Sedimentary Rocks

Description

Sedimentary rocks are one of the three main rock types (along with igneous and metamorphic rocks). They are formed on or near the Earth's surface by the accumulation and lithification of sediments. Sediments are derived from the weathering and erosion of pre-existing rocks, the remains of dead organisms, or from chemical precipitation. They are characterized by bedding or stratification, which represents successive layers of sediment accumulation. Sedimentary rocks are broadly classified into clastic, chemical, and organic types.

How to Identify

Color

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.

Luster

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).

Texture

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.

Crystal Form

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.

Cleavage

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.

Geological Environment

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.

Quick Check

• Color: Highly variable (white, gray, black, red, brown, green)
• Luster: Dull to earthy, sometimes vitreous or pearly
• Streak: Variable, often white or light-colored for clastic rocks; specific to constituent minerals for others (e.g., hematite-rich sandstone will have a reddish-brown streak)

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.

Formation

Formed from the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles are transported by water, wind, ice, or mass movement, and then lithified through compaction and cementation.

Usage

Sedimentary rocks are extensively used in construction (e.g., sandstone, limestone for building materials, cement), as sources of fossil fuels (coal, oil shale), for industrial purposes (e.g., rock salt, gypsum), and as aquifers for groundwater.

Age Distribution

From Precambrian to Holocene, depending on the specific rock type and depositional environment.

Where to Find

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Scientific Classification

Mineral Class

Not a mineral, but a rock type. Composed of various mineral classes (silicates, carbonates, sulfates, halides, oxides, organic matter).

Group

Sedimentary Rocks (Clastic, Chemical, Organic)

Crystal System

Not applicable to the rock as a whole.

Chemical Formula

No single chemical formula, as it's a mixture of various minerals and organic compounds.

Composition

Diverse, depending on the specific rock type. Examples: Quartz (SiO2) for sandstone, Calcite (CaCO3) for limestone, Clay minerals (hydrous aluminosilicates) for shale, Halite (NaCl) for rock salt, Carbon (C) for coal.

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