ROCKS ... and How to Identify Them

7/28/2019 ROCKS ... and How to Identify Them

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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

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ROCKS ... and How to Identify Them