Sandstone: Types, Formation, Classification, Uses

Sandstone is a sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. It is one of the most common sedimentary rocks, making up about 20–25% of all sedimentary rocks.

Sandstone is a clastic sedimentary rock, meaning it's formed from cemented pieces of other rocks and minerals. These pieces, called clasts, are typically sand-sized, ranging from 0.0625 to 2 millimeters in diameter. Quartz and feldspar are the most common clasts, but you can also find sand grains of rock fragments, fossils, and even volcanic glass.

Sandstone Key Features

Category: Clastic Sedimentary Rock
Texture: Clastic, with visible sand-sized grains (0.06-2 mm) (visible to the eye).
Composition: Primarily quartz and feldspar, with other minerals and a cementing material
Color: Highly variable, influenced by mineral composition, cement, and impurities (red, brown, yellow, tan, gray, pink, white, black)
Hardness: Variable, typically soft to moderately hard (depends on mineral composition and cement)

How is sandstone formed

Sandstone is formed from the accumulation and compaction of sand particles, often derived from the weathering of other rocks.

Sandstone samples

1. Weathering and Erosion

This stage breaks down existing rocks into smaller fragments, including sand-sized particles. The primary agents of weathering and erosion include:

Physical weathering: Freezing and thawing, temperature changes, and plant roots can crack and break down rocks.
Chemical weathering: Rainwater dissolves minerals in the rock, particularly feldspar and calcite, leaving behind sand-sized quartz grains.
Biological weathering: Burrowing animals, plant roots, and microorganisms can break down rocks and contribute to soil formation.

2. Transportation

Once sand grains are released, they need to be transported away from their source rock to accumulate in large enough quantities for sandstone formation. This transportation happens through:

Rivers: Flowing water carries sand grains downstream, sorting them based on size and weight. Finer grains travel farther, while coarser ones settle closer to the source.
Wind: In deserts and coastal areas, wind can pick up sand and transport it over long distances, often forming dunes.
Glaciers: Moving glaciers can grind down rocks and carry sand grains along with other debris, depositing them later when the glacier melts.

3. Deposition

Sand grains eventually settle down in areas where the transporting agent loses its energy. This can occur in:

River deltas: Where rivers meet oceans or lakes, their flow slows down, causing sand to be deposited in layers.
Beaches: Waves and currents sort and deposit sand along coastlines, creating beaches and dunes.
Deserts: Windblown sand can accumulate in large areas, forming deserts like the Sahara.
Continental basins: Areas within continents can collect sand eroded from surrounding mountains, forming sandstone deposits.

4. Lithification

Over time, the deposited sand grains undergo a process called lithification, transforming them into solid rock. This involves:

Compaction: The weight of overlying sediments presses down on the sand, squeezing the grains closer together and reducing pore space.
Cementation: Mineral-rich fluids, such as silica-rich water or calcium carbonate solutions, circulate through the sand and precipitate minerals between the grains, binding them together into a solid rock.

Sandstone Composition

The composition of sandstone can vary quite a bit, depending on its origin and history. Here's a breakdown of the main components:

Sandstone Composition

Quartz: The dominant mineral in most sandstones, often exceeding 70%. Its hard, resistant nature makes it a survivor of weathering and erosion. Imagine tiny, translucent grains like sand on a beach, but cemented together.

Feldspar: Another common mineral, comprising up to 25% in some sandstones. It comes in various colors like pink, white, or gray, and adds to the overall hue of the rock.

Rock fragments: These are bits and pieces of other rocks, like granite or volcanic rocks, that get incorporated into the sandstone during its formation. They add variety to the texture and can even tell us about the source of the sand.

Mica: Flaky minerals that can add a shimmery or glittery appearance to the sandstone.

Clay minerals: These fine-grained particles can fill the gaps between the larger grains and act as a natural cementing agent. They can also influence the color and permeability of the sandstone.

Accessory minerals: Minor players like iron oxides, hematite, magnetite, and pyrite, influencing color and sometimes used for historical dating.

Classification of Sandstone

Sandstone can be classified in a number of ways, including by their:

Texture: The texture of a sandstone refers to the size, shape, and sorting of its grains. Arenites have sand grains that are well-sorted and rounded, while wackes have sand grains that are poorly sorted and angular.
Composition: The composition of a sandstone refers to the type of mineral grains that it contains. The most common mineral grains in sandstone are quartz, feldspar, and lithic fragments.
Cement: The cement of a sandstone is the material that binds the sand grains together. The most common cements are silica, calcite, and iron oxide.

Sandstone Classification by texture

Sandstone can also be classified by their texture, which refers to the size, shape, and sorting of the framework grains. The texture of a sandstone can be described as fine-grained, medium-grained, or coarse-grained. The shape of the framework grains can be angular, subangular, or rounded. The sorting of the framework grains refers to how evenly the grains are distributed in terms of size. A well-sorted sandstone has grains that are all of similar size, while a poorly-sorted sandstone has grains of a variety of sizes.

Well-sorted sandstone have sand grains that are all about the same size.
Poorly sorted sandstone have sand grains that vary widely in size.
Well-rounded sandstone have sand grains that are smooth and have rounded edges.
Angular sandstone have sand grains that are sharp and have angular edges.

Texture can give geologists clues about the environment in which a sandstone was formed. For example, well-sorted, well-rounded sandstones are often formed in beaches and other coastal environments. Poorly sorted, angular sandstones are often formed in rivers and other streams.

Sandstone Classification by Mineral Composition

Sandstones are classified based on the composition of their framework grains, which are the sand-sized grains that make up the bulk of the rock. The three main types of framework grains are quartz, feldspar, and lithic fragments. Quartz is the most common framework grain, followed by feldspar. Lithic fragments are pieces of other rocks that have been weathered and eroded into sand-sized grains.

Sandstones can be divided into four main compositional groups:

Quartz arenite

Arenites (Latin: arena, "sand") are sandstones that are composed of more than 90% quartz. They are the most mature sandstones, meaning that they have undergone the most weathering and sorting. Arenites are typically white or light in color and are very hard and durable.

Arkosic sandstones

Arkoses are sandstones that contain more than 25% feldspar. They are typically pink or red in color and are less hard and durable than arenites. Arkoses are formed from the weathering and erosion of granitic rocks.

Lithic sandstones

Lithic sandstones contain more than 25% lithic fragments. They can be any color, depending on the composition of the lithic fragments. Lithic sandstones are formed from the weathering and erosion of a variety of rocks, including igneous, metamorphic, and sedimentary rocks.

Greywacke sandstones

Graywackes are sandstones that contain more than 15% clay and less than 25% labile minerals (feldspar and lithic fragments). They are typically gray or dark green in color and are soft and crumbly. Graywackes are formed from the rapid deposition of sediment in unstable tectonic environments, such as mountain ranges and subduction zones.

Quartz wackes are uncommon because quartz arenites are texturally mature to supermature.
Felspathic wackes are feldspathic sandstone that contain a matrix that is greater than 15%.
Lithic wacke is a sandstone in which the matrix greater than 15%.

Sandstone Classification by Cementing Material

Sandstones are cemented together by a variety of materials, including silica, calcite, iron oxides, and clay. The type of cement that binds a sandstone together can affect its strength and durability.

Silica-cemented sandstone are the strongest and most durable sandstones. Silica cement is very hard and resistant to weathering.
Calcite-cemented sandstone are softer and less durable than silica-cemented sandstones. Calcite cement is relatively soft and can be dissolved by acidic water. Calcareous sandstone
Iron oxide-cemented sandstone are typically red or brown in color. Iron oxide cement is relatively soft and can be dissolved by acidic water. Ferruginous sandstone

Sandstone Properties

Texture:

Clastic: Composed of fragments of other rocks and minerals, visible to the naked eye in some cases.
Grain size: Varies from fine (0.06 mm) to coarse (2 mm).
Sorting: Can be well-sorted (grains similar in size) or poorly sorted (mixture of sizes).
Rounding: Can be well-rounded (smooth edges) or angular (sharp edges).

Color: Highly variable depending on mineral composition and impurities.

Common colors include:

Tan and brown: Hematite and limonite (iron oxides).
Yellow and red: Hematite, goethite (iron oxides).
White and gray: Quartz and feldspar dominant.
Pink: Feldspar with potassium feldspar contributing to the color.
Black: Iron minerals or organic matter.

Hardness: Also variable, ranging from soft to hard:

Soft (Mohs scale 2-3): Sandstones with clay-rich matrix or weak cement.
Medium (Mohs scale 4-5): Most common sandstones with silica or carbonate cement.
Hard (Mohs scale 6-7): Quartz-rich sandstones with strong silica cement.

Porosity and Permeability: Sandstone's loose grain structure often grants it high porosity, allowing fluids like water or oil to flow through. This property makes it valuable for aquifers and hydrocarbon reservoirs.

Porosity: The spaces between sand grains allow fluids like water and oil to flow through, making sandstone an important reservoir rock for oil and gas.
Permeability: The ease with which fluids can pass through the rock, influencing its use in filtration systems and water aquifers.

Strength and Durability: Despite its variable hardness, sandstone can be surprisingly strong and durable, especially when well-cemented. This characteristic makes it a popular building material for centuries.

Types of Sandstone

Where Sandstone is Found

Sandstone is a widely distributed sedimentary rock found on every continent. Its location depends on several factors, including:

Depositional Environment:

Beaches and coastlines: Wind and waves erode and transport sand-sized grains, eventually depositing them as beaches, dunes, and coastal plains. Examples include the Navajo Sandstone in the southwestern United States and the Bondi Formation in Australia.

Riverbeds and floodplains: Rivers erode rocks and carry sand downstream, depositing it along their courses and floodplains. The Fountain Formation in Colorado and the Bunter Sandstone in Germany are examples.

Deserts: Windblown sand accumulates in deserts, forming vast sand seas and dunes that can later be lithified into sandstone. The Sahara Desert and the Arabian Peninsula contain notable examples.

Ancient lakes and inland basins: Similar to river systems, ancient lakes and inland basins can trap and deposit windblown or river-transported sand, forming sandstone formations. The Chinle Formation in the southwestern United States and the Sherwood Sandstone Group in the UK are examples.

Geological Age and Uplift:

Sandstone formations span various geological periods, and their current location depends on subsequent geological processes like erosion and uplift.

Uplift and erosion can expose ancient sandstone formations at the surface, while others may be buried deep underground.

Types of Sandstone

Quartz Arenite

Composition: The undisputed king of sandstones, quartz arenite is composed of over 90% quartz, making it incredibly strong and durable. Its grains are well-rounded and sorted, resulting in a smooth, almost glass-like texture.

Color: Typically white or light-colored due to the abundance of quartz.

Properties: Hard, durable, and resistant to weathering, making it a popular choice for building materials and sculptures.

Uses: Highly sought-after for building materials, monuments, and even glassmaking. Its strength and purity make it a reliable and long-lasting choice.

Examples: The stunning white cliffs of Dover in England and the iconic Ayers Rock in Australia.

Arkose

Composition: Arkose is a feldspar-rich sandstone, containing 25% or more feldspar alongside quartz. This gives it a slightly coarser texture than quartz arenite.

Color: Can range from pink and tan to red and brown, depending on the type of feldspar present.

Uses: Commonly used for building materials, decorative stones, and even road construction due to its good workability and attractive color variations.

Examples: The majestic red rocks of Sedona, Arizona, and the famous brownstone buildings of New York City.

Greywacke

Composition: Greywacke is a "dirty" sandstone, containing a significant amount (15% or more) of clay and rock fragments along with quartz and feldspar. This gives it a darker, less sparkly appearance compared to other types.

Color: Typically grey, green, or brown due to the presence of clay minerals and rock fragments.

Properties: Can be less durable than other sandstones due to the softer matrix.

Uses: Often found as building stones in older structures, but less common in modern applications due to its lower strength compared to other types.

Examples: The rugged cliffs of the Scottish Highlands and the greywacke formations of the Himalayas.

Lithic Arenite

Composition: Lithic arenite stands out with its abundance of rock fragments (over 25%) alongside quartz and feldspar. These fragments can be volcanic, metamorphic, or sedimentary in origin, giving the rock a unique and varied appearance.

Color: Highly variable depending on the type of rock fragments present, ranging from grey and brown to red and green.

Uses: Often used as a building stone, particularly in areas where it is readily available. Its unique appearance can also make it attractive for decorative purposes.

Calcareous Sandstone

Calcareous sandstone

Composition: Calcareous sandstone is unique for being cemented with calcium carbonate (calcite) instead of the more common silica or iron oxide. This gives it a softer, more porous feel compared to other types.

Color: Can be white, grey, or tan depending on the presence of other minerals.

Uses: Often used for landscaping stones, decorative elements, and even in agricultural applications due to its ability to improve soil quality.

Properties: Typically softer and more porous than other sandstones, can be susceptible to weathering by acids.

Examples: The delicate coquina rock found in Florida and the famous Indiana Limestone, widely used in buildings.

Ferruginous Sandstone

Composition: Rich in iron oxide, giving it a distinctive red, brown, or yellow color.

Appearance: Can be coarse- or fine-grained, with a distinctive rusty hue.

Properties: Can be quite hard and durable due to the iron oxide cement.

Examples: The breathtaking red rock formations of Utah's national parks and the picturesque Navajo Sandstone of the American Southwest.

Uses of sandstone

Sandstone is a versatile sedimentary rock that has been used for various purposes throughout human history. Its durability, aesthetic appeal, and workability make it a popular choice in construction, landscaping, and artistic endeavors. Here are some common uses of sandstone:

Construction and Architecture

Building Stone: Sandstone's durability and aesthetic appeal make it a popular choice for building facades, cladding, pavements, and architectural elements. Examples include the Sydney Opera House (Australia) and the Petra archaeological site (Jordan).

Decorative Elements: Sculptures, fountains, fireplaces, and countertops often utilize sandstone due to its workability and diverse colors and textures.

Landscaping and Paving: Sandstone flagstones, pavers, and walkways add natural beauty and durability to patios, gardens, and public spaces.

Industrial Applications

Glass Production: Crushed sandstone serves as a key ingredient in glass manufacturing, contributing to its strength and clarity.

Abrasives and Sandblasting: Sandstone finds use in industrial abrasives for cleaning, polishing, and sandblasting applications.

Cement and Concrete: Sandstone can be processed into aggregates used in the production of cement and concrete, contributing to their strength and texture.

Oil and Gas Extraction: Sandstone formations play a crucial role in facilitating the extraction and storage of oil and gas resources.

Other Uses

Filtration: Sandstone's porous nature makes it useful in filters for water purification and industrial processes.

Molding and Casting: Fine-grained sandstone is used in the creation of molds and casts for metalworking and other industries.

Art and Sculpture: From intricate carvings to large-scale sculptures, sandstone has been a preferred medium for artists throughout history.

Factors Affecting Use

Type of Sandstone: Different types, like quartz-rich or feldspathic sandstones, have varying properties influencing their suitability for specific applications.

Color and Texture: Aesthetics play a role in architectural and decorative uses, with unique colors and textures influencing design choices.

Availability and Cost: Local availability and processing costs can impact the competitiveness of sandstone compared to other materials.

The wide range of uses for sandstone highlights its importance in various industries and its continued relevance in modern construction and design. The rock's natural beauty, combined with its practical attributes, makes it a valuable material in both historic and contemporary contexts.

Famous Sandstone Formations

The Arches National Park, Utah

The Arches National Park, Utah:

The Arches National Park, Utah: This park is home to over 2,000 natural sandstone arches, including the Delicate Arch, Landscape Arch, and Double Arch. These arches were formed over millions of years by wind and water erosion. The world's tallest sandstone arch is Rainbow Bridge in Utah, stretching 290 feet high.

The Navajo Sandstone, Utah

The Navajo Sandstone, Utah: This iconic formation is found in Zion National Park and Bryce Canyon National Park, and is known for its vibrant red and orange colors. The Navajo Sandstone is home to some of the world's most spectacular hoodoos, which are tall, thin rock spires that have been eroded over time.

The White Sands National Park, New Mexico

The White Sands National Park, New Mexico: This park is home to a vast sea of white gypsum sand dunes, which cover over 275 square miles. The dunes are constantly shifting and changing, creating a beautiful and ever-evolving landscape.

The Uluru (Ayers Rock), Australia

The Uluru (Ayers Rock), Australia: Uluru is a massive sandstone monolith that is sacred to the Indigenous Australians. It is one of the largest rocks in the world and has been a popular tourist destination for many years.

The Petra, Jordan

The Petra, Jordan: This ancient city is carved into the side of a sandstone cliff and is one of the most popular tourist destinations in the Middle East. Petra was the capital of the Nabataean kingdom and is famous for its tombs, temples, and other structures.