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Identification of igneous rocks
1. Identification of Igneous Rocks<br />Grain sizeUsual colorOtherCompositionRock typeFineDarkGlassy appearanceLava glassobsidianFineLightMany small bubblesLava froth from sticky lavapumiceFineDarkMany large bubblesLava froth from fluid lavascoriaFine or mixedLightContains quartzHigh-silica lavafelsiteFine or mixedMediumBetween felsite and basaltMedium-silica lavaandesiteFine or mixedDarkHas no quartzLow-silica lavabasaltMixedAny colorLarge grains in fine-grained matrixLarge grains of feldspar, quartz, pyroxene or olivineporphyryCoarseLightWide range of color and grain sizeFeldspar and quartz with minor mica, amphibole or pyroxenegraniteCoarseLightLike granite but without quartzFeldspar with minor mica, amphibole or pyroxenesyeniteCoarseLight to mediumLittle or no alkali feldsparplagioclase and quartz with dark mineralstonaliteCoarseMedium to darkLittle or no quartzLow-calcium plagioclase and dark mineralsdioriteCoarseMedium to darkNo quartz; may haveolivineHigh-calcium plagioclase and dark mineralsgabbroCoarseDarkDense; always hasolivineOlivine with amphibole and/or pyroxeneperidotiteCoarseDarkDenseMostly pyroxene with olivine and amphibolepyroxeniteCoarseGreenDenseAt least 90% olivineduniteVery coarseAny colorUsually in small intrusive bodiesTypically graniticpegmatite<br />Identification of Sedimentary Rocks<br />HardnessGrain SizeCompositionOtherRock TypeHardCoarseClean quartzWhite to brownSandstoneHardCoarseQuartz and feldsparUsually very coarseArkoseHard or softMixedMixed sediment with rock grains and clayGray or dark and quot;
dirtyquot;
Wacked/GreywackeHard or softMixedMixed rocks and sedimentRound rocks in finer sediment matrixConglomerateHard or softMixedMixed rocks and sedimentSharp pieces in finer sediment matrixBresciaHardFineVery fine sand; no clayFeels gritty on teethSiltstoneHardFineChalcedonyNo fizzing with acidChertSoftFineClay mineralsSplits in layersShaleSoftFineCarbonBlack; burns with tarry smokeCoalSoftFineCalciteFizzes with acidLimestoneSoftCoarse or fineDolomiteNo fizzing with acid unless powderedDolomite rockSoftCoarseFossil shellsMostly piecesCoquinaVery softCoarseHaliteSalt tasteRock SaltVery softCoarseGypsumWhite, tan or pinkRock Gypsum<br />Identification of Metamorphic Rocks<br />FoliationGrain SizeHardnessUsual ColorOtherRock Typefoliatedfinevery softlightgreasy feelSoapstonefoliatedfinesoftdarkquot;
tinkquot;
when struckSlatefoliatedfinesoftdarkshiny; crinkly foliationPhyllitefoliatedcoarsehardmixed dark and lightcrushed and stretched fabric; deformed large crystalsMylonitefoliatedcoarsehardmixed dark and lightwrinkled foliation; often has large crystalsSchistfoliatedcoarsehardmixedbandedGneissfoliatedcoarsehardmixeddistorted quot;
meltedquot;
layersMigmatitefoliatedcoarseharddarkmostly hornblendeAmphibolitenonfoliatedfinesoftgreenishshiny, mottled surfaceSerpentinitenonfoliatedfine or coarseharddarkdull and opaque colors, found near intrusionsHornfelsnonfoliatedcoarsehardred and greendense; garnet and pyroxeneEclogitenonfoliatedcoarsesoftlightcalcite or dolomite by the acid testMarblenonfoliatedcoarsehardlightquartz (no fizzing with acid)Quartzite<br />