Sandstone

• 1. Introduction

  ›

  Sandstone is one of the most common rocks on Earth, yet most people walk past it every day without a second thought. It makes up roughly a fifth to a quarter of all sedimentary rock on the planet. It has sheltered humans since prehistoric times. It holds the water that cities drink and the oil that economies run on. And in the American Southwest, it paints entire desert landscapes in shades of red that travelers drive thousands of miles to see.

  So what is it, exactly? At its simplest, sandstone is sand that became rock. Grains between 0.0625 and 2 millimetres across, pressed and cemented over millions of years into something solid enough to build temples from. But inside that simple description lies a surprising world of chemistry, pressure, time, and transformation. How does loose sand harden into stone? Why are some sandstones almost pure crystal while others are muddy and dark? And how does the same material that forms a soaring cliff in Arches National Park end up as a grindstone for sharpening blades, or a reservoir hiding billions of barrels of oil underground? Those are the questions worth following.

• 2. From Sand to Stone

  ›

  Quartz is the grain that starts almost every sandstone story. It survives the brutal process of weathering and erosion better than nearly any other common mineral, a fact captured in what geologists call the Goldich dissolution series. Rivers and wind carry the resulting sand away from eroding highlands, such as volcanic arcs and mountain belts, toward lower basins where space exists for sediment to pile up.

  Once sand settles in those basins, burial begins. The earliest stage of transformation, called eogenesis, happens at shallow depths of just a few tens of metres. At these modest depths, organisms churn through the sediment, and the minerals begin to shift. The red color of red bed sandstones, which comes from hematite, likely forms during this early phase.

  Deeper burial brings a second stage called mesogenesis. Here the weight of overlying sediment bears down, and grains shuffle into tighter arrangements. Soft grains of mica deform. Pore spaces shrink. At points where grains press against each other, the rock actually dissolves from the strain, then re-deposits that dissolved mineral in the open spaces nearby. This pressure solution is one of the chief ways cement forms from within the rock itself.

  Mechanical compaction does most of its work at depths below 1,000 metres, while chemical compaction can continue to depths of 2,000 metres. The heat at these depths also speeds the arrival of external cements that lock the grains together permanently. When erosion later strips away the rock above, a final stage called telogenesis begins. Rainwater seeps back in and sometimes dissolves part of the cement, opening new pore spaces inside the hardened stone.

• 3. Inside the Grain

  ›

  Feldspar is the second most abundant mineral family in most sandstones, after quartz. Alkali feldspars range in chemistry from KAlSi3O8 to NaAlSi3O8, while plagioclase feldspars span from NaAlSi3O8 to CaAl2Si2O8. Geologists can tell the two families apart under a petrographic microscope, and the distinction matters for understanding where the sand originally came from.

  Lithic fragments are the third major ingredient: pieces of older rock that have not yet broken down to individual mineral grains. The most common are clasts from volcanic rocks. Their presence in a sandstone is a direct clue to a geologically active source region nearby.

  Rounding out the mineral inventory are accessory minerals, which typically make up only a small fraction of any sandstone but carry outsized information. Zircon, tourmaline, and rutile, collectively known as ZTR minerals, are dense and resistant. Their relative abundance compared to less resistant heavy minerals such as garnet and magnetite can signal how mature a sandstone is, meaning how far and how long its grains have traveled.

  Between all these grains, cement does the binding work. Quartz cement is the most common silicate binder; it wraps around existing quartz grains in a crystallographic extension called an overgrowth. Calcite cement, made of tiny interlocking calcite crystals, is the most common carbonate binder. Other materials that step in as cement include hematite, gypsum, barite, and clay minerals. When cement weathers away, the sandstone crumbles back toward loose sand, though the process can be partially reversed by applying tetraethyl orthosilicate, which deposits amorphous silicon dioxide between the grains.

• 4. Classifying the Rock

  ›

  Dott's classification scheme, published in 1964, is among the most widely used systems for sorting sandstones into types. Dott built on Gilbert's earlier work and folded in R. L. Folk's ideas about textural and compositional maturity. The goal, in Dott's own framing, was to better portray the continuous nature of variation from mudstone to arenite and from stable to unstable grain composition.

  The scheme draws a line at 15 percent clay matrix. Sandstones below that threshold are called arenites; those above are wackes. Within the arenites, quartz content sets the next boundary. A quartz arenite contains more than 90 percent siliceous grains and is the most mature type, meaning it has survived so much weathering and recycling that nearly everything except quartz has been stripped away. These clean, pure sands tend to accumulate in stable environments such as aeolian dunes and shallow marine shelves.

  Feldspathic arenites fall below 90 percent quartz and carry more feldspar than lithic fragments. Because feldspar is less stable than quartz, these sandstones tend to be immature or sub-mature and are commonly linked to granitic source rocks. Lithic arenites, by contrast, are dominated by unstable rock fragments from shales, volcanic rocks, and metamorphic terrains, and are often associated with river systems and deeper marine deposits.

  Arkose sandstones exceed 25 percent feldspar and tend to have poorly rounded, poorly sorted grains, a signature of rapid erosion from granitic or metamorphic highlands where physical weathering outpaces chemical breakdown. Greywackes are the most heterogeneous type: angular grains of quartz and feldspar mixed with lithic fragments and surrounded by fine clay matrix that formed partly from the chemical alteration and compaction of softer fragments after deep burial.

• 5. Water, Oil, and Aquifers

  ›

  Pore space is the feature that makes sandstone economically vital. Porosity measures the percentage of the rock's total volume that consists of open voids. Permeability measures how quickly fluids can actually move through those voids. Both properties depend on how the grains are packed, how much cement has filled the gaps, and how compaction has rearranged the particles over time.

  Because most sandstone formations allow fluids to percolate through them, they serve as major aquifers and petroleum reservoirs worldwide. Forearc basins, which accumulate sand rich in lithic grains and plagioclase, are one common setting for these fluid-bearing formations. Intracontinental basins and grabens along continental margins are another.

  When a sandstone has been thoroughly cemented and then exposed to rainwater during the telogenesis stage, some of that cement dissolves, creating secondary porosity. That process can increase the capacity of an existing reservoir. The interplay between cementation and dissolution, both driven by the same chemistry of mineral solubility, is what determines whether a sandstone becomes a productive aquifer or a tight, impermeable layer.

• 6. Quartzite and Its Cousins

  ›

  Regional metamorphism can push sandstone past all recognition. Under the great heat and pressure of mountain-building events, individual quartz grains recrystallize along with the cement around them. The result is quartzite, a metamorphic rock in which the original sedimentary textures and structures are largely or entirely erased. When quartzite breaks, it fractures through the grains themselves rather than around them, producing irregular or conchoidal surfaces.

  Geologists recognized by 1941 that some rocks look like quartzite but have never experienced high-grade metamorphism. Diagenesis alone can cement quartz so thoroughly that only microscopic examination reveals the difference. These rocks are called orthoquartzite, distinguished from the metamorphic variety now termed metaquartzite. Orthoquartzite in the strict sense can reach a purity of 99 percent SiO2, with only trace amounts of iron oxide and resistant minerals such as zircon, rutile, and magnetite. Unlike metaquartzite, orthoquartzite still preserves fossils and original sedimentary structures.

  The boundary between orthoquartzite and metaquartzite is placed at the point where strained quartz grains begin to be replaced by fresh, unstrained small grains, producing what geologists call a mortar texture visible in thin sections under a polarizing microscope. With increasing metamorphic grade, recrystallization produces first a foam texture, where polygonal grains meet at triple junctions, and then a porphyroblastic texture, with coarser irregular grains including scattered large crystals called porphyroblasts. A type of pure quartz sandstone in this family, with more than 90-95 percent quartz, has been proposed for nomination to the Global Heritage Stone Resource. In Argentina, orthoquartzite-faced facades are a defining feature of the Mar del Plata style of bungalow architecture.

• 7. Building with Stone

  ›

  Sandstone has served as a construction material since prehistoric times, used for temples, churches, homes, and civil engineering works across the world. Its workability is its main advantage: although its resistance to weathering varies considerably by type, it is generally easy to cut and shape.

  Not all sandstones have proved durable in service. Collyhurst sandstone, quarried in North West England, has shown poor long-term resistance to weather, forcing repair and replacement in many older buildings where it was used. That kind of failure underscores how much sandstone type matters in practice.

  Beyond construction, sandstone's grain properties make certain varieties ideal for grinding. The hardness of individual quartz grains, the uniformity of grain size, and the friability of their binding structure combine to produce a good abrasive surface. Friable sandstone makes effective grindstones for sharpening blades. Non-friable types, called gritstone, are used for grinding grain. Sandstone also appears in asphalt concrete as a paving material. Given its deep history in human construction and tool-making, it is fitting that the specific class of nearly pure quartz sandstone, orthoquartzite, is now under consideration for formal recognition as a Global Heritage Stone Resource.