Clay minerals

Japanese: 粘土鉱物 - ねんどこうぶつ（英語表記）clay mineral

The term refers to the main mineral that constitutes clay. Most of the minerals are layered silicate minerals, but there are also amorphous or low crystallinity minerals. The clay minerals that constitute soil are called soil clay minerals, which are formed by the weathering of rocks. Many clay minerals are also found in soil. Some clay minerals, such as mica and chlorite, form large crystals, but these macroscopic crystals are not called clay minerals. Conversely, quartz, which constitutes clay, is not called a clay mineral. In other words, clay minerals do not refer to only certain minerals. The definition of clay varies slightly depending on the researcher in each field, such as ceramics, soil, geology, and minerals. According to mineralogist Toshio Sudo (1911-2000), clay is a material that has at least two of the following three properties: (1) it is plastic, (2) it is made up of fine particles, and (3) it hardens when heated to red heat. However, many studies have also stated that condition (2) is the most basic property of clay. There is no strict definition of fine particles, but particles with a particle size of 2 micrometers or less are often considered to be clay.

[Satoshi Matsubara]

Occurrence

Clay minerals are found in soil and weathered rocks, and in volcanic areas such as Japan, where they are found in rocks altered by hot water or hot springs, in dikes and veins within them, and in sediments such as volcanic ash and as minerals that make up sedimentary rocks.

[Satoshi Matsubara]

Cause

It can be formed by direct precipitation from hydrothermal solutions or by reaction with parent rock. In the latter case, the type and amount of formation are affected by the chemical composition of the parent rock, the temperature at the time of formation, and the hydrogen ion concentration (pH) of the solvent. Sometimes the entire parent rock becomes argillized, but in many cases only certain minerals within it become argillized.

[Satoshi Matsubara]

kinds

Layered silicate minerals include pyrophyllite, talc, montmorillonite, nontronite, saponite, vermiculite, mica minerals, chlorite minerals, kaolin minerals, and serpentine minerals, while amorphous or low crystallinity minerals include iconia, allophane, and hisingerite.

[Satoshi Matsubara]

use

Clay minerals have a wide range of uses. They have been used since ancient times in the ceramic industry, but are also important in pesticides, medicines, paints, textiles, paper and rubber coatings, civil engineering, fertilizers, etc. Conversely, clay is known to be a harmful agent that causes disasters such as landslides.

The English word clay comes from the Greek word meaning sticky substance.

[Satoshi Matsubara]

"Clay Mineralogy" by Toshio Sudo (1974, Iwanami Shoten)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

粘土を構成する主成分鉱物をいう。大部分は層状珪酸塩鉱物(けいさんえんこうぶつ)で、ほかに非晶質ないし低結晶度鉱物もある。土壌を構成する粘土鉱物を土壌粘土鉱物というが、これは岩石の風化で生成されたものである。土壌の中にも粘土鉱物が多くみられる。粘土鉱物には、雲母(うんも)や緑泥石などのように大きな結晶になる鉱物も含まれているが、こういう肉眼的大きさの結晶に対しては粘土鉱物といわない。逆に、粘土を構成していても石英のようなものは粘土鉱物といわない。すなわち、粘土鉱物というのは特定の鉱物のみを示すものではない。粘土の定義は、窯業、土壌、地質、鉱物など各分野で研究する人々により多少の違いがある。鉱物学者須藤俊男(としお)（1911―2000）によると、粘土とは、(1)可塑性があり、(2)微細な粒からできており、(3)赤熱すると固結する、という三つの性質のうち、少なくとも二つをもつものとされている。しかし、多くの研究から、(2)の条件が粘土のもっとも基本的な性質であるとも述べている。微細な粒というのも厳密な定義があるわけではないが、粒径が2マイクロメートル以下のものが粘土とされることが多い。

［松原　聰］