How to identify Sedimentary Rocks

To correctly identify Sedimentary Rocks (Various Sedimentary Rocks), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable, depending on mineral composition and presence of organic matter or iron oxides. Can range from white (pure quartz sandstone, limestone) to black (shale, coal) through reds, browns, grays, and greens.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be vitreous in some clastic grains (e.g., quartz in sandstone) or pearly in some evaporites (e.g., gypsum).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic rocks are defined by grain size (e.g., conglomerate, sandstone, siltstone, shale). Chemical rocks can be crystalline (e.g., rock salt, gypsum) or microcrystalline (e.g., chert, some limestones). Organic rocks are often amorphous or finely layered (e.g., coal). Presence of bedding, fossils, and ripple marks are common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains within clastic sedimentary rocks often retain their original crystal forms or are rounded/sub-rounded. Chemical sedimentary rocks can exhibit euhedral to anhedral crystals depending on growth conditions. Organic rocks typically lack distinct crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage is generally not a characteristic of the rock as a whole, but rather of the individual mineral grains within it. For example, calcite in limestone has perfect rhombohedral cleavage, while quartz in sandstone has no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Form in diverse environments including oceans (deep marine, shallow marine), lakes (lacustrine), rivers (fluvial), deserts (aeolian), glaciers (glacial), and swamps/marshes (paludal). Each environment leaves characteristic textures, structures, and fossil assemblages.

Key Facts

Hardness

Highly variable, from very soft (e.g., shale, gypsum, coal) to hard (e.g., well-cemented quartz sandstone, chert). Mohs hardness can range from 1-7+.

Specific Gravity

Highly variable, typically 2.0-2.8 g/cm³ for most common types, but can be lower for organic rocks (e.g., coal ~1.2-1.5 g/cm³) or higher for some evaporites (e.g., rock salt ~2.16 g/cm³).

Crystal System

Not applicable to the rock as a whole, but individual mineral grains within the rock will have their own crystal systems (e.g., quartz is trigonal, calcite is trigonal).

Color

Extremely diverse, reflecting mineralogy, organic content, and oxidation state.

Luster

Generally dull, earthy, or granular; can be vitreous for quartz grains, pearly for gypsum, or waxy for chert.

Transparency

Opaque to translucent, depending on grain size, mineralogy, and cementation.

Fracture

Variable; can be conchoidal (chert), splintery (shale), granular (sandstone), or irregular.

Cleavage

Absent in the rock as a whole, but present in individual mineral components (e.g., calcite, feldspar).

Composition

Composed of fragments of pre-existing rocks and minerals (clastic), precipitated minerals from solution (chemical), or organic remains (organic). Common minerals include quartz, feldspar, clay minerals, calcite, dolomite, gypsum, halite, and organic carbon.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Individual mineral grains can be anhedral, subhedral, or euhedral, or amorphous.
• Cleavage Type: Not applicable to the rock as a whole. Individual mineral components may exhibit cleavage (e.g., perfect rhombohedral in calcite, basal in micas).
• Fracture Type: Variable: granular, conchoidal, splintery, irregular.
• Tenacity: Variable: friable (shale, poorly cemented sandstone), brittle (limestone, chert), sectile (gypsum), elastic (some shales).
• Luster Type: Dull, earthy, granular, vitreous, pearly, waxy.

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