Rock Classification Tables

Igneous Rocks

Figure A: Visual guide to estimating the proportions of dark minerals in light-coloured rocks.
Figure B: A simplified classification diagram for igneous rocks based on their mineral compositions. Note that glassy igneous rocks pumice and obsidian are not included on this diagram.

Sedimentary Rocks

Table A: The Udden-Wentworth grain-size scale for classifying sediments and the grains that make up clastic sedimentary rocks
Type Description Size range (millimetres, mm) Size range (microns, μm)
Boulder large 1024 and up
medium 512 to 1024
small 256 to 512
Cobble large 128 to 256
small 64 to 128
Pebble (Granule) very coarse 32 to 64
coarse 16 to 32
medium 8 to 16
fine 4 to 8
very fine 2 to 4
Sand very coarse 1 to 2 1000 to 2000
coarse 0.5 to 1 500 to 1000
medium 0.25 to 0.5 (1/4 to 1/2 mm) 250 to 500
fine 0.125 to 0.25 (1/8 to 1/4 mm) 125 to 250
very fine 0.063 to 0.125 (1/16 to 1/8 mm) 63 to 125
Silt very coarse 32 to 63
course 16 to 32
medium 8 to 16
fine 4 to 8
very fine 2 to 4
Clay clay 0 to 2
Table B: The main types of clastic sedimentary rocks and their characteristics. You are expected to be able to identify the bolded rock names in this course.
Group Examples Characteristics
Conglomerate Dominated by rounded clasts, granule size and larger (>2 mm), poorly to very poorly sorted
Breccia Dominated by angular clasts, granule size and larger (>2 mm), poorly to very poorly sorted
Sandstone quartz sandstone Dominated by sand (1/16 to 2 mm), greater than 90% quartz, range of roundness and sorting possible
arkose (feldspathic sandstone) Dominated by sand (1/16 to 2 mm), greater than 10% feldspar, range of roundness and sorting possible
lithic wacke Dominated by sand (1/16 to 2 mm), greater than 10% rock fragments, greater than 15% silt and clay, range of roundness and sorting possible
Mudrock mudstone Greater than 75% silt (1/256 to 1/16 mm) and clay (<1/256 mm), not bedded, well-sorted, grains too fine to judge roundness using hand lens
shale Greater than 75% silt (1/256 to 1/16 mm) and clay (<1/256 mm), thinly bedded, well-sorted, grains too fine to judge roundness using hand lens
Figure 5.35: A visual reference for descriptions of sorting (top) and roundness (bottom) of sediments and grains in clastic sedimentary rocks. Note that rounded grains are not necessarily spherical in shape! Grain shapes are controlled by both the extent of transportation (and abrasion) and by the physical properties of the grain.
Figure C: A visual reference for descriptions of sorting (top) and roundness (bottom) of sediments and grains in clastic sedimentary rocks.
Figure 5.3.5: A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Sandstones with more than 15% silt and clay are called wackes (e.g., quartz wacke, lithic wacke).
Figure D: A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Sandstones with more than 15% silt and clay are called wackes (e.g., quartz wacke, lithic wacke).
Table C: Classification chart for chemical sedimentary rocks
Composition Texture Distinctive Properties Rock Name
Calcite (CaCO3)

*Note that all limestones will react with dilute HCl.

 Crystalline Crystalline; fine to coarse grained Crystalline limestone
Fossiliferous Various fossil fragments well cemented together Fossiliferous limestone
Oolitic Comprised of ooids (spheroidal particles typically <2 mm in diameter) Oolitic limestone
Bioclastic Visible shell fragments weakly cemented together Coquina
Bioclastic Soft rock made of microscopic shells Chalk
Quartz (SiO2) Microcrystalline Microcrystalline; hardness of ~7 (can scratch glass); may exhibit conchoidal fracture Chert (note that dark coloured varieties may be called flint and red coloured varieties may be called jasper)
Halite (NaCl) Crystalline Crystalline; fine to coarse grained; commonly forms cubic crystals; tastes salty Rock salt
Gypsum (CaSO4·H2O) Crystalline Crystalline; fine to coarse grained; hardness ~2 (can scratch with fingernail) Rock gypsum
Organic material (plant fragments) Amorphous Black brittle rock with amorphous texture; low density Coal

Metamorphic Rocks

Figure E: Metamorphic grades, common metamorphic index minerals, and corresponding rock names for a mudrock protolith under increasing metamorphism (increasing temperature and pressure).
Table D: A rough guide to the types of metamorphic rocks that form from different protoliths at different grades of regional metamorphism. You are expected to know the rock names indicated in bold font.
Protolith Very Low Grade (150-300°C) Low Grade (300-450°C) Medium Grade (450-550°C) High Grade (Above 550°C)
Mudrock slate phyllite schist gneiss
Granite no change no change almost no change granite gneiss
Basalt greenschist greenschist amphibolite amphibolite
Sandstone no change little change quartzite quartzite
Limestone little change marble marble marble

Media Attributions

  • Figures A, B, D: © Steven Earle. CC BY.
  • Figures C, E: © Siobhan McGoldrick. CC BY.

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A Practical Guide to Introductory Geology Copyright © 2020 by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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