INDEX INTRODUCTION IMPORTANCE OF CLAY MINERALS: ORIGIN OF CLAY MINERALS. CHARGE DEVELOPMENT ON CLAYS. SILICATE CLAYS. TYPE OF CLAY MINERALS. CARBONATE AND SULFATE MINERALS USE OF CLAY MINERALS. PROPERTIES OCCURRENCE CLASSIFICATION INFORMATION CLAY MINERALS & SOILS DEFINITION OF CLAY MINERALS HISTORY CLAY MINERALS FORMATION DIAGENESIS HYDROTHERMAL ALTERATION USES CERAMICS AND BRICKS WORLD PRODUCTION AND CONSUMPTION MINING AND PROCESSING HYDROTHERMAL

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CLAY MINERALS

Introduction

Soils are made up of a complex mixture of solids, liquids and gases.The solid fraction of soils are made up of organic and inorganic components. The inorganic component of the soil makes up more than 90% of the soil solids.Inorganic components occur mainly in limited number of compounds with definite crystalline structure called minerals. The inorganic component includes both primary and secondary minerals.The secondary minerals normally are found in the clay fraction of the soil which is the fraction of the soil solids which is less the 2 micron or 0.002 mm. Clay minerals are minerals which mainly occur in the clay sized fraction of the soil.

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  Clay minerals are hydrous aluminum 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.

 Importance of Clay Minerals:

The clay minerals and soil organic matter are colloids.The most important property of colloids is their small size and large surface area. The total colloidal area of soil colloids may range from 10m2/g to more than 800 m2/g depending the external and internal surfaces of the colloid.Soil colloids also carry negative or positive charges on their external and internal surfaces. The presence of charge influences their ability to attract or repulse

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charge ions to or from surfaces.Soils colloids play a very important role in the chemical reaction which take play in soil and influence the movement and retention of contaminants, metals, and nutrients in the soil.

Origin of Clay Minerals.

Clay minerals are formed weathering a variety of minerals.The two main processes may involve slight physical and chemical alteration or decomposition and recrystallization.Clay mineral types are normally determined by the types of minerals and acidity of the leaching water.Based on their origins clays may classified as Inherited, Modified, Transformed or Neoformed

 

Charge Development on Clays.

Two main sources of charge in clay minerals are isomorphous substitution and pH-dependent charges.Charge development of on silicate clays is mainly due to isomorphous substitution. This is the substitution of one element for another in ionic crystals with out change of the structure. It takes place during crystallization and is

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not subject to change afterwards. It takes places only between ions differing by less than about 10% to 15% in crystal radii.. In tetrahedral coordination, Al3+ for Si4+ and in octahedral coordination Mg2+, Fe2+, Fe3+ for Al3+. Charges developed as a result of isomorphous substitution are permanent and not pH-dependent.In allophanes, some silicate clays e.g. kaolinite, and the metal oxides the main source of charge are termed pH -dependent charges because these charges depend on the pH of the soil. pH depend charges are variable and may either be positive or negative depending on the pH of the soil. In the metal oxides acid soils tend to develop positive charges because of the protonation of the oh ggoud on the oxide surfaces.

Type of Clay Minerals.

There are four major types of Clay minerals ( see Table 5-1).These include the layer silicates, the metal oxides and hydroxides and oxy-oxides, amorphous and allophanes, and crystalline chain silicates.

 Silicate Clays.

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The silicate clays are layers of tetrahedral and octahedral sheets.The basic building blocks of tetetrahedral and octahedral sheets are the silica tetrahedron and the aluminum octahedra.The Si+4 cation occurs in fourfold and tetrahedral coordination with oxygen whilst the Al3+ is generally found in sixfold or octahedral coordination.Layer silicate minerals are sometimes defined on the basis of the number of certain positions occupied by cations. When two-thirds of the octahedral positions are occupied , the mineral is called dioctahedral; when all 3 positions are occupied it is called trioctahedral.When one octahedral sheet is bonded to one tetrahedral sheet a 1:1 clay mineral results. Presence of surface and broken - edge OH groups gives the kaolinite clay particles their electronegativity and their capacity to absorb cations.In 2:1 clay mineral an octhehedral sheet is bonded to two tetrahedral sheets. The octahedral sheet is generally sandwiched between the two tetrahedral sheets. The 2:1 clays can be classified into expanding (smectites) and non-expanding clays (Illite and micas) on the basis of the sheet where isomorphous susbstitution is taking predominantly taking place (Refer

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to Figure 5-3).In the 2:1:1 lattice clays, a positively charge brucite sheet sandwiched between layers restricts swelling, decreases effective surface area, and decreases the effective CEC of mineral. The Idealized formular of half cell is Al Mg2(OH)6)K (Mg3(Si4-x Alx)O10(OH)2. Substitution occurs is in the tetrahedral layer and the layer change is variable but similar to mica. It occurs commonly in sedimentary rocks.

Silicate Clay Mineral Groups:

 Group Layer Type

Layer Charge (x)

 Type of Chemical Formula

Kaolinite 1:1 <0.01 [Si4]Al4O10(OH)8.nH2O (n= 0 or 4)

Illite 2:1 1.4-2.0 Mx[Si6.8Al1.2]Al3Fe.025Mg0.75O20(OH)4

Vermiculite

2:1 1.2-1.8 Mx[Si7Al]AlFe.05Mg0.5O20(OH)4

Smectite 2:1 0.5-1.2 Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4

Chlorite 2:1:1 Variable (Al(OH)2.55)4[Si6.8Al01.2}Al3.4Mg0.6)20(OH)4

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Adapted from Sposito1989. The Chemistry of Soils. Oxford University Press.

Sesquioxide Clays (Metal Oxides and Hydrous Oxides)

Also found in finer component. These tend to form in soils as Si is depleted by leaching.Gibbsite is the most common Al oxide mineral and is often found in highly weatherd sois such as oxisoils in tropical areas and ultisols found predominatly in the southeastern U.S.The most commn iron oxides are Goetihte (FeO(OH) and Hematiite (Fe2O3) Thess are also found in highly weathered soils and gives many red soils their color.The metal oxides gibbsite and goethite tend to persist in the environment because Si is readily leached than Al, or Fe and significant amount of soluble organic

matter is present.Manganese oxides are also quite common in soils. Apart from being an essential plant nutrient, they are an nutrural oxidant to certain metals such as As3+ and Cr3+. Birnessite(MnO2) is the most comon Mn oxide found in soils.Most of the charges developed on the metal oxides are pH-dependent.

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Allophanes and Imogolite

These are structurally disordered aluminosilicates.They are normally derived from volcanic ash materials and constitute a major component of volacnic soils.Allophane is is often associated with clay minerals of the kaolinite group.Imogolite has the empirical formula SiAl4O10.5H2O.

 Carbonate and Sulfate Minerals

The carboate and sulfate minerals are highly soluble compared to the alumino-silicates and are more prevalent in arid and semi arid regions.The major carbonate minerals are calcite (CaCO3) and Dolomite (CaMg(CO3)2.The major sulfate mineral is gypsum.

 Use of Clay Minerals.

Clay minerals have many industrial uses in the chemical and oil industries.Organoclays, which have the metals in the clay replaced by large surfactant cations, such as long chain alkyl amine cations can be been used as liners in

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landfills to reduce transport of contaminants. Organoclays also could be used in wastewater treatment and spill control situations.

 Properties

Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) 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 (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical

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techniques for their identification and 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 processes. 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 because they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1

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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 nacrite (polymorphs of Al2Si2O5(OH)4). Some sources include the kaolinite-serpentine

group due to structural similarities (Bailey 1980). Smectite group which includes dioctahedral smectites

such as montmorillonite, nontronite and beidellite 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.[5]

Chlorite group includes a wide variety of similar minerals with considerable chemical variation.

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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 (R3, etc.). 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.

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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 composition (Al,Si)3O4. 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

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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.

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INFORMATION CLAY MINERALS & SOILS

     Clay minerals are layer silicates that are formed usually as products of chemical weathering of other silicate minerals at the earth's surface. They are found most often in shales, the most common type of sedimentary rock. In cool, dry, or temperate climates, clay minerals are fairly stable and are an important component of soil. Clay minerals act as "chemical sponges" which hold water and dissolved plant nutrients weathered from other minerals. This results from the 15 | P a g e

presence of unbalanced electrical charges on the surface of clay grains, such that some surfaces are positively charged (and thus attract negatively charged ions), while other surfaces are negatively charged (attract positively charged ions). Clay minerals also have the ability to attract water molecules. Because this attraction is a surface phenomenon, it is called adsorption (which is different from absorption because the ions and water are not attracted deep inside the clay grains). Clay minerals resemble the micas in chemical composition, except they are very fine grained, usually microscopic. Like the micas, clay minerals are shaped like flakes with irregular edges and one smooth side. There are many types of known clay minerals. Some of the more common types and their economic uses are described here:

Kaolinite: This clay mineral is the weathering product of feldspars. It has a white, powdery appearance. Kaolinite is named after a locality in China called Kaolin, which invented porcelain (known as china) using the local clay mineral. The ceramics industry uses it extensively. Because kaolinite is electrically balanced, its ability of adsorb ions is less than that of other clay minerals. Still, kaolinite was

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used as the main ingredient for the original formulation of the diarrhea remedy, Kaopectate.

Illite: Resembles muscovite in mineral composition, only finer-grained. It is the weathering product of feldspars and felsic silicates. It is named after the state of Illinois, and is the dominant clay mineral in midwestern soils.

Chlorite: This clay mineral is the weathering product of mafic silicates and is stable in cool, dry, or temperate climates. It occurs along with illite in midwestern soils. It is also found in some metamorphic rocks, such as chlorite schist.

Vermiculite: This clay mineral has the ability to adsorb water, but not repeatedly. It is used as a soil additive for retaining moisture in potted plants, and as a protective material for shipping packages.

Smectite: This clay mineral is the weathering product of mafic silicates, and is stable in arid, semi-arid, or temperate climates. It was formerly known as montmorillonite. Smectite has the ability to adsorb large amounts of water, forming a water-tight barrier. It is

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used extensively in the oil drilling industry, civil and environmental engineering (where it is known as bentonite), and the chemical industry. There are two main varieties of smectite, described in the following:

Sodium Smectite: This is the high-swelling form of smectite, which can adsorb up to 18 layers of water molecules between layers of clay. Sodium smectite is the preferred clay mineral for drilling muds, for creating a protective clay liner for hazardous waste landfills to guard against future groundwater contamination, and for preventing seepage of groundwater into residential basements.

Sodium smectite will retain its water-tight properties so long as the slurry is protected from evaporation of water, which would cause extensive mud cracks. As a drilling mud, sodium smectite mixed with water to form a slurry which performs the following functions when drilling an oil or water well: 1) lubricates the drill bit to prevent premature wear, 2) prevents the walls of the drill hole from collapsing inwards, 3) suspends the rock cuttings inside the dense mud so that the mud may pumped out of the drill hole, and 4) when the dense

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mineral barite is added to drilling mud, it prevents blowouts caused by internal pressure encountered during deep drilling. Sodium smectite is also used as commercial clay absorbent to soak up spills of liquids. High-grade deposits of sodium smectite are found in South Dakota.

Calcium smectite: The low-swelling form of smectite adsorbs less water than does sodium smectite, and costs less. Calcium smectite is used locally for drilling muds. Much of the domestic supplies of calcium smectite are mined from the state of Georgia.

Attapulgite: This mineral actually resembles the amphiboles more than it does clay minerals, but has a special property that smectite lacks - as a drilling fluid, it stable in salt water environments. When drilling for offshore oil, conventional drilling mud falls apart in the presence of salt water. Attapulgite is used as a drilling mud in these instances. Incidentally, attapulgite is the active ingredient in the current formula of Kaopectate. 

Soils

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Soil is produced by the weathering of rocks at the earth's surface, usually requiring thousands of years. Many of our present day agricultural soils date back to the last ice age, more than 10,000 years ago. Ideally, a soil has four components, and an idealized percentage for a "good" agricultural soil would be:

1. Mineral (45%)2. Organic matter (5%)3. Water (25%)4. Air (25% void space)

    Within the mineral fraction, soils are usually divided into three size fractions: sand, silt, and clay. An ideal balance between a soil that is 100% sand ("too loose") vs. 100% clay ("too tight") has a roughly equal sand:silt:clay ratio, and this type of soil is termed a "loam." (The term "soil" as used in engineering refers to "any unconsolidated material" and does not necessarily match the geologist's definition.)    Organic matter comes from products of soil microbes which promote the decay of dead plants and animals. One of these organic materials is known as humus, which mimics the adsorptive properties of clay minerals. Organic matter is generally dark in color, and a layer of topsoil rich in organic matter is said to be the "O" Horizon.20 | P a g e

Tropical Weathering Breaks Down Clay Minerals 

    In humid tropical climates, clay minerals are unstable and break down under intense chemical weathering to become hydrated oxides of aluminum (bauxite) and

iron (goethite), which are very poor substitutes for clay minerals in retaining soil nutrients. As a result, jungle soil relies on the presence of humus, an organic substance produced by microbes that cause dead plants to decay; humus mimics the ability of clay minerals to retain soil moisture and nutrients. However, humus is much more fragile than clay minerals to chemical weathering, and is protected by the tall rainforest canopy, which softens the torrential rainfall into a gentle sprinkle. When rain forest trees are cut down, the humus is quickly washed away, leaving a barren landscape that bakes to a hard, brick-like consistency under the tropical sun. This "soil" is virtually useless for western style agriculture, and cannot be converted into useful farmland due to the lack of clay minerals. Even adding chemical fertilizers is useless - the soil cannot absorb it, and it runs off the land and pollutes the rivers.

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Why There Is High Biological Diversity In Tropical Rain Forests     It is important to note that the apparent abundance of greenery in the tropics is deceiving - there is no abundance of any single species; instead there is an abundance of different species. This is known asbiological diversity. Biological diversity may be likened to nature's efficiency plan - it allows a limited resource (soil nutrients) to be shared by a large number of different plants with different diets. The warm, mild climate of tropical rain forests has the highest species diversity in the world. It is from this diversity that most pharmaceutical herbs and drugs are obtained.

How Fertile Soils Facilitate Mono-Crop Agriculture 

    Since ancient times, farmers have noticed that growing the same crop year after produced sucessively poorer harvests. This is caused by the removal of nutrients from the soil by that same crop. By alternating a different crop each season, the soil is less depleted of

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nutrients than by growing the same crop every year.. Despite the soundness of crop rotation, for the sake of efficiency, modern U.S. agriculture practices mono-crop agriculture, where field upon field of the same crop (corn, wheat, soybeans, etc.) is grown year after year. This is only possible because of the rich agricultural soil of the American midwest, which contains abundant clay minerals, and has an optimum soil consistency. Still, mono-crop agriculture would not be possible without the intensive use of chemical fertilizers to replenish an already rich soil. A side-effect of mono-crop agriculture is that it encourages the establishment of agricultural pests. Insects which feed on a particular crop will return in greater force with a steady annual supply of food. Consequently, chemical pesticides and pest-resistant seeds are also required to support mono-crop agriculture. 

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DEFINITION OF CLAY MINERALS

Definition: Clay minerals are the characteristic minerals of the earths near surface environments.  They form in soils and sediments, and by diagenetic and hydrothermal alteration of rocks.  Water is essential for clay mineral formation and most clay minerals are described as hydrous alumino silicates.  Structurally, the clay minerals are composed of planes of cations, arranged in sheets, which may be tetrahedrally or octahedrally coordinated (with oxygen), which in turn are arranged into layers often described as 2:1 if they involve units composed of two tetrahedral and one octahedral sheet or 1:1 if they involve units of alternating tetrahedral and octahedral sheets.  Additionally some 2:1 clay minerals have interlayers sites between successive 2:1 units which may be occupied by interlayer cations, which are often hydrated.  The planar structure of clay minerals give rise to characteristic platy habit of many and to perfect cleavage, as seen for example in larger hand specimens of micas.  

The classification of the phyllosilicate clay minerals is based collectively, on the features of layer type (1:1 or 2:1), the dioctahedral or trioctahedral character of the

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octahedral sheets (i.e. 2 out of 3 or 3 out of 3 sites occupied), the magnitude of any net negative layer charge due to atomic substitutions, and the nature of the interlayer material.

The basis on which clay minerals are classified is shown below; see Hillier (2003) for a more detailed introduction to clay mineralogy.

LAYER TYPE

Layer charge (q)

Group Subgroup

Species (e.g.)

1:1 q≈0 Kaolin-Serpentine

Kaolin KaoliniteSerpentine

Berthierine

2:1 q≈0      q≈1

Increasing layer charge

Pyrophyllite-talc

Pyrophyllite

Pyrophyllite

Talc TalcSmectite (q≈0.2-0.6)

Di-smectite

Montmorillonite

Tri-smectite

Saponite

Vermiculite (q≈0.6-0.9)

Di-vermiculite

Di-vermiculite

Tri-vermiculite

Tri-vermiculite

Mica Di-mica Illite, 25 | P a g e

(q≈1.0) MuscoviteTri-mica Biotite

 q variable Chlorite Di-chlorite

Sudoite

Tri-chlorite

Chamosite

Sepiolite-Palygorskite

Sepiolite SepiolitePalygorskite

Palygorskite

Variable q variable Mixed-layer

Di-mica-di-smectite

Rectorite

Tri-chlorite-tri-smectite

Corrensite

The buttons only work when the model has been activated

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Analyses: Some types of clay minerals such as mixed-layered clay minerals can only be identified precisely by techniques such as XRD.  Although it is not unusual to have to use a variety of techniques such asXRD, infrared spectroscopy, and electron microscopy to characterise and more fully understand types of clay minerals present in a sample.  We have extensive experience of the identification of clay minerals in both soils and rocks.  Our XRD work is based is backed up by our ability to compare clay mineral diffraction data with calculated diffraction data.  This is a particularly important technique for the precise identification of mixed-layer clay minerals.  Our track record in the Reynolds Cup round robin on quantitative clay mineral analysis is testimony to the quality of our work on the identification and quantification of clay minerals. We also have wide experience of the use of electron microscopy to study the texture and petrographic relationships of clay minerals. 

You can get a 3-dimensional model of the structure that you can manipulate. It uses the Jmol applet. This may take quite a time to download if you do not already have it in cache. 

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HISTORY

Clay has been used in bricks and pottery for millennia. Sundried bricks were used from possibly over 10,000 years ago and kiln-fired bricks were used in the construction of a temple in the Euphrates region, considered to be more than 5000 years old. Sumerian and Babylonian builders constructed ziggurats, palaces, and city walls of sun-dried bricks and covered them with more durable kiln-baked bricks, often brilliantly glazed and arranged in decorative pictorial friezes. The earliest form of pottery was earthenware (porous and coarse), which has been made for at least 9000 years. The earliest pottery yet discovered in the Middle East comes from Çatal Hüyük, in Anatolia (near modern Çumra, Turkey), and dates from 8500 years ago. Stoneware, a vitrified or glassy product, dates to the Shang dynasty in China around 3400 years ago. The oldest porcelain, a vitrified

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ware that rings when tapped and is usually translucent, originated in China during the T’ang dynasty (618-907 AD), but the porcelain best known in the West (where it is called chinaware) was not produced until the Yuan dynasty (1278-1368 AD). This “hard-paste” porcelain was made from petuntse, or china stone (a crushed kaolinised granite consisting of a mixture of kaolinite, sericite, feldspar and quartz), ground to powder and mixed with kaolin, and fired at a temperature of about 1450oC. Porcelain imported from China was considered a great luxury in Europe and attempts to imitate it led to the discovery in Florence during 1575 of “soft-paste” porcelain (or frit porcelain), a mixture of clay and ground glass fired at about 1200o C. The secret of hard-paste porcelain was discovered in about 1707 at the Meissen factory in Saxony (Germany) by Johann Böttger and Ehrenfried von Tschirnaus. English bone china was first produced around 1800, when Josiah

Spode added calcined bones to the hardpaste porcelain formula. The use of clays (probably smectite) as soaps and absorbents was reported in Natural History by the Roman author Pliny the Elder (c. AD 77). The use of a kaolin-bearing surface on paper began in China about

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400 AD when powdered kaolin was added to the pigment of paper coating. In New Zealand, brickmaking and pottery were among the first established industries. Small brick works were established in many parts of New Zealand. There were 37 in 1867, but the total number expanded to 127 by 1880. Most of these works ceased production after WWII, when road transport improved. In addition to bricks and clay pipes, many of the brick works produced a limited range of domestic pottery and tableware, for example Amalgamated Brick and Pipe eventually had a “Specials Department” for pottery manufacture, which was later formed into a subsidiary company, Crown Lynn Potteries (1948-1989) (Bathurst, 1999). These companies were the original producers of the legendary railway cups. Other major pottery manufacturers were based in Christchurch, Milton and Temuka, of which the factory in Temuka is the only survivor. Studio pottery was established from the 1960s in Nelson (e.g. Crewenna and Waimea) and Coromandel (Driving Creek), and has developed into a large number of small operations, reviewed by Grzelewski (1999).

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ORIGIN OF NAMES

                                       Attapulgite (palygorskite) is for Attapulgus, Georgia, USA. Ball clay is from the tradition of extracting clay by cuffing it into 1-cubic-foot blocks, which became rounded to form balls while rolling the clay to the cart. The resulting ball had a diameter of about 25 cm and weighed 13-22 kg. Bentonite is named after the Benton Shale Formation in Wyoming, USA, in which the first bentonite mine in 1897 was located. The Benton Shale drew its name from Fort Benton, Montana, USA. Ceramic is from the Greek keramos for potter’s clay. China clay is a commercial term for kaolin, and was derived from its origin in China. Clay is derived from Latin and Old English words meaning “to stick”. Fuller’s earth originated from the practice of textile workers (or fullers) who cleaned raw wool by kneading it in a clay-water mixture that adsorbed oil, dirt, and other contaminants from the fibres. Halloysite was named after Baron Omalius d’Halloy (1707-1789), a Belgian geologist who first noted the mineral. Hectorite is named after Hector, California, USA. Illite is for the State of Illinois, USA. Kaolinite is named

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after kaolin, from the Chinese Kau-ling (or Gaoling), for a high ridge near the town of Jingdezhen in northwest Jiang Xi Province, China, where deposits of white kaolin were probably first worked over 2200 years ago. Meerschaum is from the German for sea-froth, which it resembles, because its low density allows the mineral to float on water. Montmorillonite was named in 1897 after Montmorillon, Vienne, France. Natronite for the locality in the Arrondissement of Norton, near the village of Saint Paradoux, France. Palygorskite is from Palygorskaja, Urals, Russia. Porcelain is from porcellana, used by Marco Polo to describe the pottery he saw in China. Pyrophyllite is from the Greek pyr meaning fire and phyllite, a rock or stone.

Saponite is from the Latin sapo (-idos) = soap for its soaplike appearance. Sepiolite is from the Greek sepion = bone of the cuttle-fish, which is light and porous, similar to the clay mineral, and the Greek lithos for stone. Vermiculite is from the Latin word meaning “to breed worms,” alluding to the worm-like shape resulting from its expansion on heating.

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CLASSIFICATION AND PROPERTIES

Clay structure an important factor contributing to the properties of the different clay minerals is their molecular structure. Most clay minerals are based on two types of structure, the silica tetrahedral sheet and the alumina-magnesia octahedral sheet. Silica tetrahedral

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sheets are each about 4.7Å thick, consist of silica tetrahedra arranged in a plane and have a composition of Si4 O6 (OH)4 . The sheets are bound together by aluminium and/or magnesium. Alumina-magnesia octahedral sheets are each about 5.05Å thick, consist of octahedra arranged in a plane, and have compositions of either A12(OH)6 (gibbsite) or Mg3(OH)6 (brucite), depending on whether aluminium or magnesium are incorporated in the structure. The tetrahedral and octahedral structural units can be joined or stacked in several configurations of composite layers, producing various hydrated aluminosilicates that form layer-lattice minerals with a plate-like shape (e.g. kaolinite, smectite, illite and vermiculite) or chain-lattice minerals with an elongate shape (e.g. palygorskite and sepiolite). The layerlattice structures are grouped as 1:1 layer structures containing one tetrahedral sheet linked with one octahedral sheet, and 2:1 layer structures with two tetrahedral sheets linked with one octahedral sheet. Less common clay minerals are either amorphous (non-crystalline; allophane) or have a structure based on double tetrahedral chains similar to that of amphibole minerals.

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CLAY MINERALS

Clay minerals may be classified into eight main groups on the basis of variations in structure and composition: (1) kaolinite, (2) smectite, (3) vermiculite, (4) illite, (5) pyrophyllite, (6) chlorite, (7) palygorskite, and (8) allophane (Table 1). Some clay minerals are intermediate between the clay mineral groups, formed by mixtures of the different clay structural layers, resulting in mixed-layer clay minerals such as interlayered illite-smectite and interlayered chloritekaolinite. The clay minerals are very similar in physical properties (Table 2), and many can be distinguished only by X-ray diffraction, infrared spectroscopy, electron microscopy, or differential thermal analysis. Kaolinite group includes the minerals kaolinite, halloysite, dickite and nacrite, which are all dioctahedral 1:1 layer silicates. Kaolinite is by far the most common mineral of the group. Halloysite is much less common, and dickite and nacrite are comparatively rare. All of these minerals have essentially the same composition, apart from a hydrated form of halloysite, which differs from the more common metahalloysite by

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having an extra two water molecules per unit cell. On heating to 100oC, hydrated halloysite dehydrates to metahalloysite irreversibly. Halloysite crystallises as elongated tubular or, in some cases, spheroidal shapes, whereas the other kaolinite group minerals form pseudohexagonal platelets or stacks of platelets. Kaolinite group minerals are the principle constituents of kaolin. Smectite group clays have a 2:1 sheet structure and include the dioctahedral minerals smectite (also known as montmorillonite), beidellite and nontronite, and the trioctahedral minerals hectorite (Li-Mg-smectite) and saponite (Mg-smectite; also known as bowlingite and soapstone). These are expanding lattice clays that swell in water, are thixotropic and possess high cation-

exchange capacities. Smectites are the principal constituents of bentonite and fuller’s earth. Vermiculite is similar to smectite in structure and, in some cases, composition. When heated rapidly above 400o C, interlaminar water turns to steam and causes the mineral layers to exfoliate or separate into worm-like pieces. The increase in bulk volume is typically 8-20 times in commercial grades, but individual flakes can expand by

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as much as 30 times. Its specific gravity may be reduced to as low as 0.09.

Illite group clays have a dioctahedral 2:1 mica-like sheet structure, but differ from true mica by having more water and fewer inter-layer cations (mostly potassium), resulting in weak forces between layers and irregularity of stacking. Illite clays are intermediate in composition and structure between muscovite and smectite. Palygorskite group (also known as palygorskite-sepiolite group; formerly hormite group) includes the minerals, palygorskite, also known as attapulgite, and sepiolite, also known as meerschaum. These minerals have a chain-like structure and form fibrous, lath- or needle-like crystals. The structure incorporates channels of approximately 6Å and features a high surface area (sepiolite has the highest surface area of all the clay minerals), porosity, surface charge and cation exchange capacities, resulting in excellent sorptive, colloidal and thixotropic (gelling) properties in water. Amorphous clays are formless to X-ray diffraction because of their fine grain size or irregularity in the arrangement of their layers. Allophane is a hydrous aluminosilicate (SiO2 )1- 2Al2 O3 (H2 O)+ 2.5-3.0 gel, formed from volcanic glass, but transforms to halloysite with time. It consists of

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hollow, irregular spherical particles with diameters of 35 to 50Å. The walls of the spheres are 7 to 10Å thick and contain openings that permit the passage of water molecules

(Wada and Wada, 1977). The space within the walls is filled with water (10% by weight) which is strongly retained.

Clay rocks

Clay rocks are classified and named on the basis of their dominant constituent clay mineral (e.g. palygorskite, smectite, as listed above) or other names based on their use (as listed below). Kaolin, also called China clay, is principally kaolinite, with lesser quantities of illite, smectite, quartz, feldspar, muscovite and other non-kaolinite minerals, and has a low total iron content. It is a soft white clay of variable but usually low plasticity and dry strength, that retains its white colour when fired. Ball clay, flint clay and refractory clay (also known as fire clay) are varieties of kaolin. Ball clay has high plasticity and strength, but inferior whiteness compared with kaolin. Flint clay is a compact microcrystalline to crystalline clay that breaks with a pronounced conchoidal

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or “flinty’ fracture, resists slaking, and has almost no plasticity. Refractory clay has a high temperature fusion point, typically above 1425o C. It is non-white burning. Generally the higher the level of alumina, the more refractory the clay. In some instances, this level can be enhanced to as much as 60% by the addition of bauxite minerals such as gibbsite, diaspore, or pure alumina. Chamotte is a refractory clay formed by calcining clay such as kaolin, flint clay or fireclay. Bentonite is a clay consisting predominantly of smectite (montmorillonite) minerals. It is characterised by exchangeable Na+, Ca2+ or Mg2+ cations which greatly influence the properties of the clay (and therefore its commercial applications). There are two types of naturally occurring bentonite: a swelling bentonite which has a high sodium-to-calcium ratio (sodium bentonite or Wyoming bentonite) and is typically associated

with marine sediments, and a non-swelling bentonite with a low sodium to calcium ratio (calcium bentonite) that is typically associated with freshwater sediments. The swelling variety has the ability to absorb water and swell many times its original volume to form gel-like masses. Calcium bentonite can be converted to a sodium-type

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(termed sodium exchange bentonite) by treatment with soda ash to improve swelling capacity. It can also be used to produce acid-activated bentonite by treatment with inorganic acids to replace divalent calcium ions with monovalent hydrogen ions and to leach out ferric, ferrous, aluminium and magnesium ions, thus altering the crystal structure, and increasing the specific surface area and porosity. Fuller’s earth is a group of clays that have a substantial ability to adsorb impurities or colouring bodies from fats, grease, or oils. In the United Kingdom, the term was introduced for clay in which the principal clay mineral is calcium smectite, but other minerals such as kaolinite, palygorskite and sepiolite may also be present and account for its variable chemical composition. In the USA, clays that are termed fuller’s earth are predominantly palygorskite or sepiolite. Fuller’s earth is fine-grained, found in a wide range of natural colours, from brown or green to yellow and white, and has a high water content. It crumbles into mud when mixed with water, so it has little natural plasticity.

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FORMATION

Formation process Clay minerals are formed by the processes of weathering, diagenesis and hydrothermal alteration (Table 3). Weathering and soils Soils and other residual clay deposits are formed by in situ weathering. Controlling factors include the nature of the parent rock, climate, topography, vegetation, and the time period during which these factors operated. Different environments, particularly different climatic and hydrologic regimes, may produce different clay minerals from the same parent rock type. Large kaolinite deposits formed by weathering are common around the world. Commercially exploited resources occur in the United States, Brazil, Guyana, Surinam, Ghana, Australia, and Europe. Clays in sediments Clay minerals occur widely in sedimentary rocks, particularly those with fine particle size such as mudstones and shales (argillaceous or clay-rich rocks). Illite and smectite, including mixed-layer clay minerals, kaolinite and chlorite are the principal clay mineral components of recent deep-sea sediments. Smectite and kaolinite are less abundant in pre-Devonian argillaceous sediments, which are composed largely of

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illite and chlorite. Kaolinite and illite are found in some coal measures. Most ball clays are Eocene to Lower Oligocene in age and formed in swampy sedimentary environments under subtropical to tropical conditions, assisted by post-depositional diagenesis and the presence of organic components. Palygorskite and sepiolite clay deposits are mostly associated with mid-Tertiary or younger shallow lagoonal sediments formed in sub-tropical to tropical environments.

DIAGENESIS

As temperature and pressure increase with the progression of diagenesis, clay minerals in sediments change to those stable under given conditions. Therefore, certain sensitive clay minerals may serve as indicators for various stages of diagenesis. Typical examples are the crystallinity of illite, the polytypes of illite and chlorite, and the conversion of smectite to illite. With deeper and longer burial, ball clay becomes lithified to form fireclay. Fireclay is a sedimentary clay found in coal measures as “underclays”, situated immediately beneath a coal seam. Coal measures may consist of alternating sequences of 42 | P a g e

coal and clay. Whereas ball clays are associated with lignite, fireclays are usually associated with higher rank coals, reflecting the greater lithification of their formation. Flint clay is typically sedimentary kaolin that has been subject to prolonged leaching and recrystallisation (e.g. most USA examples; Missouri, Kentucky) or metamorphism (e.g. some European flint clays).

HYDROTHERMAL ALTERATION

Clay minerals are formed as alteration products associated with geothermal areas and hot springs, and as aureoles around hydrothermal ore deposits. There is typically a zonal arrangement of the clay minerals around 43 | P a g e

the source of the alteration as a result of decreasing temperature and changes in fluid composition along the fluid flow and reaction path. The zonal arrangement varies with the type of parent rock and the nature of the hydrothermal fluid. For example, in epithermal ore deposits, near-neutral hydrothermal fluids alter rocks to illite, chlorite, and smectite, whereas acid hydrothermal fluids result in the formation of kaolinite, dickite and pyrophyllite. Furthermore, there is typically a temperature dependent zonation of illite, interlayered illite/ smectite and smectite with decreasing temperature in many epithermal/geothermal systems. Pyrophyllite is mainly found associated with hydrothermally altered volcanic rocks, particularly in Japan and Korea. Bentonite deposits typically originate through the hydrothermal alteration and/or weathering of tuffaceous material rich in volcanic glass, particularly ash falls, which provide the open macro-structure (high-surface area) necessary for efficient devitrification. This includes the alteration of volcanic ash deposited in lacustrine environments, alteration by groundwater of deeply buried tuffs, the surface weathering of tuffs, and hydrothermal alteration, either at depth or in hot springs.

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USES

Commercially, the most important clays are kaolin (mainly kaolinite) and bentonite (smectite), with palygorskite, sepiolite, and vermiculite constituting small, more specialised markets (Table 3). Illite, the most abundant clay mineral in nature, is unimportant commercially as an individual mineral, but it is a prime constituent of common clay and shale.

CERAMICS AND BRICKS

Whiteware ceramics may be classified as: porcelain, including hard porcelain, soft porcelain, vitreous china (largely used for making tableware) and technical porcelain (such as electrical or insulator porcelain, and high alumina porcelain); stoneware (e.g. rustic tableware and art ware); and earthenware. Kaolin is used extensively in the ceramics industry, because of its high fusion temperature and white burning characteristics. Kaolin intended for firing as a ceramic must have a high Al2 O3 content as well as low content of fluxing (K2 O, Na2 O) and colouring (Fe2 O3, FeO, TiO2 )

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agents. In the manufacture of whiteware, the kaolin is usually mixed with approximately equal amounts of silica, feldspar and talc, and a somewhat smaller amount of ball clay to obtain the proper properties of plasticity, shrinkage and vitrification, for forming and firing the ware. Premium-grade halloysite may be utilised to add whiteness and translucency to porcelain and bone china, and for strength in technical ceramics and ceramic catalyst support bodies. The composition of ceramic pipes is similar to whiteware, but contains more silica, fluxes and colouring agents. Potter’s clay is less pure than pipe clay and sculptor’s clay or modelling clay consists of a fine potter’s clay, sometimes mixed

with fine sand. Bricks are made from an admixture of clay and sand with some ferruginous (iron-containing) matter. The main clay minerals used in brickmaking are kaolin and illite. Kaolin type clays are also used in the manufacture of refractory products such as firebricks and blocks, insulating bricks, refractory mortars and mixes, and monolithic and castable materials. Refractory clays have little or no lime, alkaline earth or iron (which act as fluxes), and are therefore infusible or highly refractory. Plastic clays, like kaolin and ball clay, are not so

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refractory as are the flinty, harder varieties, but are useful for bonding. Where flint clays are scarce, plastic kaolin may be calcined to form a hard, dense, refractory aggregate known as chamotte or refractory grog.

WORLD PRODUCTION AND CONSUMPTION

World-wide kaolin production capacity is more than 27 Mtpa from more than 50 countries. More than half of this total is relatively low-cost unprocessed “common clay” used in lightweight aggregate, cement, brick, civil engineering, sealing, and refractories. The remainder is the various forms of processed industrial grade kaolin, including ball clay and refractory clay. Production of commercial grade ball clay is concentrated in the south-central United States (Tennessee, Kentucky and northern Mississippi; 1 Mt), the UK (Devon and Cornwall; 0.8 Mt), Germany (the Westerwald; 2 Mt) and the Czech Republic (Cheb basin), although many other countries produce plastic clays of lower quality including France (Provins and Charente), Portugal, Thailand, China (Pearl River Delta), and Ukraine. Refractory clays are produced in virtually every industrialised country, although there are four main areas of production, namely the USA, Europe, China and South Africa. Flint-clay production is restricted largely to the United States,

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France, South Africa, Australia, Hungary, Commonwealth of Independent States, and China. The commercial development of high purity halloysite resources is restricted to New Zealand, Korea and Japan. Lower grade resources are exploited in Japan, USA and, to a smaller extent, the Czech Republic, France, the Philippines and Morocco. About half of the world’s bentonite production is from the USA mainly in Upper Cretaceous and Tertiary rocks.

MINING AND PROCESSING

                   Clays are generally mined by highly selective open pit methods using hydraulic excavators, front-end loaders, or draglines. The clay is processed using either a dry (air flotation) or a wet process (water washing). The wet process produces a higher cost and higher quality product than the dry process. The dry process involves crushing, drying, pulverising, and air flotation, to remove the grit particles (mostly quartz and feldspar). In the wet process, the first step is to remove the non-clay minerals, usually by extracting the grit from a clay slurry in drag boxes, classifiers, and/or hydrocyclones. The clay slurry is centrifuged and then thickened to about 30% solids in settling tanks. Further processing may involve

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ultraflotation and screening/filtering. In some cases flotation or high-intensity magnetic separation is used to remove iron and titanium impurities. Bentonite may be further processed or treated. For example, swelling sodium bentonite, may be produced by treating calcium montmorillonite, the nonswelling bentonite, with soda ash. Acid-activated smectite is manufactured through the reaction of inorganic acids with smectite. The physical effects of acid activation include opening up the edges of the platelets, increasing pore diameters, and enlarging surface area. Also some bentonite and kaolinite are surface coated with organic compounds to make organoclays.

HYDROTHERMAL

alteration Halloysitic and kaolinitic clays produced by hydrothermal alteration are found in Northland, Coromandel and the Taupo Volcanic Zone. Halloysite clay, reputed to be “the world’s whitest clay”, is produced from deposits at Matauri Bay, Northland, by NZ China Clays Limited (Townsend, 1989; Harvey et al., 1990; Harvey and Murray, 1993; Luke, 1997). Two pits are worked on the Matauri Bay and Mahimahi rhyolite domes respectively, located 2 km apart (Figs 2 and 3). NZ China Clays Limited also has deposits at Shepherds Hill, 6.5 km

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to the west of Matauri Bay, and Maungaparerua, 8 km west of Kerikeri. The clay is formed by hydrothermal alteration and subtropical weathering of Pliocene to Pleistocene rhyolite domes (Putahi Rhyolite) to material comprising approximately 50% clay and 50% quartz, cristobalite and minor feldspar. The clay is predominantly halloysite, but at Maungaparerua, Murray et al. (1977) also noted allophane and kaolinite. The degree of clay development is generally greatest at the surface, because of the effects of surficial weathering superimposed on the hydrothermal alteration. The presence of basalt flows partly overlying the domes may have been an important factor in the alteration process. Several other rhyolite domes are present in Northland, but most do not show extensive development of halloysitic clays (Bowen, 1974). Matauri Bay clay deposit is derived from the alteration of a small (about 29 ha in area) rhyolite dome of low relief. It is completely surrounded and partly onlapped by thick (up to 60 m) flows of basalt. The raw clay is generally covered by 1 to 3 m of iron-stained material, which is removed together with soil and vegetation, before mining. The deposit is mined

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selectively by hydraulic excavators, and the material is transported by motor scrapers to stockpiles on concrete pads.

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The Northland halloysite deposits have been worked since 1969. About 80,000 tpa of raw clay is mined from the Matauri Bay and Mahimahi deposits with 50% of plant feed from each. Plant capacity is about 25,000 tpa of processed halloysite, with clay products being exported to more than 20 countries. Sufficient resources exist at the Matauri Bay and nearby Mahimahi deposits to sustain production for over 30 years at current rates. Potential resources are present in other deposits at Shepherds Hill and at Maungaparerua. In addition to the halloysite deposits of Matauri Bay and Maungaparerua, kaolin and halloysite deposits formed by alteration and weathering 52 | P a g e

of volcanic rocks are widespread in Northland, but are generally of small extent. They include deposits of weathered and altered Pukekaroro Rhyodacite, Maungarei Dacite and Putahi Rhyolite.

Kaolin deposits have been recorded from Kaeo (Te Pene; Quennell, 1963, 1964), Kauri (Parekiore), Whangarei Heads (Parua Bay and Ocean Beach), McLeods Bay (Munroe Bay) and Kaiwaka. All are white clays of low plasticity and approach the clay minerals kaolin and halloysite in composition. Some are of excellent quality, having low iron and alkali (Na2O + K2O) content and have been used in the manufacture of china ware or porcelain ware, and in the case of Kauri pit, for the manufacture of refractory bricks (Kamo Green Refractories). Bowen (1966, 1974) has given resource estimates for some of these deposits, based on geological mapping and limited drilling. Thompson (1989) summarised the result of exploration at Ocean Beach and Kaiwaka. At Mount Mitchell, potters clay or potters earth has been recorded from a highly siliceous hot spring deposit.

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