Atoms, Elements and Minerals

Rocks are made up of one or more types of mineral. But what exactly is a mineral? To a geologist, a mineral is a naturally occurring solid of a single chemical composition, usually in the form of crystals. Diamond, for example, is a mineral formed from only one element: carbon. But most minerals are formed from two or more elements that bond together to form compounds.

An example is quartz, which consists of the elements silicon and oxygen that bond together as silicon dioxide (SiO2). Minerals have characteristic crystal shapes and structures because the individual atoms that make them up (in the case of quartz, silicon and oxygen atoms) are arranged in a set pattern. More than 3,000 minerals have already been described, and about a hundred new ones are added each year. How can we identify different minerals? The chemical composition is the most definitive indicator, but this can be confirmed only through laboratory analyses. In studying hand specimens, one must rely on visual differences such as colour, streak, lustre, hardness and crystal shape.

The colour of a single mineral can be extremely variable. Quartz can be colourless and as clear as glass, in which case it is termed rock crystal. But minute amounts of impurities (elements other than silicon and oxygen) produce dramatically different colours.

Amethyst, one of the most beautiful varieties of quartz, owes its deep violet hue to the presence of iron. Traces of manganese or titanium impart a beautiful pink tint to rose quartz. And the brown to grey variety, smoky quartz, has been affected by natural radioactivity.

The most startling variety of quartz is agate, which is multicoloured, the different coloured bands reflecting layers of microscopic quartz crystals. Agate eggs, which can be purchased from many rock and mineral shops, are breathtaking in their beauty.

Although colours can be misleading, some minerals have a remarkably consistent hue. The coppercarbonate mineral malachite is always green. Another carbonate of copper, azurite, is always blue. And sulphur is yellow.

The streak of a mineral is its colour when it is in powdered form. To determine this, geologists scratch a socalled "streak plate", a piece of unglazed porcelain, with the mineral. The mark left is the colour of the streak, which is constant for individual minerals. Thus, different oxides of iron can have different streak colours: magnetite is black and hematite is brownish to cherry red. One drawback of this technique is that it usually does not help in identifying light coloured or very hard minerals. Lustre is the way in which the surface of a mineral reflects light. Those minerals that look like metal, for example pyrite ("fool's gold"), are said to have a metallic lustre. Non metallic lustres can be described as vitreous (glassy, like quartz), brilliant (like gems) and earthy (dull, like howlite).

Perhaps the most commonly used property of a mineral is its hardness, which is expressed as a value of 1 (softest) to 10 (hardest). This scale was the brainchild of the German mineralogist Frederick Mohs (1773-1839), and is thus called Mohs' Scale. Mohs chose a representative mineral for each number. Thus 1 is talc, 2 is gypsum, 3 is calcite, 4 is fluorite, 5 is apatite, 6 is orthoclase feldspar, 7 is quartz, 8 is topaz, 9 is corundum and 10 is diamond. Hence, quartz will scratch calcite but not topaz, and so on. Most rock and mineral stores sell hardness kits, but there are also simple, quick tests for hardness.

A fingernail has a hardness of 2.5, a copper cent coin is 3, a knife made of steel is 5.5 and emery cloth is about 8. Hardness generally reflects the mineral's structure rather than its composition. For example, carbon forms one of the softest minerals (graphite, with a hardness of 1), as well as the hardest (diamond, with a hardness of 10), depending upon the arrangement of its atoms. The crystal shape often adds a final touch to the elegance of a mineral. It is simply an external expression of the orderly internal arrangement of the atoms. For example, pyrite occurs as cubic crystals, diamond is diamond shaped and quartz forms elongate prismatic crystals.

Another group of minerals with striking crystal shapes is the zeolites. Zeolites are common in basalts (solidified lava flows) found around parts of the Bay of Fundy. They usually occur in holes, or vesicles, that formed as gases escaped from the cooling lava. Seeping groundwater later precipitated minerals, including zeolites, in these vesicles.

The zeolite group of minerals includes natrolite, which has needlelike crystals that occur in radiating masses, mesolite, with its delicate fibrelike crystals , and stilbite, with platy crystals that tend to occur in masses resembling oldfashioned haystacks. Some minerals, such as gypsum, can have several different crystal shapes.

Gems are special minerals. They have attractive colours, but to be truly considered a gem, a mineral also must be hard and uncommon. Most gems are cut from one of only about 20 minerals. Thus blue sapphires and red rubies are both made of corundum, one of the most durable of minerals, with a hardness of 9.

Although there are over 3,000 known minerals, it is perhaps surprising that there are not more, especially considering that there are 92 naturally occurring elements. But all elements are not found in equal abundance. The bulk of the Earth's crust is made up of only eight elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. Since oxygen and silicon together make up about threequarters of the crust, silicates are by far the most common minerals. This explains why quartz, essentially pure silica, is so common. Another group of common minerals is the carbonates (with carbon and oxygen), which includes calcite and dolomite.

Calcite is the primary mineral in limestone, and also forms many seashells. Other important groups are the sulphides, which contain sulphur (for example the copper sulphide mineral, chalcopyrite), sulphates (for example the calcium sulphate mineral, gypsum), oxides (for example the iron oxide mineral, hematite), and halides (for example the sodium chloride mineral, halite, otherwise known as table salt).

Minerals are vital to the economy since they are found in nearly all manufactured products. Canada is one of the leading producers of minerals such as diamonds, gold, gypsum, nickel, potash, silver and sulphur. In the Maritimes, economic minerals have been a major source of economic growth.