Sandstones are a type of clastic (terrigenous) sedimentary rocks deriving, as the name suggest, from the consolidation of sandy sediments. Sand consists of clastic particles produced by the erosion of pre-existing rocks with grain size comprised between 0.0625 and 2 mm. The upper boundary of 2 mm arbitrarily separates sand (and sandstones) from the coarser gravel and their lithified counterparts: conglomerate and breccia. The lower boundary set at 0.0625 mm may apparently sound arbitrary but represents the limit of the human eye to discern grains. Indeed, our eye can only recognize grains larger than 0.0625 mm in diameter. Smaller particles, known as mud (silt + clay), are not visible by the human eye and the rocks consisting of them, like mudstones, appear homogeneous to us. To qualify itself as a sandstone, a clastic sedimentary rock must contain more than 25% sand over mud. Moreover, if enough particles with grain size > 2 mm (ruditic) are present, in general more than 5%, the rock is a conglomerate (or a breccia).
The sand-sized grains in sandstones are known as framework grains, of detrital origin. The empty spaces between grains (i.e. pores) are filled by matrix and/or cement. Matrix is the fine-grained grain fraction, consisting of mud, whereas cement is made of minerals precipitates in pore spaces after deposition. The commonest cements in sandstones consist of siliceous material (quartz, chalcedony, opal…) or carbonates (i.e. calcite), but other minerals like oxides, feldspars, zeolites, and authigenic clays can also form cement. Pores can be only partially filled by matrix and cement, leaving some empty pore spaces (i.e. porosity), which at depth can contain liquids such as groundwater or hydrocarbons.
Framework grains in sandstones can be monomineralic or lithic. Monomineralic grains consist of a single mineral, while lithic grains are fragments of rocks that preserve their internal texture. Most monomineralic grains in sandstones consist of quartz and feldspar, particularly K-feldspar and Na-rich plagioclase. This happens because these minerals are the most resistant to weathering and erosion among rock-forming minerals. Other accessory minerals that are resistant to erosion (e.g. zircon, garnet, muscovite…) can be present, while the presence of minerals that are susceptible to alteration (e.g. pyroxene, olivine) generally indicates transport from a nearby source. Framework grains can be in contact with each other and, in this case, the sandstone is grain-supported (or framework-supported). On the other hand, if grains ‘float’ in the matrix, the rock is matrix-supported.
Composition and compositional maturity of sandstones
When erosion reduces rocks to the grain size of sand, their mineralogical composition changes significantly. Chemical weathering and erosion tend to destroy the minerals that are unstable and alter easily on the surface of the Earth compared to those that are stable or that undergo weathering at slow pace. Among rock-forming minerals, femic minerals like olivine, pyroxene, and amphibole, and Ca-rich sialic minerals like plagioclase break down to clay minerals very fast and have little chance to survive erosion and transport as grains. Alkali feldspar, Na-rich plagioclase, and the micas (biotite and muscovite) also alter to clay minerals but at a slower pace and are, hence, present in many sands and sandstones. Quartz is the most stable rock-forming mineral, since it does not dissolve in water and its high hardness make it resistant to physical erosion. Prolonged transport concentrates quartz in the sediment, since feldspars and other less stable minerals are progressively destroyed by erosion. For example, beach and aeolian sand is the result of a very long transport and tends to be very quartz-rich. On the other hand, deep sea turbidites are rapidly deposited sediments that generally still contain abundant feldspars and even micas. Quartz-rich sediments may contain many erosion-resistant minerals like zircon, tourmaline, and rutile.
Texture and textural maturity of sandstones
Beynd grain size, four major parameters are used to describe sandstones and understand the sedimentary environment where they deposited and the type of transport they experienced:
grain shape: the shape of clasts.
sorting: the variability in grain size in a clastic sedimentary rock.
roundness: a parameter defining how much the external outline of a clast has been rounded during transport.
packing: the disposition of clasts with respect to one another and the surrounding matrix.
Together, these parameters allow to define the textural maturity of a sandstone. The idea behind the concept of textural maturity is that the texture of a sandy sediment continues to evolve and be modified during transport, as a result of the total kinetic energy it experienced before deposition. The higher is the energy, the lower the matrix content (clay particles and silt < 30 μm according to Folk, 1951), the higher the sorting and degree of rounding. Immature sandstones still contain matrix > 5%. Submature sandstones have matrix < 5% but grains are not well sorted, contrarily to mature sandstones, where grains are well sorted but still angular to subrounded. Finally, supermature sandstones have matrix < 5%, well sorted and rounded grains. Folk (1951) linked the degree of maturity of sandstones to the sedimentary environment where they deposited (see figure below).
Note: compositional and textural maturity are two completely different things. A sandstone can be compositionally very mature and at the same time immature from a textural point of view and vice versa. For example, the Numidian Sandstones in North Africa are compositionally mature, as they were sourced from quartz-rich eolian sediments, but texturally immature because, as most turbidites, they contain much clay.
Classification of sandstones
There are more than 50 classification schemes for sandstones but the diagram by Dott (1964) is the most widely used. This classification can be used on sand and sandstones. It is based on (1) the percentage of matrix, defined as the grain size < 30 μm and (2) the proportion of quartz, feldspar, and lithic fragments in the > 30 μm framework grains. Based on this diagram, if the percentage of matrix > 75%, the rock is a mudstone. Sandstones are divided in arenites (< 15% matrix) and wackes o graywackes (> 15% matrix). Arenites and wackes can be further classified based on their composition in terms of quartz (Q), feldspar (F), and rock or lithic fragments (L). If quartz > 95% (F + L < 5%), they can be classified as quartzarenite e quartzwacke respectively. Arkosic arenite e arkosic wacke occur when feldspars dominate, whereas if lithic fragments are more abundant, sandstones are classified as lithic arenite e lithic wacke. Feldspathic can be used instead of arkosic. Further subdivisions of arenites are subarkose (quartz between 75 and 95%, feldspar > lithic fragments) and sublitarenite (quartz between 75 and 95%, lithic fragments > feldspar).
Tips to recognize arenites from wackes: all matrix-supported and most grain supported sandstones with matrix are wackes. Indeed, grain supported textures with point contacts still allow > 25% matrix (see packing to find out why). Arenites either contain a lot of cement or have dominant long, concavo-convex, and sutured contacts between grains, that do not permit much matrix to be present between grains.
Recognizing grains (clasts) in sandstones
In order to classify sandstones properly, it is necessary to recognize their three main components: quartz (Q), feldspar (F), and lithic fragments. Quarzo is recognizable thanks to its grey color and for its transparency, which persists even in sand grains. Quartz can lose its transparency, however, if it is coated by other minerals, like clays or oxides, which is very common in sedimentary environment. Quartz lacks cleavage planes and breaks along conchoidal fractures, a feature which is visible on fresh, broken surfaces of quartz grains with a hand lens. Differently from quartz, feldspars show characteristic, well-developed cleavage planes. It is challenging to observe traces of cleavage planes on the surface of sand-sized feldspar grains (although possible, especially in coarse-grained sand). More commonly, cleavage planes are visible because even small feldspar grains tend to produce sharp and smooth broken surfaces that reflect light very well. By comparison, quartz grains show a more greasy or waxy luster. Moreover, feldspars alter to clay minerals, acquiring white to pale pink colors and often becoming opaque. Lithic fragments group all the clasts consisting of more than one crystal. There is an infinite variety of igneous, sedimentary, and metamorphic rocks that can be preserved in sandstones as lithic fragments. In general, lithic fragments can be identified because: (1) they contain more than one crystal, and (2) they show an internal texture inherited from the parent material (e.g. igneous, metamorphic, and sedimentary textures). Care if needed when dealing with lithic fragments of very fine-grained rocks, because they can be misidentified as monomineralic grains, for example clasts of cherts of fine-grained limestones. In this case, a thin section is required to identify them with confidence.
Above: family portrait. Feldspar grains with white to transparent color, showing evident cleavage planes (highlighted by black dashed lines). Quartz grains are transparent (grey). Two lithic fragments of schists, preserving internally a metamorphic foliation, are visible. Width: about 2 cm. Macigno Sandstone. Navacchio, Pisa, Italy.
Some time ago I have drawn this diagram to bring students on sandstone in the field. I post it here, free to use. Be careful to print it at the right size.
Dott, R. H. (1964). Wacke, graywacke and matrix; what approach to immature sandstone classification?. Journal of Sedimentary Research, 34(3), 625-632.
Folk, R. L. (1956). The role of texture and composition in sandstone classification; discussion. Journal of Sedimentary Research, 26(2), 166-171.
Folk, R. L. (1980). Petrology of sedimentary rocks. Hemphill publishing company.
Garzanti, E. (2019). Petrographic classification of sand and sandstone. Earth-science reviews, 192, 545-563.
Okada, H. (1971). Classification of sandstone: analysis and proposal. The Journal of Geology, 79(5), 509-525.
Pettijohn, F. J., Potter, P. E., & Siever, R. (2012). Sand and sandstone. Springer Science & Business Media.
Sandatlas.org – Sand