There have been over fifty classification schemes proposed for sandstone. Most of those currently in use involve a ternary QFL (quartz, feldspar, lithic) plot for framework grains and a major division based on the relative amount of matrix.

Classification of Sandstone

Dott Classification

Because of its simplicity and added detail, I prefer this version (with some modification of terminology) of the Dott classification.

Point Counting

Sandstone composition is typically determined microscopically by identifying mineral composition at 300 equally spaced points across a thin-section.

Total Rock Composition

Component Proportion (%)Quartz 78

Feldspar 60

Rock fragments 36

Matrix 126

Total: 300

Total Q, F, and L: 174

Framework Grain Composition

Component Proportion (%)Quartz (78/174) 45

Feldspar (60/174) 34

Lithic (36/174) 21

Total: 100

Sample Point Count

Composition at 300 points is identified. The relative abundance of framework grains to matrix determines whether the rock is an arenite, a wacke, or a mudstone. The framework composition is normalized to determine the type (quartz, feldspathic, lithic) of arenite or wacke.

Ternary Diagrams

The normalized framework percentages are plotted on ternary diagrams to determine rock composition.

This sample plots as lithofeldspathic. Because of its high matrix content, it would be a lithofeldspathic wacke or lithic arkose wacke.

Quartz

Monocrystalline

Polycrystalline

Quartz grains occur in three forms, monocrystalline (igneous source), polycrystalline (metamorphic source) and chert (sedimentary source). Some workers consider chert to be a lithic fragment.

Chert

FeldsparsOrthoclase crystal undergoing dissolution

Feldspar grains occur in four forms, orthoclase, plagioclase, microcline, and sanidine and repesent a variety of igneous and metamorphic sources. Feldspar rapidly alters to clay (kaolinite, sericite) and calcite and is highly subject to dissolution.

Plagioclase altering to calcite

Dissolution along cleavage planes

Lithic Grains

Volcanic Rock Fragment

Chert Plutonic Rock Fragment

Metamorphic Rock Fragment

Sedimentary Rock Fragment (shale)

Lithic fragments are grains that are recognizable as particles of pre-existing rocks and are classified as igneous, sedimentary, or metamorphic. Some workers consider plutonic and gneissic rock fragments to be feldspar grains.

Provenance of Sandstone (framework grains)

Influence of Source Rock Influence of Climate

The primary control on sandstone composition is the source rock. However, climate and transport distance also have significant impacts.

Provenance of Accessory Minerals in Sandstones

Igneous Metamorphic Ig/Mc mixed

Aegerine Actinolite Biotite

Augite Andalusite Enstatite

Chromite Chloritoid Hornblende

Ilmenite Cordierite Hypersthene

Muscovite Diopside Magnetite

Topaz Epidote Sphene

Garnet Tourmaline

Sedimentary Glaucophane Zircon

Calcite Kyanite

Chert Rutile

Sillimanite

Staurolite

Tremolite

Accessory minerals typically have a much more restricted range of formation than quartz or feldspar; therefore, even minor occurrences can be very useful in determining the sediment source.

Provenance of Sandstone (accessory [mostly heavy]

minerals)

Matrix

The matrix portion of sandstone consists of mud-size particles, particularly of clay and mica in composition, though quartz and feldspar can play a significant role.

Cement

Common cements:• Quartz• Calcite• Dolomite• Siderite• Hematite• Limonite• Mn oxides• Pyrite• Gypsum• Barite

Cement is a diagenetic precipitate formed mostly during burial and reflects changes in temperature, pressure, and ion concentration.

Quartz sandstone with calcite, silica, and pyrite cements

Quartz Arenite

• At least 95% quartz• Most common along passive continental margins and in

epicontinental seas• Often found in eolian, beach, and shelf depositional systems• Are typically well-cemented and texturally mature to

supermature• Usually represent multiple recycling events (polycyclic)• Can be first generation if the result of intense weathering

Feldspathic Arenite/Wacke (arkose)

• At least 25% feldspar (any type)• Arenites have less than and wackes more than 15% matrix• Most common in proximity to granitic source terranes, particularly

those in arid or cold climatic regions or humid regions with high relief

• Often found in alluvial fan depositional systems• Are typically immature to sub-mature texturally• Usually from first generation sediment

Lithic Wacke (greywacke)

• At least 25% lithic framework grains• At least 15% matrix• Most common in proximity to volcanic source terranes,

particularly those in arid or cold climatic regions or humid regions with high relief

• Often found in alluvial fan depositional systems• Almost always immature to sub-mature texturally• Nearly always from first generation sediment

Glauconitic Sandstone

Glauconite is an iron-rich clay mineral related to illite that forms on marine shelves under mildly reducing conditions and very slow sedimentation rates prior to burial, often coating the surfaces of shells, fecal pellets, and sedimentary (clastic and carbonate) particles. Deposits are often some times referred to as “green sands.”

Phosphatic Sandstone

Phosphate nodules and cements develop through dissolution of organic materials, such as bones and teeth, under conditions of low pH. Phosphate is commonly found in association with glauconite.

Porosity

Porosity is a measure of the open space within a rock and can be either primary (intergranular) or secondary (dissolution) in nature.

Primary Porosity Secondary porosity

Porosity and Grain Size

Grain size and sorting significantly affect the primary porosity of sand.