Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks
Identifying Rocks Christie Rowe
20 April 2010
Metamorphic Rocks
Eleven minerals to know
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks
I
Quartz (straight up silica)
Sedimentary Rocks
I
Plagioclase Feldspar (NaAlSi3 O8 - CaAl2 Si2 O8 )
Metamorphic Rocks
I
Alkali Feldspar (KAlSi3 O8 )
I
Muscovite (KAl2 (AlSi3 )O10 (OH,F)2 )
I
Biotite (K(Mg,Fe)3 (Al,Fe)Si3 O10 (OH,F)2 )
I
Chlorite ((Fe,Mg,Al)6 (Si,Al)4 O10 (OH)8 )
I
Carbonates: calcite (CaCO3 ), dolomite (CaMg(CO3 )2 )
I
Amphibole ((Ca,Na)2−3 (Mg,Fe,Al)5 (Al,Si)8 O2 2(OH)2 )
I
Pyroxene ((Fe,Mg)SiO3 or Ca(Fe,Mg)SI2 O6 )
I
Sulfides + oxides (assorted)
Identifying Rocks
Plagioclase Feldspar
Christie Rowe 11 Minerals
I
White or gray
I
Aluminum silicate with some combo of Calcium and Sodium
I
90◦ cleavage so when it breaks, it makes blocks
I
May have a line down middle of crystal (albite twinning)
I
or planar laminations of transparent/transluscent material
Igneous rocks Sedimentary Rocks Metamorphic Rocks
Identifying Rocks
Alkali Feldspar
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks
I
White or pink (rarely red)Metamorphic Rocks
I
Aluminum silicate with some combo of Potassium and Sodium
I
90◦ cleavage so when it breaks, it makes blocks
I
May have a wavy laminations (perthitic twinning
Identifying Rocks
Muscovite Mica
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
Clear or silvery
I
Flaky (perfect thin cleavage planes)
I
forms small hexagonal stacks = “books”
Identifying Rocks
Biotite Mica
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
Dark brown or black
I
Flaky (perfect thin cleavage planes)
I
forms small hexagonal stacks = “books”
Identifying Rocks
Clorite Mica
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
Green to gray, often shiny
I
Flaky (perfect thin cleavage planes)
I
Usually too fine grained to form books
Identifying Rocks
Carbonates
Christie Rowe 11 Minerals Igneous rocks
I
Usually white, yellow, or Sedimentary Rocks gray but can be any color Metamorphic Rocks
I
Calcite fizzes vigorously with acid
I
Dolomite fizzes weakly, but vigorously if scratched or powdered
I
Cleavage and softness (4) are best indications
I
Lots of other carbonate minerals exist but much more rare
Identifying Rocks
Amphibole
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
Dark brown or green to black
I
Long thin crystals with diamond shaped cross section
Identifying Rocks
Pyroxene
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
Dark brown, green or black
I
Stubby, rounded or long thin crystals
I
90◦ cleavages
Identifying Rocks
Sulfides and oxides (assorted)
Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
I
(Mostly) metallic minerals
I
Usually in small euhedral crystals in trace amounts
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – felsic plutonic rock
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – felsic volcanic rock
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Mafic rock classification
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – mafic plutonic rock
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – mafic volcanic rock
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Sedimentary Rocks
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks
I
Clastic sedimentary rocks have some mixture of: I I I
I I
I
Clay (single grains of small platy minerals) Silt (small single grains of quartz and feldspar) Sand (medium to large (sub-mm) grains of quartz, feldspar and rock fragments (lithics)) And on up! pebbles, cobbles...boulders! May be cemented by minerals grown in pore spaces
Chemical sediments I
I
Cherts (SiO2 ) condense from volcanic gasses and microfossils in still water (deep marine or lakes) Carbonates (CaCO2 ) precipitate from warm water or built by microorganisms, coral, shells, etc (warm marine, playa lakes)
Sedimentary Rocks Metamorphic Rocks
Classify Clastic seds by % clay, quartz, feldspar, lithics
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – Shale, siltstone, sandstone, conglomerate
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – Chert
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Metamorphic Rocks
Identifying Rocks Christie Rowe
I
I
Schist has well aligned micas and/or amphiboles making a foliation
11 Minerals
Gneiss has aligned minerals but doesn’t split easily along the foliation
Sedimentary Rocks
I
Given a starting rock, different pressure/temperature will give different minerals – these minerals give names to “facies”
I
“facies” are named for minerals that form in meta-mudstones and meta-mafic rocks because sandstones and felsic rocks don’t change that much with P/T –quartz and feldspar are too stable!!
I
Name that rock: Mineral(s) + texture (e.g. biotite schist)
I
Another way to name that rock: meta-protolith (e.g. metabasalt)
Igneous rocks
Metamorphic Rocks
Metamorphic facies
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – greenschist metabasalt = Greenstone
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – blueschist metabasalt = Blueschist
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – Amphibolite metabasalt
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – muscovite schist = mudstone at greenschist P/T
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
Example – migmatite = mudstone at granulite P/T
Identifying Rocks Christie Rowe 11 Minerals Igneous rocks Sedimentary Rocks Metamorphic Rocks
