Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocksIdentifying Rocks

Christie Rowe

20 April 2010

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Eleven minerals to know

I Quartz (straight up silica)

I Plagioclase Feldspar (NaAlSi3O8 - CaAl2Si2O8)

I Alkali Feldspar (KAlSi3O8)

I Muscovite (KAl2(AlSi3)O10(OH,F)2)

I Biotite (K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2)

I Chlorite ((Fe,Mg,Al)6(Si,Al)4O10(OH)8)

I Carbonates: calcite (CaCO3), dolomite (CaMg(CO3)2)

I Amphibole ((Ca,Na)2−3(Mg,Fe,Al)5(Al,Si)8O22(OH)2)

I Pyroxene ((Fe,Mg)SiO3 or Ca(Fe,Mg)SI2O6)

I Sulfides + oxides (assorted)

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Plagioclase Feldspar

I White or gray

I Aluminum silicate withsome combo of Calciumand Sodium

I 90◦ cleavage so when itbreaks, it makes blocks

I May have a line downmiddle of crystal (albitetwinning)

I or planar laminations oftransparent/transluscentmaterial

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Alkali Feldspar

I White or pink (rarely red)

I Aluminum silicate withsome combo of Potassiumand Sodium

I 90◦ cleavage so when itbreaks, it makes blocks

I May have a wavylaminations (perthitictwinning

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Muscovite Mica

I Clear or silvery

I Flaky (perfect thincleavage planes)

I forms small hexagonalstacks = “books”

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Biotite Mica

I Dark brown or black

I Flaky (perfect thincleavage planes)

I forms small hexagonalstacks = “books”

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Clorite Mica

I Green to gray, often shiny

I Flaky (perfect thincleavage planes)

I Usually too fine grainedto form books

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Carbonates

I Usually white, yellow, orgray but can be any color

I Calcite fizzes vigorouslywith acid

I Dolomite fizzes weakly,but vigorously if scratchedor powdered

I Cleavage and softness (4)are best indications

I Lots of other carbonateminerals exist but muchmore rare

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Amphibole

I Dark brown or green toblack

I Long thin crystals withdiamond shaped crosssection

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Pyroxene

I Dark brown, green orblack

I Stubby, rounded or longthin crystals

I 90◦ cleavages

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Sulfides and oxides (assorted)

I (Mostly) metallic minerals

I Usually in small euhedralcrystals in trace amounts

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – felsic plutonic rock

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – felsic volcanic rock

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Mafic rock classification

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – mafic plutonic rock

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – mafic volcanic rock

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Sedimentary Rocks

I Clastic sedimentary rocks have some mixture of:I Clay (single grains of small platy minerals)I Silt (small single grains of quartz and feldspar)I Sand (medium to large (sub-mm) grains of quartz,

feldspar and rock fragments (lithics))I And on up! pebbles, cobbles...boulders!I May be cemented by minerals grown in pore spaces

I Chemical sedimentsI Cherts (SiO2) condense from volcanic gasses and

microfossils in still water (deep marine or lakes)I Carbonates (CaCO2) precipitate from warm water or

built by microorganisms, coral, shells, etc (warm marine,playa lakes)

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Classify Clastic seds by % clay, quartz, feldspar,lithics

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – Shale, siltstone, sandstone,conglomerate

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – Chert

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Metamorphic Rocks

I Schist has well aligned micas and/or amphibolesmaking a foliation

I Gneiss has aligned minerals but doesn’t split easilyalong the foliation

I Given a starting rock, different pressure/temperaturewill give different minerals – these minerals give namesto “facies”

I “facies” are named for minerals that form inmeta-mudstones and meta-mafic rocks becausesandstones and felsic rocks don’t change that muchwith P/T –quartz and feldspar are too stable!!

I Name that rock: Mineral(s) + texture (e.g. biotiteschist)

I Another way to name that rock: meta-protolith (e.g.metabasalt)

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Metamorphic facies

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – greenschist metabasalt = Greenstone

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – blueschist metabasalt = Blueschist

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – Amphibolite metabasalt

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – muscovite schist = mudstone atgreenschist P/T

Identifying Rocks

Christie Rowe

11 Minerals

Igneous rocks

Sedimentary Rocks

MetamorphicRocks

Example – migmatite = mudstone at granuliteP/T