Kaolinite

• 1. Introduction

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  Kaolinite is a soft, earthy, usually white clay mineral with the chemical composition Al2Si2O5(OH)4. In 2021 the world produced an estimated 45 million tonnes of it, worth around US $4.24 billion. Most listeners have handled it without knowing. It coats glossy paper, fills toothpaste, soothes upset stomachs, and gives porcelain its body.

  The name itself was borrowed in 1727, lifted from a French missionary's reports on Chinese porcelain. The mineral traces back to a village called Gaoling, meaning High Ridge. How did a clay named after one Chinese hillside become one of the most common minerals on Earth?

  This documentary follows kaolinite from its quiet molecular architecture to the kiln, from the soils of tropical rainforests to a stretch of Georgia that calls itself a belt of white gold. Along the way it slows blood, settles stomachs, and gets eaten on purpose by people who say it is a habit like smoking.

• 2. A Borrowed Name

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  François Xavier d'Entrecolles wrote his French reports on the manufacture of Jingdezhen porcelain in 1712. The English name kaolin was borrowed from those reports in 1727. He was transcribing a Chinese term, now romanized as gaolingtu in pinyin.

  The village of Gaoling, whose name means High Ridge, sits near Ehu in Fuliang County. That area is now part of Jiangxi Province's Jingdezhen Prefecture. Over the course of the Qing dynasty, the land around the village became the main source of Jingdezhen's kaolin. The mineralogical suffix -ite was later added to generalize the name, so it could cover nearly identical minerals found elsewhere.

  Kaolinite once carried older labels too. It was occasionally discussed under the archaic names lithomarge and lithomarga, from a Latin combination of litho-, meaning stone, and marga, meaning marl. In more proper modern use, lithomarge now refers specifically to a compacted and massive form of kaolin. The same word, then, narrowed to name a particular texture rather than the mineral as a whole.

• 3. One To One

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  Compared with other clay minerals, kaolinite is chemically and structurally simple. Mineralogists call it a 1:1 or TO clay mineral, because its crystals consist of stacked TO layers. Each layer pairs one tetrahedral sheet with one octahedral sheet.

  The T sheet is built from silicon and oxygen. Each silicon ion sits surrounded by four oxygen ions, forming a tetrahedron, which is where the letter comes from. The O sheet holds oxygen, aluminium, and hydroxyl ions. Each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. Inside a layer, the two sheets are strongly bonded by shared oxygen ions. Between layers, hydrogen bonds link the oxygen of one sheet to the hydroxyl of the next.

  A kaolinite layer carries no net electrical charge. Because of that, there are no large cations such as calcium, sodium, or potassium wedged between the layers, unlike most other clay minerals. This explains kaolinite's relatively low ion exchange capacity, measured at 1 to 15 milliequivalents per 100 grams. The close hydrogen bonding also keeps water molecules from slipping between layers, which is why kaolinite does not swell when wet.

• 4. The Memory Of Water

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  When moistened, the tiny platelike crystals of kaolinite acquire a layer of water molecules. That film makes the crystals adhere to one another and gives kaolin clay its cohesiveness. The bonds are weak enough to let the plates slip past each other while the clay is being molded.

  Those same bonds are strong enough to hold the plates in place once the shape is set, so molded clay keeps its form. When the clay dries, most of the water molecules are removed. The plates then hydrogen bond directly to each other, leaving the dried clay rigid but still fragile.

  Moisten that dried clay again, and it will once more become plastic. This reversibility holds only up to a point. The behavior changes entirely once the clay meets serious heat, which is where the next chapter begins.

• 5. Trial By Fire

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  Below 100 degrees Celsius, exposure to low humidity air slowly evaporates any liquid water in the kaolin. At low moisture content the mass is described as leather dry, and at near zero percent moisture it is called bone dry. Above 100 degrees, any remaining free water is lost.

  Above around 400 degrees Celsius, hydroxyl ions leave the kaolinite crystal structure in the form of water. The material can no longer be plasticised by absorbing water. This step is irreversible, and so are the transformations that follow. The process is called calcination.

  Endothermic dehydration begins at 550 to 600 degrees, producing disordered metakaolin, though hydroxyl loss continues up to 900 degrees. Researchers once disagreed sharply about what metakaolin actually is. The consensus now holds that it is not a simple mixture of amorphous silica and alumina, but a complex amorphous structure that keeps some longer-range order from the stacking of its hexagonal layers.

  Further heating to 925 to 950 degrees converts metakaolin into an aluminium-silicon spinel, sometimes called a gamma-alumina type structure. Above 1050 degrees, that spinel nucleates and transforms into platelet mullite and highly crystalline cristobalite. Finally, at 1400 degrees, the needle form of mullite appears, bringing substantial increases in structural strength and heat resistance. That last change is structural but not chemical, the same atoms rearranged into a tougher form.

• 6. Soil And White Gold

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  Kaolinite is one of the most common minerals, mined as kaolin across at least nineteen named countries, from Australia and Brazil to China, India, Iran, and Vietnam. It gathers in abundance in soils formed by the chemical weathering of rocks in hot, moist climates, such as tropical rainforest areas.

  Move toward cooler or drier climates and the balance shifts. The proportion of kaolinite decreases, while illite rises in cooler climates and smectite rises in drier ones. Because of this, scientists read clay mineral content in ancient buried soils to infer past climates. In the INERA classification system, soils whose clay fraction is predominantly kaolinite are called kaolisol, from kaolin and soil.

  In the United States, the main deposits sit in central Georgia, along the Atlantic Seaboard fall line between Augusta and Macon. This stretch of thirteen counties is called the white gold belt. The town of Sandersville is known as the Kaolin Capital of the World for its abundance of the mineral. United States kaolin production in 2011 reached 5.5 million tons.

  Georgia is not the whole American story. In the late 1800s an active kaolin surface-mining industry ran in the extreme southeast corner of Pennsylvania, near the towns of Landenberg and Kaolin, in what is now White Clay Creek Preserve. The product traveled by train to Newark, Delaware, on the Newark-Pomeroy line, where many open-pit clay mines can still be seen. Those deposits formed between the late Cretaceous and early Paleogene, roughly 100 to 45 million years ago.

• 7. A Mineral Of Many Trades

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  In 2009, up to 70 percent of kaolin went into making paper. Demand then fell, pressed by competing minerals and the rise of digital media. By 2016 the market split into paper at 36 percent, ceramics at 31 percent, paint at 7 percent, and other uses at 26 percent.

  Paper work rewards specific qualities: high brightness, low abrasion, and delaminated kaolins. In coatings it sharpens gloss, brilliance, and smoothness, and can make up 25 percent of a paper's mass. As a filler it extends pulp and raises opacity, accounting for around 15 percent. In whiteware ceramic bodies, kaolin can form up to half the raw materials, lending green strength and plasticity before firing, then helping form mullite in the kiln.

  Kaolinite reaches into medicine and the body as well. It is used to soothe an upset stomach, echoing how parrots and later humans in South America used it. It treats diarrhea, works as a mild abrasive in toothpaste, and induces blood clotting. In April 2008 the US Naval Medical Research Institute announced the successful use of a kaolinite-derived aluminosilicate infused into traditional gauze, which remains the hemostat of choice across all branches of the US military.

  Some people simply eat it. The practice, called geophagy, is done for pleasure or to suppress hunger. In Africa the kaolin used this way is known as kalaba in Gabon and Cameroon, and as calaba and calabachop in Equatorial Guinea. Consumption is greater among women, especially during pregnancy, and is sometimes described by women of the region as a habit like cigarette smoking among men. The practice has also been observed among a small population of African-American women in the Southern United States, especially Georgia, where the kaolin is called white dirt, chalk, or white clay.