Most likely our planet originally was a mass of molten material. When this material cooled, a thin film solidified on the surface just as a film forms on the surface of milk after boiling. As our planet was cooling but still fluid, heavy materials such as metals sank towards the center of the planet, so that the other materials that were left on the surface were of a fairly uniform composition. They cooled and hardened, forming what is now called igneous rocks and minerals. Here are some interesting facts about the earth itself:
- The core of our earth, which is about 3.500 km thick, is made of liquid iron and nickel;
- Around this core is a viscous layer made of nickel and magnesium, called the mantle. It is approximately 2.800 km thick;
- On top of this mantle is the planet's surface, a very thin layer called the "skin", only 10 to 70 km thick. This surface area is called the rind or bark. It is solid, and consists of three layers:
1 - The deepest layer, called the basaltic layer, is composed mostly of magnesium and silicon, and forms the bottom of our oceans;
2 - On top of the basaltic layer is a layer of granite, called the continental crust. It is composed of at least 75% silicon and aluminium. These two oxides are the main elements found in clay;
3 - Above these two layers lies the sediment surface area.
According to geologists (Millot, 1983; Galàn, 2006) there are three main geological processes at the origin of clay. These are:
* inheritance : (local or moved) Clay forms from silicate rocks in the deep and is or is not moved to the surface by tectonic movement. Silicate rocks are granite's, gneiss, mica schist, shale, lava, feldspar and mica;
* transformation : (second generation clay) Clay is transformed from the original inheritance clay by evolving and adapting to a new environment through a process called aggradation or degradation;
* neoformation : clays form a new from dispersed elements such as ions, combining at the bottom of a lake for example.
In addition to the above three mechanisms, other processes called diagenesis and metamorphism can change the clay further
Bernal first suggested the role of clay minerals in the origin of life because of the ordered arrangement of the clay mineral particles, the large adsorption capacity, shielding against ultraviolet radiation, ability to concentrate organic chemicals and ability to serve as polymerization templates. Clay minerals were proposed as possible genetic material. This appealing hypothesis was not yet supported by experimental data. Many experiments were run to correlate biological one-handedness to clay mineral chemistry but convincing data were not obtained. However, clay minerals act as very efficient catalysts in the polymerization of amino acids and nucleotides, thus supporting the possible existence of an RNA world preceding a cellular world. RNA adsorbed to clay minerals can be encapsulated within vesicles. Once formed, such vesicles could grow by incorporating fatty acidsand divide, thus mediating vesicle replication through cycles of growth and division. The data obtained so far suggest that clay minerals played an active role in the abiotic origin of life.
Clay is produced by the erosion of the rock composing the earth's crust. This erosion is universal, permanent and continuous. Every day more clay forms than man could extract for all its industrial uses.
Originally the Earth was a mass of matter in fusion. The heaviest materials, like metals, sunk in the deeper layers. This process formed surface layers of nearly uniform chemical composition. The Earth then cooled down, and the upper layer solidified forming a crust, surrounding a boiling interior mass. The vertical thrust of the rock, compressed and lifted during the cooling process, created the first mountains. The rock formed by the cooling of this matter in fusion is called igneous rock.
Average composition of the igneous rock:
Silica 59.0 %
Alumina 15.3 %
Iron oxide 7.0 %
Calcium oxide 5.0 %
Sodium oxide 3.8 %
Magnesium oxide 3.5 %
Potassium oxide 3.1 %
Titanium oxide 1.0 %
Water 1.0 %
Various 1.1 %
The earth's crust is essentially composed of a very small number of oxides. Silica and alumina account for 75 % and these two oxides are also the principal elements composing clay. During the
cooling of the Earth various minerals were formed. Variations in the composition of the matter in fusion and various conditions of cooling created a great variety of minerals. Of the hundreds of
identified minerals, the majority are relatively rare and only five of them make up more than 90 % of the earth's crust. Feldspar is by far the most common mineral.
Approximate percentages of the minerals forming the earth's crust:
|Iron-magnesium group||16.8 %|
|Titanium and composites||1.5 %|
Approximately two billion years ago began the process of geological transformation affecting igneous rocks; the gaseous atmosphere and the earth's crust reacted with each other. The moisture contained in the atmosphere, which was initially vapoured, condensed into torrential rain which fell during millions of years.
This rain was the most significant factor of the geological transformations. Water gradually washed out of the rock all the soluble matter that it contained and transported it to the sea, filling the oceans. The presence of salt in the oceans demonstrates the capacity of rainwater to dissolve minerals. In addition to its chemical action like a solvent, water also has a mechanical movement. The abrasive effect of precipitations, freezing and thawing, as well as the crushing of the rock by torrents, streams and rivers disintegrated the rock by splitting it in increasingly smaller particles.
Bed of common clay in Mistook, QC