This article explains the most important properties, characteristics and uses of smectite, one of the most important groups of micaceous minerals in the oil, construction and agricultural industries.
The term smectite actually refers to a group of dioctahedral micaceous and clay minerals that include montmorillonite, beidelite, and nontronite, as well as the trioctahedral minerals hectorite (rich in Li), saponite (rich in Mg), and sauconite (rich in Zn).
These minerals are important because they are used in the petroleum industry as fillers, carriers, absorbents, and a component in drilling fluids.
Therefore, the smectites mineralogically they are considered as clay minerals belonging to the group of silicates, specifically to the phyllosilicates.
Distinctive features
The smectite, refers to any mineral in the smectite group, and is typically found in altered volcanic and sedimentary rocks as a replacement mineral in association with low-temperature zeolites, or less commonly, as a filler for open spaces.
Volcanic glass is especially susceptible to being replaced by smectite.
Smectite in hand sample
Replacement of rock-forming minerals and volcanic glass with smectite can produce a bleached appearance, with tan to light-medium green colors ; specimens scratch easily, and some splinter in water.
Some physical and chemical properties of smectite
- It is a highly plastic laminated clay
- Size: naturally occurring smectite ranges in size from 0.25 microns to 0.75 microns at its broadest dimension.
- Burden: smectite platelets have a negative ionic charge within the body of the platelet and a positive charge at the edge of the platelet.
Chemical composition
The varieties of smectite group they are differentiated by variations in chemical composition involving substitutions of Al for Si at tetrahedral cation sites and Al, Fe, Mg, and Li at octahedral cation sites.
The smectite clays they have a variable net negative charge, which is balanced by externally adsorbed Na, Ca, Mg, and/or H on the interlaminar surfaces.
Occurrence, genesis and geological setting
This mineral group is generally formed by the weathering of basic rocks such as basalt or gabbro.
The formation conditions are favored or enhanced by flat land or very gently sloping land with poor drainage, in addition, it must be slightly alkaline (as in marine environments) and must have a high Si and Mg content.
Other factors that favor its genesis include the availability of Ca and the scarcity of K, in the field.
Why is poor drainage necessary?
This condition is necessary, because if it is not so, the water could leach magnesium ions (Mg) released in the alteration reactions.
In hydrothermal alterations
Smectite is a general term for a group of clay minerals of which montmorillonite is the most common.
These clays form in the presence of both neutral and acidic, relatively cold hydrothermal fluids, and are stable up to about 140 °C.
Consequently, the smectite group is commonly found above or adjacent to the mineralization of epithermal deposits.
The illite/smectite or interlocked illite association prevails at higher temperatures. Smectite can also occur as fill in open spaces, where its accuracy as a geothermometer is less certain.
It is worth mentioning that an X-ray diffraction analysis is necessary to confirm its presence or occurrence in a rock.
Smectite uses
- Large volumes of smectites together with Fuller’s earth (calcium bentonites with high absorptive power) are used directly in the smelting and oil drilling industries.
- Large quantities are also destined for use in civil engineering with the aim of preventing the movement of water in the soil where buildings are located.
- A large volume of sodium-rich smectites are used for various purposes in the manufacture of many industrial, chemical, and consumer products.
- Large volumes of Ca-Mg-rich smectites are used directly in iron smelting and in the agricultural industry.
- Large volumes of acid or Fuller’s earths are used commercially along with smectite to make animal litter trays, oil and grease absorbents, and as insecticide containers.