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Apr 08 2013

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Rock Classification

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To be honest, this is one of those “grey” areas in science. The classifications (naming) of rocks are usually based on empirical observations. Rarely a Geologist would name a rock based on chemical analysis. The qualitative observations of minerals in a sample is commonly used in place of chemical analysis. Therefore, we can categorically argue naming conventions are based on the chemical properties of the rock. Depending on the scheme, the rock name may be sightly varies. This is why I recommend recording the modal proportions of minerals when naming a rock (or should include a plotted diagram). This will avoid confusion in different classification schemes.

Identification of Igneous Rocks

Grain Size Usual Color Other Composition Rock Type
fine dark glassy appearance lava glass Obsidian
light many small bubbles lava froth from sticky lava Pumice
dark many large bubbles lava froth from fluid lava Scoria
fine
or mixed
light contains quartz high-silica lava Felsite
medium between felsite and basalt medium-silica lava Andesite
dark has no quartz low-silica lava Basalt
mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine Porphyry
coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite
light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite
light to medium little or no alkali feldspar plagioclase and quartz with dark minerals Tonalite
medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite
medium to dark no quartz; may have olivine high-calcium plagioclase and dark minerals Gabbro
dark dense; always has olivine olivine with amphibole and/or pyroxene Peridotite
dark dense mostly pyroxene with olivine and amphibole Pyroxenite
green dense at least 90% olivine Dunite
very coarse any color usually in small intrusive bodies typically granitic Pegmatite

Identification of Sedimentary Rocks

Hardness Grain Size Composition Other Rock Type
hard coarse clean quartz white to brown Sandstone
coarse quartz and feldspar usually very coarse Arkose
fine very fine sand; no clay feels gritty on teeth Siltstone
fine chalcedony no fizzing with acid Chert
hard
or soft
mixed mixed sediment with rock grains and clay gray or dark and “dirty” Wacke/
Graywacke
mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate
mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia
soft fine clay minerals splits in layers Shale
fine carbon black; burns with tarry smoke Coal
fine calcite fizzes with acid Limestone
coarse or fine dolomite no fizzing with acid unless powdered Dolomite rock
coarse fossil shells mostly pieces Coquina
very soft coarse halite salt taste Rock Salt
coarse gypsum white, tan or pink Rock Gypsum

Identification of Metamorphic Rocks

metamorphic rock naming flow chart

Foliation Grain Size Usual Color Other Rock Type
foliated fine light very soft; greasy feel Soapstone
fine dark soft; strong cleavage Slate
fine dark shiny; crinkly foliation Phyllite
coarse mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite
coarse mixed dark and light wrinkled foliation; often has large crystals Schist
coarse mixed banded Gneiss
coarse mixed distorted “melted” layers Migmatite
coarse dark mostly hornblende Amphibolite
non foliated fine dark soft; massive structure Argillite
fine greenish soft; shiny, mottled surface Serpentinite
fine or coarse dark dull and opaque colors, found near intrusions Hornfels
coarse red and green dense; garnet and pyroxene Eclogite
coarse light soft; calcite or dolomite by the acid test Marble
coarse light quartz (no fizzing with acid) Quartzite

Few tricks…

Igneous rocks

  • If grains can not be observed with little or no magnification aid, then
    • It must be aphanitic AND based on colour
      -dark = mafic
      -medium = intermediate
      -light = felsic
      -volcanic glass
      …use QAPF Aphenetic diagram
  • If grains can be observed with little or no magnification aid, then
    • It must be phaneritic AND based on colour
      record the crystal shapes, sizes and mineral inclusions
      …use QAPF Phaneritic diagram

Sedimentary rocks

  • Based on the modal proportions of quartz-feldspars-lithics (Q-F-L diagram);
    1. quartzarenite
    2. graywacke
    3. arkose
    4. lithic arenite

Indicates metamorphism

  • If it has a shale protolith AND it is foliated (regional), then
    • Must be Barrovian sequence
      Chloorite (low grade)
      -Biotite
      -Garnet
      -Staurolite
      -Sillimanite
      -Sillimanite and K-feldspar (high grade)
  • If it has a shale protolith AND it is not foliated (regional), then
    • Must be Buchan sequence
      -Spotted phyllite (low grade)
      -Cordierite-andalusite hornfels
      -K-feldspar hornfels
      -Sillimanite (high grade)
  • If it has a basalt protolith AND it is foliated, then
    • What are the mineralogical characteristics?

Classification Charts

Carbonates Classification

For Carbonates: Dunham (1962) and modified by Embry and Klovan (1971)


For Classifications of Sandstones

For Classifications of Sandstones


Sandstone classification triangle - Folk (1974)

Sandstone classification triangle – Folk (1974)


For Aphenetic rocks

For Aphenetic rocks


For Phaneritic (plutonic) rocks

For Phaneritic (plutonic) rocks


This one is specific to rocks in fault zones.

This one is specific to rocks in fault zones.

References

BGS Rock Classification Scheme
Structural Geology by Haakon Fossen

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