Rock Classification Tables
Igneous Rocks
![](https://pressbooks.openeducationalberta.ca/app/uploads/sites/66/2020/06/dark-minerals2-1024x189.png)
![](https://pressbooks.openeducationalberta.ca/app/uploads/sites/66/2019/06/ingeous-rocks2-300x139.png)
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 |
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.](https://pressbooks.openeducationalberta.ca/app/uploads/sites/66/2020/07/sorting-roundness-01-300x168.png)
![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).](https://pressbooks.openeducationalberta.ca/selfpublishguide/wp-content/uploads/sites/66/2020/06/arenite-sandstones.png)
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
![](https://pressbooks.openeducationalberta.ca/app/uploads/sites/66/2020/07/metachart-v2-01-300x231.png)
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.