Clay Minerals
Introduction
Clay minerals are the dominant component of matrix in sandstones and the principal mineral group in shales. They belong to the phyllosilicate mineral group, characterised by two-dimensional layer structures arranged in indefinitely extending sheets. Their small grain size - below 0.03 mm in the matrix context - means that routine petrographic microscopy cannot identify them. Specialised analytical methods are required. Despite this analytical difficulty, clay minerals are fundamental to understanding diagenesis, weathering, and the environmental conditions of ancient depositional systems.
Occurrence in Sandstones
Grains in sandstones smaller than about 0.03 mm, which fill interstitial spaces among framework grains, are referred to as matrix minerals. Clay minerals make up the bulk of these matrix grains, though fine-size micas, quartz, and feldspars may also be present. [1]
Because of their small size, clay minerals are difficult to identify by routine petrographic microscopy. They must be identified by X-ray diffraction techniques, electron microscopy, or other non-optical methods. [1]
Crystal Structure
Clay minerals are compositionally diverse. They belong to the phyllosilicate mineral group, characterised by two-dimensional layer structures arranged in indefinitely extending sheets. [1]
The sheet structure is the defining feature of all phyllosilicates. Silicon-oxygen tetrahedra link into flat networks, and these tetrahedral sheets are bonded to octahedral sheets containing aluminium, magnesium, or iron. In two-layer clays, one tetrahedral sheet pairs with one octahedral sheet; in three-layer clays, the octahedral sheet is sandwiched between two tetrahedral sheets. The number and nature of these stacked layers determine the mineral’s properties - including its ability to absorb water and cations, its swelling behaviour, and its diagenetic stability.
The Four Major Clay Mineral Groups
The most common clay mineral groups in siliciclastic sedimentary rocks are illite, smectite (montmorillonite), kaolinite, and chlorite. [1]
Their formulas are:
- Illite: K2(Si6Al2)Al4O20(OH)4 [1]
- Smectite (montmorillonite): (Al,Mg)8(Si4O10)3(OH)10·12H2O [1]
- Kaolinite: Al2Si2O5(OH)4 [1]
- Chlorite: (Mg,Fe)5(Al,Fe3+)2Si3O10(OH)8 [1]
Kaolinite is a two-layer clay; illite, smectite, and chlorite are three-layer clays. Smectite is a clay-mineral group, of which montmorillonite is a principal variety. [1]
The distinction between two-layer and three-layer structure has important practical consequences. Three-layer smectite clays have a charged interlayer space that can absorb water molecules and cations, causing the mineral to swell when wetted. This swelling is the reason why smectite-rich shales can be difficult to drill through in the petroleum industry - they absorb drilling fluid, swell, and close the borehole. Kaolinite, a two-layer clay with no interlayer charge, does not swell and is therefore a far less problematic matrix mineral in reservoir sandstones.
Formation Pathways
Clay minerals form principally as secondary minerals during subaerial weathering and hydrolysis, but they can also form by subaqueous weathering in the marine environment and during burial diagenesis. [1]
This three-pathway origin explains why clay minerals can be present in sandstones in two fundamentally different contexts. Detrital clay minerals are transported into the basin along with sand and silt grains and accumulate in the matrix. Authigenic clay minerals grow in place within the pore system during diagenesis, precipitating from pore fluids at the expense of less stable minerals like feldspars. Authigenic chlorite rims on grains, for example, can dramatically reduce effective porosity even in otherwise clean sandstones.
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References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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