Clay minerals

Clay is a subset of the omnipresent material called “soil”. Soil is essentially the waste
material originating from rocks due to weathering and decayed plant and animal matter.
Modifications in the nature and composition of soil may be caused by a combination of
physical, chemical and biological processes. It constitutes the top layer of the ground.
Clay is the finest grained fraction of the soil, and it comprises very fine minerals
composed of hydrous silicates called “clay minerals”. But, defining clay has been a subject of
research and controversy for a long time, and even now there seems to be no universal
agreement on a standardized definition. According to Wentworth’s scale (1922), clay
comprises particles of size below 1/256 mm (cf., silt: 1/256- 1/16 mm, sand: +1/16 – 2 mm,
granular gravel: +2 – 4 mm, gravel: +4 mm). Twenhofel (1937) considered that in clay, the
clay minerals must constitute at least one-fourth of the total matter. Bateman (1942) laid
emphasis on chemical composition, colloidal nature, plasticity and fired properties.
According to him, clay is an earthy substance consisting chiefly of hydrous aluminium
silicates with colloidal material and specks of rock fragments, which generally become plastic
when wet and stone-like when fired. However, as we see today, not all types of clay are
plastic, and all of them are not necessarily fired before use. According to Pettijohn (1949),
undue reliance on grain size may often be misleading. The US Bureau of Mines (USBM) and
the US Geological Survey (USGS) have accepted a simplistic and general definition that clays are hydrous aluminium silicates of a great many mineral species, containing varying
proportions of impurities.
All these controversies stem from the fact that no two clays are similar, and industries
have never cared for definition of the clays that they use. Instead, they have all along relied
on the trial tests and on the past records of performance of the clays drawn from historically
well-known sources. Nevertheless, clay minerals form an important component of any clay.
These are the nanometre-sized products of decomposition by weathering of primary silicate
minerals. All clay minerals (some of which are similar enough to form broad groups) are
essentially hydrous aluminium silicates, but they show variations in mineralogy, in chemical
composition and in crystal form.