Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium,
alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay minerals form in the presence of water and have been important to life, and many
theories of abiogenesis involve them. They have been useful to humans since ancient times
in agriculture and manufacturing.
Properties Clays form flat hexagonal sheets similar to
the micas. Clay minerals are common weathering products and low temperature hydrothermal
alteration products. Clay minerals are very common in fine grained sedimentary rocks such
as shale, mudstone, and siltstone and in fine grained metamorphic slate and phyllite.
Clay minerals are usually ultrafine-grained and so may require special analytical techniques
for their identification/study. These include x-ray diffraction, electron diffraction methods,
various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, Raman
Spectroscopy, and SEM-EDS or automated mineralogy solutions. These methods can be augmented
by polarized light microscopy, a traditional technique establishing fundamental occurrences
or petrologic relationships. Occurrence
Given the requirement of water, clay minerals are relatively rare in the Solar System, though
occur extensively on Earth where water has interacted with other minerals and organic
matter. Clay minerals have been detected at several locations on Mars including Echus
Chasma and Mawrth Vallis and the Memnonia quadrangle and the Elysium quadrangle. Spectrography
has confirmed their presence on Ceres and Europa.
Classification Clay minerals can be classified as 1:1 or
2:1, this originates from the fact that they are fundamentally built of tetrahedral silicate
sheets and octahedral hydroxide sheets, as described in the structure section below.
A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples
would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched
between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.
Clay minerals include the following groups: Kaolin group which includes the minerals kaolinite,
dickite, halloysite, and nacrite4).Some sources include the kaolinite-serpentine group due
to structural similarities.
Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite
and trioctahedral smectites for example saponite. In 2013, analytical tests by the Curiosity
rover found results consistent with the presence of smectite clay minerals on the planet Mars.
Illite group which includes the clay-micas. Illite is the only common mineral.
Chlorite group includes a wide variety of similar minerals with considerable chemical
variation. Other 2:1 clay types exist such as sepiolite
or attapulgite, clays with long water channels internal to their structure.
Mixed layer clay variations exist for most of the above groups. Ordering is described
as random or regular ordering, and is further described by the term reichweite, which is
German for range or reach. Literature articles will refer to a R1 ordered illite-smectite,
for example. This type would be ordered in an ISISIS fashion. R0 on the other hand describes
random ordering, and other advanced ordering types are also found. Mixed layer clay minerals
which are perfect R1 types often get their own names. R1 ordered chlorite-smectite is
known as corrensite, R1 illite-smectite is rectorite.
History Knowledge of the nature of clay became better
understood in the 1930s with advancements in x-ray diffraction technology necessary
to analyze the molecular nature of clay particles. Standardization in terminology arose during
this period as well with special attention given to similar words that resulted in confusion
such as sheet and plane. Structure
Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of
corner sharing SiO4 tetrahedra and/or AlO4 octahedra. The sheet units have the chemical
composition3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra
forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another
tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared
vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always
bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, and
coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms
part of one side of the octahedral sheet, but an additional oxygen atom is located above
the gap in the tetrahedral sheet at the center of the six tetrahedral. This oxygen atom is
bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized
depending on the way that tetrahedral and octahedral sheets are packaged into layers.
If there is only one tetrahedral and one octahedral group in each layer the clay is known as a
1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the
unshared vertex of each sheet pointing towards each other and forming each side of the octahedral
sheet. Bonding between the tetrahedral and octahedral
sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal
distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes
the overall bond-valence distortions of the crystallite.
Depending on the composition of the tetrahedral and octahedral sheets, the layer will have
no charge, or will have a net negative charge. If the layers are charged this charge is balanced
by interlayer cations such as Na+ or K+. In each case the interlayer can also contain
water. The crystal structure is formed from a stack of layers interspaced with the interlayers.
See also Clay
Expansive clay Mudstone/clays on planet Mars
Aeolis quadrangle Composition of Mars
Timeline of Mars Science Laboratory
The Clay Minerals Society References
^ http:www.clays.orgarchive1-1-19.pdf ^ Clays On Mars: More Plentiful Than Expected
Science Daily. Dec. 20, 2012 ^ A.S. Rivkina, E.L. Volquardsenb, B.E. Clark.
'The surface composition of Ceres: Discovery of carbonates and iron-rich clays'
^ http:www.nasa.govnews/europa-clay-like-minerals-20131211.html ^ a b c d Amethyst Galleries. "The Clay Mineral
Group." 2006. February 22, 2007 ^ Agle, DC; Brown, Dwayne. "NASA Rover Finds
Conditions Once Suited for Ancient Life on Mars". NASA. Retrieved March 12, 2013.
^ Wall, Mike. "Mars Could Once Have Supported Life: What You Need to Know". Space.com. Retrieved
March 12, 2013. ^ Chang, Kenneth. "Mars Could Once Have Supported
Life, NASA Says". New York Times. Retrieved March 12, 2013.
^ Moore, D. and R.C. Reynolds, Jr., 1997, X-Ray Diffraction and the Identification and
Analysis of Clay Minerals, 2nd ed.: Oxford University Press, New York
^ a b c Bailey, S. W., 1980, Summary of recommendations of AIPEA nomenclature committee on clay minerals,
American Mineralogist Volume 65, pages 1-7. [1]