Mohs Scale Trivia Quiz

Answer: Talc

Like many minerals, talc does not have a definite chemical formula (and they look very strange when subscripts are not possible), so suffice to say it contains magnesium, hydrogen, silicon and oxygen. It is rarely found in crystalline form, and is more familiar in the foliated form we call talcum powder. As well as being used as a cosmetic or for preventing diaper rash in babies, talc is used as a lubricant and as a filler to help bind the fibres in papermaking. Talc is often the main ingredient in tailor's chalk, and it has a variety of uses in producing ceramic artworks.

The Mohs scale of hardness (named after the German geologist Friedrich Mohs who developed it) uses ten minerals that are given arbitrary numbers between 1 and 10, and compares others to them. One mineral is said to be harder than another if the first mineral can produce a scratch on the second when they are rubbed across each other. All minerals can be assigned hardness numbers in rank order by comparing them with each other. A sample of an unknown mineral is roughly given a hardness by comparing it to the ten standards - its hardness is between the hardest one it scratches and the one next on the scale which it fails to scratch. More precise ratings can be obtained if a larger range of test samples is available, so that known intermediate values are represented. This scale, however, is not linear, nor is it exponential - the ten fixed values were chosen because the minerals are widely available in the same form (since different crystal structures of the same substance can have different hardness) and spread across the range between the softest and hardest minerals known at the time the scale was established. There are other hardness scales, some of which use numbers that indicate how much harder one substance is than another (based on sclerometric testing). On these quantitative scales, talc is still usually given the number 1.

Although talc is the softest known mineral, there are some elements which are softer - all of the alkali metals (lithium, sodium, potassium, caesium and rubidium) are softer than talc. Because they are such active metals, they are not found as pure metals in the earth's crust, but as compounds whose hardness is greater than 1.

Answer: Gypsum

When gypsum is heated strongly, most of the water is eliminated from the crystal structure, forming a substance called calcined gypsum, gypsum plaster, or plaster of Paris (a name deriving from the fact that there was a large gypsum mining industry in the Montmartre district of Paris). If this powder is mixed with water, it turns back into gypsum and becomes hard - gauze strips impregnated with plaster of Paris are used to produce the plaster casts used to protect broken bones as they heal. Artists call plaster-impregnated material modroc, and use it to produce sculptures which can simulate the textural appearance of wood, stone or metal, depending on how it is manipulated.

Elements with a hardness of 2 include calcium, selenium, cadmium, sulfur, tellurium and bismuth. Ice is between 1 and 2. On an absolute scale of hardness, gypsum is approximately 3 times as hard as talc.

Answer: Calcite

Calcium carbonate can form several crystal structures, which are given different names, including aragonite and vaterite, but the others are much less stable, and tend to reform into calcite under suitable conditions. Pure calcite is clear or white in color, but it is often found with embedded impurities that give it a tinge of color - every color of the rainbow, plus brown, grey and black.

Calcite is the main substance found in limestone, as it is formed from the shells of dead marine organisms whose shells were composed of calcite. (Trilobites had calcite lenses in their eyes!) Since marble is formed from limestone by metamorphosis, it is also essentially calcite. You may also be familiar with the calcite cave formations, such as stalactites and stalagmites, which form when calcium carbonate crystallizes from the water in which it has been dissolved.

Elements with a hardness of around 3 include copper, arsenic and antimony. On an absolute scale of hardness, calcite is about 9 times as hard as talc.

Answer: Fluorite

Fluorite was originally called fluorospar, the first part of its name (from a Latin word meaning flow) referring to its use as a flux - an agent which helps remove chemical impurities during the process of extracting metals from their ores. It also gave its name to the property of fluorescence, the process in which substances emit light they have absorbed, so can be seen (briefly) to glow in the dark. The mineral's name was also the reason why the element fluorine, one of its two primary components, got its name.

Elements with a hardness of three include iron and nickel. On an absolute scale of hardness, fluorite is around 20 times as hard as talc.

Answer: Apatite

Apatite is actually a mixture, in varying proportions, of crystals of calcium phosphate with hydroxyl or fluoride or chloride ions incorporated into the crystal, given the more specific names of hydroxylapatite if it contains OH- ions, fluorapatite if it contains F- ions, and chlorapatite if it contains Cl- ions. The name for the mineral comes from a Greek word (apatein) meaning to deceive - it is easily mistaken for other minerals.

Apatite is of significance in human physiology, as hydroxylapatite is a main component in tooth enamel. The fact that fluoroapatite is more resistant to acid attack than is hydroxylapatite is the science behind adding fluoride to water and/or toothpastes, as the fluoride ions replace the hydroxyl ions, and produce a stronger tooth enamel which is less likely to undergo decay. (Using your tooth as a makeshift hardness 5 test is not recommended - unless you want to help your dentist enjoy a world tour at your expense.)

Another material with a hardness of 5 is obsidian (volcanic glass). On an absolute scale, apatite is almost 50 times as hard as talc.

Answer: Orthoclase feldspar

Tectosilicates are minerals with a three-dimensional structure based on silicon dioxide - between them all, they form around three-quarters of the material found in the earth's crust. The feldspars (along with most tectosilicates other than the quartz family) are aluminosilicates, meaning that they incorporate aluminum ions in the crystal structure, along with 3 silicon atoms for every 8 oxygen atoms. (This looks clearer to me written as a chemical formula, but without being able to use subscripts, they look awfully messy!) Orthoclase is a potassium feldspar - it also has potassium ions in its crystal - which can be identified because the crystal splits neatly along two axes which are at right angles (orthogonal) to each other. Feldspar is one of the main constituents of granite, responsible for its common pink coloring.

Elements with a hardness of 6 include titanium and uranium (assuming you feel like handling a sample of the pure metal). On an absolute scale of hardness, it registers as being 72 times as hard as talc.

Answer: Quartz

Quartz is second only to feldspar in its abundance on the surface of the earth. It is composed of tetrahedral units of one silicon atom and four oxygen atoms, connected in such a way that overall there are only two oxygen atoms for each silicon atom. Variations in the crystalline structure produce varieties with different names (and often different colors). Some of these include amethyst (generally with a purplish color), citrine (yellow to brown due to iron impurities) and rose quartz (pink, as its name suggests).

Quartz is piezoelectric, meaning that its ability to conduct electricity varies as pressure is applied. This property made quartz crystals in early phonographs. The stylus travelling in a record groove would vibrate with an amplitude governed by the shape of the groove, placing a varying pressure on the crystal which then created a varying electric signal which could be amplified and used to drive a speaker, thus changing record grooves into sounds. This tidbit is for all of you out there who remember record players - quartz crystals do not figure in the workings of CDs, DVDs, Blue-Ray, etc., but they are still used in crystal oscillators used to produce signals of a precise frequency.

The unglazed porcelain streak plate used for testing minerals in the field by rubbing them to produce a powder residue whose color is characteristic has a hardness of around 7, so is only useful for testing substances whose hardness is no more than 7. Iron pyrite (fools' gold), silicon, opals and jade are all between 6 and 7 on the Mohs scale. On an absolute scale of hardness, quartz is 100 times as hard as talc.

Answer: Topaz

Pure topaz is colorless, but it is usually found with impurities included in the crystal structure that give it color. Perhaps the most familiar is orange topaz, traditionally the birthstone for the month of November and also the state gemstone of the US state of Utah. Blue topaz is the state gemstone of Texas - but beware, most samples are not natural (as that is quite rare) but produced by heating and irradiating samples of colorless or yellow topaz.

Cubic zirconia, commonly synthesized for use in jewellery as a diamond substitute, is another mineral with a hardness of 8. Emeralds are softer, rated between 7 and 8 on the Mohs scale. On an absolute hardness scale, topaz is 200 times as hard as talc.

Answer: Corundum

While you may not be familiar with the name corundum, you will almost certainly recognise the common names of some of its colored forms, which include rubies and sapphires. Rubies are aluminium oxide with chromium atoms included in the crystal structure to produce the characteristic red color. Although most people associate sapphires with a blue color, they can actually occur in a range of colors (identified by a preceding color, as green sapphire, if not blue) depending on the metal ion which is included as an impurity on the crystal structure. Iron produces a blue gem, titanium a yellow one, chromium a purple one, copper an orange one, and magnesium a green one. Because corundum is so hard, it is commonly used as an abrasive - small crystals of corundum are the "sand" in sandpaper.

We are reaching the extremes of hardness now. One of the few substances with which you may be familiar which has a hardness of 9 is tungsten carbide, often used for the rolling tips of ballpoint pens. On an absolute scale of hardness, corundum is 400 times as hard as talc.

Answer: Diamond

Carbon atoms can join to form solids in a number of ways (called allotropes of each other), the most familiar of which are the three-dimensional tetrahedral structure of diamond, which is one of the hardest substances known, and graphite, made of two-dimensional networks loosely joined to each other. This structure means the planes can easily slide past each other, which is why your pencil leaves a black trail of carbon when you move it across the page (if you still have any pencils around, that is). Technically, diamond is a less stable structure than graphite, but under normal conditions the decay from diamond to graphite is virtually non-existent - you don't need to worry that you may wake up to find your engagement ring is now more suited for use as a writing implement!

No naturally occurring substances are as hard as diamond, which is why it was chosen to be 10. Some synthetic materials (such as aggregated diamond nanorods, known as ADNRs) formed by compression of graphite or fullerene (another carbon allotrope which deserves a quiz of its own) have been produced with a hardness over 10. On an absolute hardness scale, diamond is 1600 times as hard as talc.