SEDIMENTARY ROCKS/Mineralogy and Classification 27 by the vast volumes of flint (chert) gravels to be found on the beaches of north-west Europe These flints originate in rare horizons in the Cretaceous Chalk As Charles Lyell wrote: ‘‘The entire mass of stratified deposits in the Earth’s crust is at once the monument and measure of the denudation which has taken place.’’ Gravels and conglomerates are described in greater detail in Sedimentary Rocks: Rudaceous Rocks Sandstones Sandstones are composed of particles with an average size of between 2.00 and 0.0625 mm in diameter They have four constituents: grains, matrix, cement, and, sometimes, porosity (Figure 1) Sand-sized particles form the framework of the rock Matrix, the finer grained material that may infill space between the framework grains, was deposited at the same time as the framework grains Cement is the term that describes minerals precipitated in pores after the deposition of the sediment Thus, matrix is syndepositional and cement is postdepositional Cement and porosity are described in Sedimentary Rocks: Sandstones, Diagenesis and Porosity Evolution, which deals with the diagenesis of petroleum reservoirs Framework grains and matrix are described below The framework grains of sandstones are normally composed of varying amounts of the mineral quartz (silica, SiO2) In order of decreasing abundance, sandstones also contain feldspar (a suite of calcium, potassium, and sodium silicates), micas (sheet silicates, with varying amounts of iron, magnesium, and aluminium), a complex of ferromagnesian minerals, informally termed ‘mafics’, and heavy minerals (those with densities significantly greater than that of quartz (2.65 g /cc), examples of which include iron ores, mica, siderite, zircon, and apatite) Sandstone may also contain sand-sized grains composed of more than one mineral or crystal These are termed ‘rock fragments’ or ‘lithic grains’ Sand-sized rounded green grains of the complex mineral glauconite are a common constituent of shallow marine sands (glauconite is described in Minerals: Glauconites) Sandstones often contain fossil fragments Teeth, fish scales, and bone are largely phosphatic The most common fossils, however, are shells, mainly composed of lime, calcium carbonate (CaCO3) With increasing lime content, sandstones grade into calcareous sandstones, then to sandy limestones, and finally to pure limestone, composed entirely of calcium carbonate, and with negligible quartz Thus, although typically composed of quartz, sandstones also contain a range of other minerals These are used as a basis for naming and classifying sands and are important because of their impact on geophysical well-log interpretation The syndepositional matrix that may occupy some of the space between the framework sand grains consists of silt, clay, and organic matter Heavy mineral grains are commonly silt sized, and so technically they may form part of the matrix The composition of typical sandstone may be as follows Framework grains: quartz, 45%; rock fragments, 5%; feldspar, 10%; mafics, 5%; mica, 5%; heavy minerals, 2% Matrix: clay, 7% Cement: calcite, 5% Porosity, 16% Total: 100% Two parameters are used to name and classify sandstones: chemical mineralogy and physical texture When sediment is first eroded from its parent outcrop, it is generally immature in both its composition and texture That is to say, it will still contain a range of chemically unstable mineral grains that surface processes have yet to break down and dissolve Similarly, the debris first transported down a hillside will be very poorly sorted, consisting of a range of particles, varying in size from boulders to clay When looking at an ancient lithified sandstone, its maturity may be described in terms of its chemical and physical properties (mineralogy and texture) Four main types of sandstone may thus be recognized as shown in Table This table also employs four commonly used names to describe sandstones Table A classification of sandstones based on textural and mineralogical maturity Mineralogical maturity Figure Diagram of a thin section of sandstone showing the four components of framework grains, matrix, cement, and pores Only the first of these is always present Reproduced with per mission from Selley RC (2000) Applied Sedimentology, 2nd edn London: Academic Press Texturally immature Texturally mature Immature Mature Greywacke Figure Arkose Figure Quartz wacke Figure Quartzite Figure