of the biotite) for collection purposes.

http://en.wikipedia.org/wiki/Biotite

Phlogopite is often found in association with ultramafic intrusions as a secondary alteration phase within metasomatic margins of large ultramafic to mafic layered intrusions. In some cases the phlogopite is considered to be produced by autogenic alteration during cooling. In other instances, metasomatism has resulted in phlogopite formation within large volumes, as in the ultramafic massif at Finero, Italy, within the Ivrea zone. Trace phlogopite, again considered the result of metasomatism, is common within coarse-grained peridotite xenoliths carried up by kimberlite, and so phlogopite appears to be a common trace mineral in the uppermost part of the Earth's mantle. Phlogopite is encountered as a primary igneous phenocryst within lamproites and lamprophyres, the result of highly fluid-rich melt compositions within the deep mantle."

http://en.wikipedia.org/wiki/Phlogopite

　 Though

  (Source: http://www.uwgb.edu/dutchs/EarthSC102Notes/102ROCKS.HTM) 

     In the photos below,

     Delaware has

  (Source:) 

     For example,

     Our Piedmont

     Therefore,

Three lithologies have been identified

within the Baltimore Gneiss in the Mill Creek Nappe:

granitic gneiss, hornblende-biotite gneiss, and amphibolite

with or without pyroxene. 

Lack of exposure prevents these

lithologies from being mapped separately. Granitic gneisses

with swirling leucosomes and irregular, biotite-rich, restite

layers constitute approximately 75 to 80 percent of the

exposed rocks. Pegmatites are commonly associated with

the granitic gneisses and are exposed in a series of abandoned

quarries along the northwestern boundary of the

H o c k e s s i n - Yorklyn anticline (Wo o d r u ff and Plank, 1995).

P e t ro g r a p h y. The granitic gneiss is composed of

quartz, plagioclase, biotite, and microcline. Minor and

accessory minerals are garnet, muscovite, magnetite,

ilmenite, sphene, apatite, and zircon (Wo o d r u ff and Plank,

1995). The plagioclase is chiefly oligoclase with myrmekite

concentrated along grain boundaries. Much of the granitic

gneiss is migmatitic.

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,

------

P e t ro g r a p h y. The Setters Formation and the

Wissahickon Formation are difficult to differentiate macroscopically,

but they can easily be distinguished in thin sections

by identifying feldspars. Microcline with cross hatch

twinning and perthitic lamellae is the most abundant

feldspar in both pelitic and psammitic facies of the Setters

Formation. This distinguishes the Setters Formation from

the Wissahickon in which plagioclase is the most abundant

feldspar and untwined orthoclase is present in small

amounts. Other minerals in the pelitic and psammitic rocks

of the Setters Formation are garnet, muscovite, sillimanite,

quartz, and biotite with minor plagioclase. Accessory minerals

are iron-titanium oxides, zircon, sphene, and apatite.

Myrmekites of quartz and feldspar cluster along grain

boundaries of microcline. Heteroblastic grains of quartz are

commonly recrystallized to smaller subgrains. The plagioclase

shows extensive alteration to green clay, and biotite

may show alteration to chlorite. Sillimanite occurs as fibrolite,

acicular grains, and large prisms that lie across the foliation.

Modal analyses are published in Kuhlman (1975),

Woodruff and Plank (1995), and Plank and Schenck (1997).

Plank and Schenck (1997) compare the Setters in Delaware

and Pennsylvania with the Setters in Maryland.

ironrich

biotite is unstable and breaks down to form garnet suggesting

a reaction such as:

biotite + sillimanite + quartz = garnet + orthoclase +

water (Tracy 1978) or the incongruent melting of biotite:

biotite + sillimanite + quartz + Na-plagioclase = garnet

+ Ca-plagioclase + melt (Calem, 1987; Alcock, 1989).

Mineral assemblages of biotite,

microcline, quartz, and plagioclase indicate the rocks were

metamorphosed to the amphibolite facies.

The biotite tonalite occurs as rounded boulders

with light gray to buff colors on fresh and weathered surfaces.

The tonalite is leucocratic and contains 15-25 percent

mafic minerals. The biotite tonalite can be distinguished from

the Ardentown Granitic Suite in hand specimen by a finergrained

equigranular texture and the scarcity or absence of Kf

e l d s p a r. Chemically, the biotite tonalite differs markedly

from Ardentown Suite rocks of comparable silica content (70

percent SiO2 by weight) in having significantly lower K2O

(0.69 vs. about 3.5 weight percent), as well as some diff e rences

in trace element abundances (Srogi, 1988).

In this report we have formally named eleven new rock

units, informally recognized two rock units, and redefined previously

mapped rock units in the Delaware and southeastern

Pennsylvania Piedmont. We have redefined the Wi l m i n g t o n

Complex to include all rocks associated with the development

of a Paleozoic magmatic arc; five metaplutonic lithodemes recognized

and herein named the Brandywine Blue Gneiss,

Montchanin Metagabbro, Mill Creek Metagabbro, Barley Mill

Gneiss, and the Christianstead Gneiss; three metavolcanic

units, the Windy Hills Gneiss, Faulkland Gneiss, and the

Rockford Park Gneiss; three igneous plutons, the redefined

Arden Plutonic Supersuite (comprising the Ardentown Granitic

Suite, Perkins Run Gabbronorite Suite, and one lithodeme of

biotite tonalite), the Bringhurst Gabbro, and Iron Hill Gabbro.

The systematic changes that reflect increasing intensity

of metamorphism can be observed in the field as pelitic

assemblages record the breakdown of muscovite to sillimanite

and orthoclase, the breakdown of biotite and sillimanite

to garnet, and the incongruent melting of biotite to garnet

and cordierite. Mafic assemblages in the Wi l m i n g t o n

Complex record the change from blue-green amphibole to

brown amphibole to pyroxene. Precise estimates of peak

temperatures are more difficult because the reaction temperatures

are a function of water pressure, which is unknown

and probably variable, and a higher pressure metamorphic

overprinting is present in the Wissahickon east of the

Wilmington Complex (Crawford and Mark, 1982;

Bosbyshell, et al., 1999). Attempts to estimate peak metamorphic

temperatures and pressures based on mineral

assemblages in garnet-bearing biotite gneisses using petrogenetic

grids and various geothermometers and geobarometers

find:

(1) Brandywine Blue

http://www.dgs.udel.edu/publications/pubs/reportofinvestigations/ri59.pdf

The layering in the Wissahickon wall rock is irregular and defined by stringers of garnet, biotite and sillimanite in a mass of quartz and feldspar. The garnets are dark red, either oval or round, and may be as large as three quarters of an inch in diameter.

The Wissahickon rocks appear to have been melted and recrystallized to form granites with thin layers of garnets. The biotite and sillimanite that occur in the Wissahickon gneisses are replaced by tiny garnets. This reaction in which garnet replaces biotite and sillimanite occurs only at very high temperatures.

http://www.dgs.udel.edu/geology/geoadventures/bluerocks.aspx

http://www.dgs.udel.edu/publications/pubs/bulletins/bulletin7e.pdf

comprising the Ardentown Granitic Suite,

Perkins Run Gabbronorite Suite, and one

lithodeme of biotite tonalite; and three

metavolcanic units named the Rockford Park

Gneiss, Windy Hills Gneiss, and Faulkland

Gneiss."

http://www.dgs.udel.edu/publications/pubs/fsg/v19no1.pdf

"A mineral with perfect basal cleavage that easily separates into

rocks except the high-grade gneisses of the Wilmington Complex."

http://www.dgs.udel.edu/publications/pubs/specialpublications/sp20.pdf

     More sp

  (Source: http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_delaware.html)

     A rare igneous garnet is found in the pegmatite of the Woodlawn Quarry.  The locale may be found
in the red area of the Generalized Geologic Map of Delaware, below.

    Why not try this one: Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

 
                   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

 

     Yes, right in o

     As BSP is adjacent to our clubhouse at Historic Greenbank Mill, and drillin

    The fabri

  (Source: http://www.udel.edu/dgs/Publications/pubsonline/report38/31.html) 

     If you like the smalle

     If,

     As the Delaware Greenways Project expands, we’ll be able to hike directly to other geologic locations
rather efficiently on future MOTM fieldtrips. Thanks to the State of Delaware and its partners, our educational
and recreational experiences will be enhanced.

     Now, to the park-at-hand. Follow me.  I’ll guide us with the DGS's GeoAdventure directions, so that we
may tread lightly, and leave the squirrels and woodpeckers to their business.  Don't worry, we'll see garnets!

     "To see the contact, you need to follow the stream to the confluence of Hurricane Run and Rocky Run
and stay on the northeast side of Rocky Run. (E, Figure 2). The exposed contact is difficult to recognize
and probably interesting only to geology students at the high school or college level. It is exposed in a ten
foot area along the northeast side of Rocky Run where dark, fine grained Wilmington Complex gneisses are
interlayered with light colored Wissahickon gneisses. The Wissahickon rocks appear to have been melted
and recrystallized to form granites with thin layers of garnets. The biotite and sillimanite that occur in the
Wissahickon gneisses are replaced by tiny garnets. This reaction in which garnet replaces biotite and
sillimanite occurs only at very high temperatures. The Wilmington Complex layers vary in thickness between
3 inches and 2 feet, and are dark solid, massive rocks.”

  (Source:  http://www.udel.edu/dgs/Education/bluerocks.html) 

     Let's zoom in on our gneiss garnets. 

Other Quartz Experiences

     Now that we have sampled views of some of the largest garnets in Delaware, let's have lunch.  I'll trade
you an extra pomegranate for half of your peanut butter and jelly sandwich.  We can discuss other garnet
experiences over dessert, if you like.   

     Of course, our club visits collecting sites for garnet all around our area. Please do inquire upon how to
join us as members, and can benefit from our club fieldtrips. Our trips our setup and led by our own Bob Asreen,
Geologist, and Vice-President of Fieldtrips.

(Top): Gem Trails cover by Mark Webber (Right): Close-up of Wissahickon Valley garnets in schist matrix from Fairmount Park		Gem Trails of Pennsylvania and New Jersey by Scott Stepanski and Karenne Snow, suggests that very small, nicely faced, and deep red Almandine garnets may be found weathered out of their host schist at Fairmount Park, Philadelphia. This author visited there about three years ago, and found small quantity on the trail--enough to fill a thimble or two.      I was fortunate enough to meet Ms. Snow a couple of years ago at one of the Cape-Atlantic Rockhounds events.  She was kind enough to sign my copy of her book.  I think it brought me luck, since I've had some good experiences collecting from locations that she and Scott Stepanski had recommended.      These Wissahickon Valley garnets may be collected from Fairmount Park in Philadelphia. According to the book, garnets wash out of the surrounding schist, to be collected on the trail.  Good Luck!

     If

Pyroxenes (and amphiboles), Tourmaline and Garnet (UC Berkeley)

Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

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