26 ROCKS AND MINERALS OF PENNSYLVANIA
OXIDES AND HYDROXIDES
MAGNETITE: Fe3O4. Black. Metallic luster. Attracted to magnets.
Crystals and irregular masses. Large deposits in Lebanon and Berks
Counties served as the principal ore mineral of iron mines. Also found
in small quantities in rocks and surficial deposits statewide. Can be
collected from sand in many streams by using a magnet.
CHROMITE: (Mg,Fe)Cr2O4. Black. Metallic luster. Irregular masses
and small crystals. Was mined for chromium in southeastern Pennsylvania.
Southern Lancaster County was the world’s leading source of
chromium in the 1800s. Chromite is found in sand in streams in areas
of serpentine bedrock.
HEMATITE: Fe2O3. Irregular masses are typically red and earthy; crystals
can be gray and have metallic luster. Was used as an iron ore in
colonial times in eastern Pennsylvania. Found in small quantities in
sedimentary rocks statewide. Thought to be what makes Triassic and
some other sedimentary rocks red.
GOETHITE: FeO(OH). Commonly yellowish-brown and earthy irregular
masses. Principal mineral in “limonite,” which is a mixture of iron
oxides. Limonite formed from the oxidation of pyrite has the same
form as the pyrite crystals. Was mined for iron in small operations from
colonial times to the late 1800s in eastern and central Pennsylvania.
HALITE: NaCl. Normally white. Vitreous luster. Soluble in water. Salty
taste. Large, deep, subsurface deposits of halite are found in northern
and western Pennsylvania. Rocks consisting primarily of halite are
called evaporites. Some sedimentary rocks in Pennsylvania contain
casts of halite crystals that dissolved long ago.
FLUORITE: CaF2. Purple is the common color in Pennsylvania. Also
colorless, white, green, blue, or yellow. Vitreous luster. Small crystals
and cleavage fragments are found in several locations in south-central
and eastern Pennsylvania.
CALCITE: CaCO3. Colorless and transparent, white to gray and opaque,
other colors; may be yellowish. Vitreous luster. Found as crystals in
some places. The principal mineral forming large limestone deposits
in many areas of the state. Also forms marble in the regions of metamorphic
rock in southeastern Pennsylvania.
DOLOMITE: CaMg(CO3)2. Colorless and transparent, white to gray
and opaque, other colors; may be yellowish. Vitreous luster. Found as
crystals and masses. Forms large deposits of dolomite rock in many
areas, especially in south-central and southeastern Pennsylvania.
SIDERITE: FeCO3. Yellowish-brown to reddish-brown. Vitreous luster.
Widespread occurrence as nodules in the coal-producing regions of
western Pennsylvania. Some crystals. Also found in several places in
CELESTINE: SrSO4. Pale blue or colorless. Vitreous luster. Fibrous
masses and small crystals. The world’s first discovery of celestine was
in Blair County in 1791. Proposed to be the state mineral of Pennsylvania,
awaiting approval as of May 2004.
GYPSUM: CaSO4⋅2H2O. Colorless or white. Vitreous or pearly luster.
Small crystals found in small scattered deposits associated with shales
and carbonate rocks. Probably exists in larger subsurface deposits associated
MELANTERITE: Fe2+SO4⋅7H2O. White. Vitreous luster. Metallic taste.
This and some other sulfates are commonly seen as powdery coatings
or small crystals on shale. Forms from the oxidation of pyrite in the
BARITE: BaSO4. White to gray, blue, red, or brown. Vitreous luster. High
specific gravity. Found as irregular masses and crystals. Most common
in southeastern and south-central Pennsylvania.
APATITE GROUP: Carbonate-fluorapatite (Ca5(PO4,CO3)3F), fluorapatite,
(Ca5(PO4)3F), and carbonate-hydroxylapatite (Ca5(PO4,CO3)3OH)
are probably the most common members of this group in Pennsylvania.
Various shades of green, white, brown, and red. Subresinous to vitreous
luster. Apatite minerals are found in a number of places. Fluorapatite
is found as tiny crystals in igneous and metamorphic rocks of
southeastern Pennsylvania. Carbonate-fluorapatite has been reported
in fossilized bone from sedimentary rocks in central Pennsylvania.
WAVELLITE: Al3(PO4)2(OH,F)3⋅5H2O. Colorless, white, or green. Vitreous
or pearly luster. Radiating clusters of crystals are found in southcentral
and southeastern Pennsylvania.
QUARTZ: SiO2. Commonly colorless and transparent; many other
colors possible. Vitreous luster. Varieties include amethyst, chalcedony
(flint or chert), jasper, smoky quartz, and rose quartz. Very abundant
in most sedimentary, metamorphic, and igneous rocks statewide.
Crystals are found in numerous locations.
FELDSPAR GROUP: Plagioclase (a continuous series of compositions
ranging from albite, NaAlSi3O8, to anorthite, CaAl2Si2O8), and
orthoclase and microcline (both KAl(Si3O8)) are the most common
feldspars in Pennsylvania. Various shades of white, gray, pink, and
other colors. Vitreous luster. Abundant in many sedimentary, metamorphic,
and igneous rocks statewide.
KAOLINITE–SERPENTINE GROUP: Kaolinite (Al2Si2O5(OH)4), antigorite
((Mg,Fe2+)3Si2O5(OH)4), and clinochrysotile and lizardite (both
Mg3Si2O5(OH)4) are the most common members of this group in Pennsylvania.
Kaolinite exists as tiny particles in clay. It is normally whitish
and has an earthy texture. It is found in shale and other fine-grained
rocks in many places in Pennsylvania. The other three minerals are
commonly green and have a waxy or silky luster. Clinochrysotile is
commonly fibrous; antigorite and lizardite are massive. Serpentine
minerals make up the bulk of the altered ultramafic rocks known as
serpentinite, which are found in Delaware, Chester, and Lancaster
Counties. Serpentine is also found in metamorphic rocks in Northampton
MICA GROUP: In Pennsylvania, the most common members of
this large group are muscovite (KAl2(AlSi3)O10(OH)2) and biotite
(K(Mg,Fe2+)3AlSi3O10(OH)2). Muscovite is colorless or pale shades of
green or brown; biotite is darker. Both have a vitreous or pearly luster
and are easily separated into very thin sheets that spring back
when bent. Mica is abundant in many sedimentary, metamorphic,
and igneous rocks statewide. Large sheets of muscovite are found in
some rocks in southeastern Pennsylvania. The weathered form of
mica, illite, is a clay mineral and a major component of shale.
CHLORITE GROUP: Clinochlore ((Mg,Fe2+)5Al(Si3Al)O10(OH)8) and
chamosite ((Fe2,+Mg,Fe3+)5Al(Si3Al)O10(OH,O)8) are the most commonly
found chlorites in Pennsylvania. Both tend to be green or brown.
Vitreous to earthy luster. Large specimens can be easily separated
into very thin sheets that are easily bent. Chlorite is abundant in metamorphic
rocks, some of which contain large sheets. It is also abundant
as a clay mineral in shale.
AMPHIBOLE GROUP: There are 65 minerals belonging to the amphibole
group. Many amphiboles are found in Pennsylvania. Among
them are two hornblende minerals, ferrohornblende and magnesiohornblende
(Ca2[(Fe2,+Mg)4(Al,Fe3+)]Si7AlO22(OH)2), and actinolite
(Ca2(Mg,Fe2+)5Si8O22(OH)2). The hornblendes are usually dark green
to black and have a vitreous luster. They form short columnar crystals
in the metamorphic rocks of southeastern Pennsylvania. Actinolite
is white or green, has a vitreous luster, and forms long, thin
crystals. It is also found in the metamorphic rocks of southeastern
PYROXENE GROUP: The most common pyroxenes in Pennsylvania
are thought to be augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) and diopside
(CaMgSi2O6). Both minerals are white to dark green or black. Vitreous
luster. Short crystals or massive forms. Pyroxenes are important minerals
in the mafic rocks of southeastern Pennsylvania, especially in
the Jurassic diabase, where they make up half the rock.
TOURMALINE GROUP: Schorl (NaFe3Al6(BO3)3Si6O18(OH)4) is the
most common species of tourmaline found in Pennsylvania. Black.
Vitreous luster. Occurs as crystals found in metamorphic rocks of
southeastern Pennsylvania. It is also found as tiny, radiating grains in
OLIVINE GROUP: Forsterite (Mg2SiO4) and fayalite (Fe2+
2 SiO4) are
the two most common species of olivine. Normally olive-green. Vitreous
luster. Olivine in Pennsylvania commonly has a composition between
those of forsterite and fayalite, but is closer to forsterite. It is found in
southeastern Pennsylvania, where most of it has altered to serpentine.
Small grains of olivine can be found in many serpentine deposits.
GARNET GROUP: Almandine (Fe2+
3 Al2(SiO4)3) is one of several kinds
of garnet found in Pennsylvania. Red color most common, but can be
other colors. Vitreous to resinous luster. Garnets are found as crystals
or irregular forms in the metamorphic rocks of southeastern Pennsylvania.
Garnet sand can be panned from many streams in the glaciated
regions of northwestern and northeastern Pennsylvania.
ZIRCON: ZrSiO4. Colorless or brown. Vitreous or adamantine luster.
Occurs as crystals. Very small zircons are found in the igneous and
metamorphic rocks of southeastern Pennsylvania and in many sedimentary
rocks statewide. Because they are highly resistant to erosion,
even the zircons found in sedimentary rocks commonly show their
crystal form. Tiny zircons can be panned from streams throughout