Minerals and organic solids
As well as being broadly categorised as either crystalline or amorphous, solids are also often described as being either minerals or organic solids. It is therefore probably useful at this point to briefly look at what these terms mean.
We will look first at minerals. These are generally defined as being naturally occurring materials that are solid at room temperature, can be described using a chemical formula, have an ordered atomic arrangement, and are not abiogenic (in other words, they are inorganic; they do not occur as the result of the activities of living organisms).
Most minerals are crystalline in form (we'll discuss what that means in due course), and are created as the result of geological processes. The word mineral is thought to be derived from the Medieval Latin term minerale, which loosely translated means "something mined" or "substance obtained by mining".
Even though, by definition, minerals are naturally occurring, they can be either elemental substances or chemical compounds. Diamond and graphite, for example, are both naturally occurring allotropes of the element carbon (interestingly, diamond is the hardest natural mineral, while graphite is one of the softest). Common non-elemental minerals, some of which you may have heard of, include talc, gypsum, quartz, topaz and corundum.
An uncut diamond(left) and a sample of crystalline vein graphite (right)
The hardness of a mineral substance is one of the things that is of interest to mineralogists (a mineralogist is someone who studies minerals), because it is one of the ways in which a mineral substance can be identified. One of the most widely used methods for testing the hardness of a mineral substance is known as the Mohs hardness test, named after the German geologist and mineralogist Carl Friedrich Christian Mohs (1773-1839) who developed the technique in 1812.
The Mohs hardness test is used to determine the resistance of a mineral substance to being scratched, which is essentially a measure of how easily atoms are dislodged from the surface of a substance. The hardness of a sample, on a scale of one to ten, is tested using ten reference materials collectively known as the Mohs Hardness Scale. The materials used, together with their relative values in the Mohs Hardness Scale, are listed in the table below.
|Mohs hardness||Mineral name||Chemical formula||Absolute hardness|
|1||Talc||Mg3 Si4 O10 (OH)2||1|
|2||Gypsum||CaSO4 ·2H2 O||3|
|5||Apatite||Ca5 (PO4 )3 (OH -,Cl -,F -)||48|
|6||Orthoclase feldspar||KAlSi3 O8||72|
|8||Topaz||Al2 SiO4 (OH -,F -)2||200|
The Mohs hardness test itself is a relatively unsophisticated process. The piece of material to be tested (let's call it sample A) is placed on a flat surface like a table or workbench. A sample of one of the materials listed in the table above (we'll call it sample B) is chosen to test the hardness of sample A. One edge of sample B is dragged sharply across a flat area of sample A, and the result noted.
Essentially, if sample B can produce a scratch on sample A, sample A is softer than sample B. The test can be repeated with different materials from the Mohs hardness scale until the relative hardness of the test sample has been determined.
Note that we have included absolute hardness values in the table for reference purposes. These values are obtained using far more sophisticated testing techniques, and give a much more accurate idea of the relative hardness of the different materials. The Mohs hardness test is still widely used, however, because of its simplicity and relatively low cost.
Organic solids are organic chemical compounds that exist as solids at room temperatures. The Encyclopedia Britannica defines organic compounds as "a . . . class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen." In fact, only a relatively small number of chemical compounds found in nature are not organic.
We saw above that minerals are generally described as naturally occurring substances that are solid at room temperatures, and are not produced as the result of the activities of living organisms. Organic solids, by contrast, are frequently produced directly as the result of biological processes. Organic compounds of various kinds are found in all living organisms, their waste products, and their physical remains. Many organic solids, such as synthetic rubbers and plastics, are man-made.
Examples of organic solids: a living magnolia (left) and plastic anemones (right)
Both naturally occurring and man-made organic solids are a very important feature of our everyday lives. Wood, for example, is a complex natural organic material that has found endless applications throughout human history, including: the construction of dwellings; the building of ships, boats, and other forms of transport; furniture making; the creation of tools, weapons, and countless other artifacts; and the manufacture of paper.
Plastics, on the other hand, are man-made materials that have only emerged in the last century or so. Nevertheless, they are now a ubiquitous part of our existence and can be found all around us, in food and drink packaging, in construction materials, in furniture, in motor vehicles, and in all manner of consumer products.
The one thing all organic solids have in common is that they are all complex chemical compounds based on carbon. Carbon atoms have four valence electrons, and thus tend to form covalent bonds with atoms of other elements, like hydrogen or nitrogen. The individual molecules of the resulting chemical compounds are called monomers, and they have a strong tendency to form chemical bonds with other monomers. The resulting structure is like a daisy-chain made up of a large number of individual monomers, and effectively forms a much larger molecule called a polymer.
The process of forming a polymer is called polymerisation. Polymers that occur through natural processes are called biopolymers. There are three main kinds of biopolymer: Polynucleotides like deoxyribonucleic acid (DNA) are chains of nucleotides; Polyamides are chains of proteins; and Polysaccharides are chains of sugars. Artificial polymers like synthetic rubber and plastics, which are manufactured using industrial processes, are called industrial polymers or synthetic polymers.