rocks ... and how to identify them
TRANSCRIPT
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Rocks ... and how to identify them
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THE ROCK CYCLE
Rocks are naturally occurring combinations or coherent
aggregates of minerals, fossils or other hard materials.
They are classified by the way in which they form. Thethree rock types are: igneous, sedimentary and
metamorphic.
All rocks on Earth are locked into a system of cycling and
re-cycling known as the ROCK CYCLE.
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METAMORPHICETAMORPHIC SEDIMENTARYEDIMENTARY
IGNEOUSGNEOUS
meltin
g
heat
,pre
ssur
e,
ions
weathering,tra
nsp
orta
tion
lithific
ation
melting
weathering, transportation,lithification
heat, pressure, ions
THE ROCK CYCLE
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IGNEOUS ROCKSIGNEOUS ROCKSare born of
fire. In other words, they
were once molten and upon
cooling, themagma(molten
rock) crystallized into solid
rock. Igneous rocks may
form deep inside the Earth or
at the Earths surface when a
volcano erupts.
magma
conduit
Anatomy of a Volcano
vent
lava
landsurface
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IGNEOUS ROCKS
Slow cooling deep beneath the
Earths surface allows crystals
to grow to large size (1/8 or
more). These crystals are
easily visible and distinguish
this group of igneous rocks as
INTRUSIVE.
Rapid cooling near or at theEarths surface, producesmany small crystals that are
not readily seen by theunaided eye. This group ofigneous rocks is calledEXTRUSIVEand are typicallyvolcanic in origin. Cooling
may be so rapid that crystalsdo not have a chance to formand instead a glass isproduced.
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LIGHT COLORED
Felsic
INTERMEDIATE
COLOREDDARK COLORED
Mafic
VERY DARK
COLORED
Ultramafic
COARSE-
GRAINED
(You can seedifferent
minerals)
GRANITE: Can see crystals. Usually
gray or pink. Can seequartz - gray, glassy grains.Can see feldspar - pink,buff, or white.
DIORITE:Can see crystals with
somewhat more light coloredfeldspar grains than dark
colored minerals. A mix of lightand dark but with no quartz. Salt& pepper appearance.
GABBRO:Can see crystals lots of
flat shiny cleavage surfaces.
Usually black to greenishblack.
FINE-
GRAINED
(You can NOTsee crystals, forthe most part)
RHYOLITE:Usually gray, pink,
pastel. Might see smallclear, rectangular crystals.
Sometimes banded.
ANDESITE:Light to dark gray. Normally has
small black crystals
BASALT:Usually black or rust red.
May have some or lots ofgas bubble holes, someholes may be filled. Maysee small green grains.
GLASSY OBSIDIAN: Black, red, green, GLASSPERLITE: Usually pearly gray. May contain Apache Tears.
PUMICE: LOTS of gas bubble holes, very lightweight, will float on water. Abrasive.
PORPHYRITIC
(2 grain sizes)ALL CRYSTALLINE IGNEOUS VARIETIES may exhibit porphyritic texture
FRAGMENTAL
TUFF: Compacted volcanic fragments generally less than 4mm diameter (ash)
VOLCANIC BRECCIA: Mixed tuff and angular large (>32mm diameter) fragments
AGGLOMERATE: Mixed ash and rounded/sub-rounded large (>32mm diameter) fragments
PERIDOTITE:Composed of 90-
100% olivine
PYROXENITE:Composed of pre-
dominately pyroxene
AMPHIBOLITE:Composed of pre-
dominately amphiboles(such as hornblende)
CLUES TO
IGNEOUS ROCKS
COLOR
TEXTURE
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Granite - intrusive
GRANITE is a coarse to medium-
grained rock that forms from
the cooling of magma deep
within the Earth (intrusive). Itis made up mainly of varying
amounts of the minerals:
quartz, orthoclase, muscovite,
biotite and hornblende. Thename is from the Latin
granum, for grains.
quartz
feldspar
biotite
mica
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Granite - intrusive
Pegmatite
Graphic Granite
Porphyritic
mica
feldspar
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Granite - intrusive Pegmatite a Special Case
tourmaline
feldspar
mica
quartz
PEGMATITESareclassified as intrusive igneous
rocks, but there is a
difference. They are VERY
coarse grained and strictly
speaking are not crystallizingout of a magma.
The coarse grained nature is
the result of crystal growth in
aqueous solutions rather than
in the molten liquid state.
The resulting freedom of ion
motion allows the crystal to
grow much larger in a shorter
length of time.
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Diorite - intrusive
DIORITE is very similar to
granite, but is distinguished
in the hand specimen by theabsence of visible quartz.
Generally it has a salt and
pepper appearance (about
black and white).
feldspar
biotite
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Gabbro - intrusive
GABBROis a coarse-grained
rock that is high in iron &
magnesium-bearing minerals(pyroxenes, amphiboles,
plagioclase feldspar, olivine).
The rocks will be dark in color,
somewhat heavier thangranitic rocks and devoid of
quartz.Black minerals are primarily amphibole (like
hornblende) and plagioclase feldspar.
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Peridotite -intrusive
PERIDOTITE or DUNITE is
composed of 90-100%
olivine. As a result it ischaracteristically olive-greens
in color.
This material is thought to
have originated in the upper
mantle of the Earth.
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Rhyoliteextrusive
RHYOLITES name comes from
the Greek rhyo, from rhyax,
stream of lava. It is formed
when molten rock with thesame composition as a high
silica granite oozes (rhyolite is
VERY viscous and does not
really flow) to the Earthssurface; and therefore cools
quickly so only microscopic-
sized crystals develop. The
volcanoes that produce
rhyolite are very explosive
varieties such as Mt. St.Helens, Krakatoa and
OLeary Peak (AZ).
Frequently it is banded due
to flow alignment of differentassociated minerals (quartz,
feldspar, mica, and
hornblende).
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This is an example of
banded rhyolite.
Rhyoliteextrusive
If you look closely, you might see tiny clear phenocrysts of
feldspar.
This is another sample of rhyolite.
This has a crystal of garnet that
grew after the rhyolite was
deposited.
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Andesite - extrusive
ANDESITE is the fine-
grained equivalent of
DIORITE. It tends to bea darker gray than
rhyolite and is often
porphyritic, with visible
hornblende.
hornblende
phenocrysts
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Basalt - extrusive
BASALT occurs as thin to
massive lava. flows,
sometimes accumulating to
thicknesses of thousands offeet and covering thousands
of square miles. The
volcanoes that produce
basaltic lavas are relativelyquiet, such as the Hawaiian
Islands volcanoes. Basalt
is dark, fine-grained andoften vesicular(having gaspockets). The pockets may
be filled with secondaryminerals, e.g. quartz, zeoliteminerals, calcite, opal, etc.and then it is calledamygdaloidal(a-mig-duh-loy-
dal) basalt.The name may have originated
with Pliny who used theEthiopian word basalfor iron-bearing rocks.
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Basalt - extrusive
Gases released near the surface
of a lava flow create bubbles or
vesiclesthat are frozen in
stone.
Vesicular (Scoria)
Amygdaloidal
Volcanic Bomb
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Basalt - extrusive
Peridotite
Peridotite xenolith
Basalt
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Basalt - extrusivePahoehoe is a feature
that forms on the
surface of very fluid
basalt flows. Much
like the skin on a bowl
of tomato soup the
surface in contact with
the air begins to
crystallize, while the
fluid lava belowcontinues to flow. This
drags the upper, still
plastic, surface into a
series of smooth
wrinkles. (*)
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Obsidian - extrusive
OBSIDIANis volcanic glass (an
acrystalline solid actually a
supercooled liquid). Its
glassy, lustrous andsometimes banded
appearance makes it rather
easy to distinguish from all
other rocks. It is composedof the elements that make
quartz, feldspar and iron/
magnesium minerals that
have cooled so quickly that
the minerals could not
develop and crystallize.
Colors vary from black to red,
black & red (mahogany),
gray, green, iridescent,
snowflake.
Apache Tears are little nodules
of obsidian.
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Obsidian - extrusiveFlow banding
Apache tear
Snowflake
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Perlite - extrusive
PERLITE is a light gray volcanic
glass, having numerous
concentric cracks which give
rise to a perlitic or onion skinstructure. It is generally of
rhyolitic composition. It also
exhibits apearly luster.
Apache Tears are some times
found embedded in perlite.
Apache Tear
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Pumice - extrusive
PUMICE is highly vesicular
(i.e. it has lots of gas
bubble holes) and is ofrhyolitic composition.
Due to the many small
holes, it is lightweight
and will often float inwater.
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Tuff - extrusive
Tuffis the accumulation of ash
and small pyroclastic debris
(
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Volcanic Breccia - extrusive
VOLCANIC BRECCIA ispyroclastic (fire-formedfragments) and forms inexplosive eruptions. It isa mix of large angularfragments and small ash.
Often, the material is hotwhen it comes to restand cools (welds) into avery hard rock.
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SEDIMENTARY ROCKS
SEDIMENTARY ROCKS are composed of particlesderived from pre-existing rocks or by thecrystallization of minerals that were held in solutions.
A general characteristic of this group is the layering orstratification, as seen in the outcrop.
Those sedimentary rocks that are
composed of particles of pre-existing
rocks are considered FRAGMENTALorCLASTIC. These fragments show
evidence of transport rounding of the
grains and size sorting.
CHEMICAL sedimentary rocks are the
result of either precipitation of solids
from solutions (like salt from water) orby organic process, like shells from
marine organisms.
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CLUES TO
SEDIMENTARY ROCKS
FRAGMENTAL: Composed of pieces of rocks and minerals
LARGE PIECES
(Boulders,cobbles,pebbles)
BRECCIA: Composed of large angular pieces and clay
CONGLOMERATE: Composed of large rounded pieces andclay
SMALL PIECES
(sand)
SANDSTONE: Looks sandy (may shed sand grains), feelsrough (like sandpaper); may be tan, white, red, gray.
VERY SMALL PIECES(clay, silt, mud)
SHALE: Has very thin layers; often black. May have fossilsusually impressions (no shell, but indentation with pattern ofshell) or carbonized film (as for plants). Has a dull luster. Issoft. When tapped with a rod or on a table, it generally makesa dull thunk.
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Breccia - fragmental
A BRECCIA is made of varyingsizes ofangularfragmentscemented together. The
name is from the Italian wordfor broken stones orrubble.
Many form as the result of faultmovement; others form as
the result of rapid and shorttransportation, such aslandslides.
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Conglomerate - fragmental
CONGLOMERATES are very
similar to breccias, but the
fragments are rounded. The
name is from the Latinconglomeratusfor heaped,
rolled or pressed together.
These rocks form in alluvial
fans, stream beds andpebble beaches.
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Sandstone -fragmentalSANDSTONE is made up of
fine-grained particles (1/162 mm). The sand grains(often quartz) are commonly
cemented by silica,carbonates, clay or ironoxides. Sandstone isidentified by its sandy texture which often translates intoa gritty feel.
Environments in whichsandstones form includebeaches, sand bars, deltas
and dunes.
Coconino Sandstone,
the result of a Permian
age coastal dune field
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Shale - fragmental
SHALE is a very common rockmade of silt and clay sizedparticles. It is generally verythin-bedded and splits alongthe bedding planes. In fact,
the name is probably fromthe Old English scealu, shellor husk. Normally gray toblack, shale may be brown todark red, depending on the
amount of included ironoxide.
Shales form in quiet environ-ments, such as lakes,swamps, deltas and offshoremarine.
Black shale, deposited in a offshore
basin in a Middle Cambrian sea.
Wheeler Shale with trilobite fossil
(Elrath ia king i i )
Fish scales
Utah
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MORE CLUES TO
SEDIMENTARY ROCKS
CHEMICAL: Rocks are crystalline
LIMESTONES: Composed of calcite and all WILL FIZZ vigorously in acidCrystalline Looks sugary, usually gray or tanFossiliferous Contains seashells (usually) or other aquatic organismsTravertine Looks sugary with bands of various colorsChalkWhite and soft (comes off on hands)Coquina Contains almost nothing but seashells or seashell fragments
CHERT: Cryptocrystalline QUARTZVery fine can NOT see crystals
Waxy lusterConchoidal fracture (breaks like glass)Very hard will easily scratch glassMay be ANY COLOR (Red = Jasper, Black = Flint, includes
Silicified Wood)
GYPSUM: White, gray, clearVery soft you can scratch with fingernailClear sheets, fibrous or sugary
ROCK SALT: White to clear cubic shape
Soft
you can scratch it with fingernailTastes like table salt (IT IS TABLE SALT!)
COAL: Black; Brittle; LightweightMay contain plant fossilsVarieties: peat, lignite (incl. jet), sub-bituminous, bituminous
DIATOMITE: WhiteVery soft (comes off on your hands)Lightweight
Will NOT fizz in HCl acid (unlike chalk)
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Limestone -chemicalUses:
Manufacture of lime andPortland cement & toneutralize smokestackgases.
Finely ground, used as a
functional filler in productssuch as paint, countertops& plastics.
The dust on chewing gum isground limestone.
Mild abrasive additive to
toothpaste. Soil conditioner
Flux in processing iron andcopper ores.
Building and ornamental
stone.
LIMESTONE is composedprimarily of calcite. Generally
it is dense, fine-grained, and
usually white to dark gray. Its
most distinguishing feature is
its solubility in weakhydrochloric or acetic acid
accompanied by brisk
effervescence.
The environment of deposition ifgenerally warm, shallow seas.
Therefore, marine invertebrate
fossils are common.
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LIMESTONE - chemical
Redwall Limestone
Pleistocene, Rocky Point,
Mexico
Crystalline
Fossiliferous
Coquina
Travertine
Mayer, AZ
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CHERT - chemical
CHERT is crypto-crystallinequartz. It is often the result ofthe dissolution of volcanic ash
and is sometimes found inextensive beds, such as thenovaculite of Arkansas.
It has waxy luster, is translucentand fractures conchoidally.
Chert can be any color, butextensive beds are generallywhite to gray.
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GYPSUM - chemical
GYPSUM(calcium sulfate) isfound in geographically wide-spread deposits resulting fromthe evaporation of a body ofwater, such as ocean basin or
playa lake.It is soft (H=2) & usually white to
gray. Three varieties are:Alabaster, Satin Spar andSelenite.
Gypsum is mined for use inwallboard and plasters, as anagricultural amendment and tocontrol the set/cure time of
Portland cement.
AlabasterSatin Spar
Selenite
Gypsum & Anhydrite (water-less
calcium sulfate), Carlsbad, NM
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ROCK SALT -chemical
ROCK SALT (halite sodiumchloride) is also a depositresulting from evaporation of
a marine basin or playa lake.It has cubic cleavage and tastes
salty.
Rock salt is used as a source ofchlorine and sodium, as a
food supplement, in watersofteners and as a road de-icer.
Halite Trona, CA
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COAL - chemical
COALis considered a rock,although it is not composed ofminerals, but rather the
decomposed remains of largevolumes of vegetation thataccumulated in a wet, lowoxygen environment, such as aswamp or marsh.
Peat, Lignite and Sub-Bituminous& Bituminousare sedimentaryvarieties of coal and are usedas fuels.
Coal (sub-bituminous) out of the Cretaceous Dakota
Formation of north-eastern Arizona.
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DIATOMITE - chemical
DIATOMITE, also known as
diatomaceous earth, is
composed of the siliceous
shells of microscopic algacalled diatoms. It is light
weight and is generally white.
Diatomite is used as an
abrasive, insecticide, filteringmedium, and paint flattener.
San Manuel, AZ
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METAMORPHIC ROCKSMETAMORPHIC ROCKS have changed (meta) their form (morphic). Under
the influence of heat, pressure and fluids, pre-existing rocks are modifiedin form and even in internal atomic structure to produce new rocks stableat the new conditions. This is done within the solid state, i.e. without
melting.Changes that occur include: increase in grain size, new minerals andfoliation (parallel alignments).
Metamorphic rocks that exhibitparallel alignments ofminerals are calledFOLIATED. In these rocks,the minerals all line upperpendicular to the exerted
pressure.
Metamorphic rocks composed ofminerals that are not elongatedor flat, do not exhibit parallelalignments and are calledNON-FOLIATED.
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FOLIATED: Rocks have layers or banding
SLATE: Rock breaks into very thin layersBeginning to look polished; Is harder than shale
Cannot see crystalsBlack , gray or red
PHYLLITE: Like slate, but shinier (phyllitic sheen similar to satin)
SCHIST: Very shiny you can SEE CRYSTALS (usually MICA)Is layered
May have crystals (of garnet, tourmaline, etc.) growingwith the mica
GNEISS: Crystalline
Black & White BANDING (due to segregation of minerals)
CLUES TO
METAMORPHIC ROCKS
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SLATE -foliatedSLATEis derived from shale. It
is a dense, microcrystallinerock, but one in whichparallel planes are veryevident in its slaty foliationa feature resulting from thealignment of clay and micaminerals,which allows it tosplit readily into sheets. It
may be gray, black, green orred.
Uses include roofing, flagstone,pool table tops andblackboards.
Note the relatively dull luster of slate.
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PHYLLITE -foliated
PHYLLITEis somewhat more
metamorphosed than slate.
The platy crystals of mica
have grown and the rockdisplays a subtle, satiny
shine referred to as phyllitic
sheen. The name comes
from its leaf-like (many finelayers) appearance. Note the phyllitic sheen.
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SCHIST -foliatedSCHISTis medium to coarse-
grained, crystalline, withprominent parallel mineralorientation. Typically, it ispredominately muscovite mica,which lends a silvery white togray sparkly appearance. It isnot unusual for accessoryminerals (such as garnets,
staurolite, tourmaline) to grow inthe rock.
Schist is added to clay mixtures asa strengthening material invitreous pipe (red sewer) and
clay roof tiles.
Crumpling of schist due to
pressure and collapse of
mica crystals
garnet
porphyroblast
tourmaline
porphyroblast
note alignment
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GNEISS -foliated
GNEISS formed underconditions of hightemperatures and
pressures at great depthduring regionalmetamorphism. It ischaracterized byfoliation expressed as
black and white banding.Because the rockbecomes plastic, thebanding is oftencontorted (squiggly).
Gneissic graniteseparation of dark
& light minerals is
just beginning
Well banded gneiss
Augen Gneiss
Augen = quartz
pebble resistant tocompression
kink in gneiss
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Shale
metamorphism of shaleSHALE is the most common sedimentary rock.
Through the agents of metamorphism it changes to rocks that
are stable at higher temperatures and pressures.
These changes take place in the solid state.
Increasing Temperature and Pressure
SlatePhyllite
SchistGneiss
GRANITE
MELTING Produces
GRANITE
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METAMORPHICETAMORPHIC SEDIMENTARYEDIMENTARY
IGNEOUSGNEOUS
me
lting
heat
,pre
ssur
e,
ions
weathering,tra
ns
portatio
n
lithific
ation
melting
weathering, transportation,lithification
heat, pressure, ions
THE ROCK CYCLE
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CLUES TO
METAMORPHIC ROCKS
NON-FOLIATED: Shows NO layers or banding
MARBLE: Sugary looking
Will fizz in HCl (acid)Often is multi-colored, may be white
Soft will not scratch glass
QUARTZITE: Very dense
MAY look a bit sandyVery hard will easily scratch glass
METACONGLOMERATE: Looks like sedimentary conglomerate,
BUT it is harder (BREAKS THROUGH PEBBLES) and often thepebbles are squished & aligned (it is at this point foliated)
SERPENTINITE: Composed of members of the serpentine family(includes chrysotile asbestos)
Generally light greenish gray to greenish blackWaxy lusterOften exhibit curved and slickensided surfaces
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MARBLEnon-foliatedMARBLEis metamorphosed
limestone or dolomite. Thecolors can vary from pure whiteto gray, green, yellow, brown,black, red or any combinationthereof, depending on theimpurities in the parent
limestone. Bands or streaksresult from plastic flow duringextreme deformation, due to highpressure and temperature.
It is calcite or dolomite and will fizz inweak acids.
Marble is used for building facades,floors, countertops, statuary,landscaping, paving/roofing,poultry grit, and as filler/extenderfor paint, plastics, paper and
adhesives.
Aguila, AZ
Hewitt Canyon, AZ
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QUARTZITEnon-foliated
QUARTZITE is metamor-
phosed quartz sand-
stone. It is a very
dense,durable, massive,
microcrystalline rock (but
still may retain a slightly
sandy look). It can beany color, but tends to be
white, tan or pink.
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SERPENTINITEnon-foliated
SERPENTINITEis composed ofone or more minerals in theserpentine family. It is
common where wet basaltsor mantle rocks aremetamorphosed, such as atconvergent boundaries.
Its green colors, waxy luster,
often associated asbestosand common slickensidedsurfaces are clues to itsidentity.
Chrysotileasbestos
Salt River Canyon, AZ
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METACONGLOMERATEnon-foliatedMETACONGLOMERATE is
metamorphosedconglomerate. It retains itspebbly appearance, butwhile a sedimentaryconglomerate will break
around the pebbles, ametaconglomerate willbreak through the pebbles.
If temperatures are high
enough in the presence ofpressure, the pebbles maybecome squished orflattened and will beelongated parallel to each
Conglomerate