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