MINERALS

Anyone who has looked closely at rocks or sand knows that they are made up of individual grains of different looking materials. These we call minerals and say that rocks are aggregates of grains of different minerals. When we consider carefully what a mineral is we find it necessary to make a fairly accurate definition. First of all, they are naturally occurring substances. This distinguishes them from man-made or synthetic materials. Secondly, they have some sort of definite internal arrangement of the elemental materials of which they are made. Thus coal, water and organic materials are eliminated. Thirdly, they have distinctive chemical and physical properties by means of which they can be recognized.

TABLE OF MINERALS

Name	Common Colour	Lustre	Hardness *1	Specific Gravity	Cleavage *2	Where it is commonly found	Special Features
1. Quartz	Colourless or white	Glassy	+ knife	Average	None	Sand grains; sandstone; veins; grains in igneous rocks.	Often occurs in six-sided, pointed crystals. Most common of all sand grains.
2. Feldspar	White, pink or grey	Shiny to dull	+ knife	Average	Good (2)	Major part of igneous rocks; occasional sand grains; in some veins.	Light coloured minerals with shiny faces in igneous rocks are largely feldspars.
3. Biotite (black mica)	Black	Shiny	— fingernail	Average	Excellent (1)	Igneous rocks; veins; flakes in sands and sandstone; schists and gneisses.	Black flakes in sand; tight books of cleavage flakes in other rocks.
4. Muscovite (white mica)	White to transparent	Pearly to shiny	— fingernail	Average	Excellent (1)	As above for biotite.	Often as flakes in sands where they are sometimes golden.
5. Hornblende	Black or dark green	Dull to sub-metallic	— knife	Average	Fair (2)	Igneous rocks; gneisses and schists; occasional grains in dark sands.	Needles and grains in igneous rocks or fragments in sands.
6. Pyrite	Pale yellow	Metallic	+ knife	Twice the average	None	Almost everywhere, sometimes in perfect cubes.	"Fool's Gold"; unlike real gold because hard, brittle and often in cubes.
7. Calcite	White or light colours	Glassy	+ nail	Average	Good (3)	Limestone; veins, pockets.	White and fairly soft with three cleavages not at right angles.
8. Magnetite	Black	Metallic to submetallic	+ knife	Twice the average	Poor in several directions	Common in sand grains and sandstone; some igneous rocks.	Magnetic so can be picked up with point of magnetized knife blade or magnet.
9. Limonite	Yellow or brown	Dull	—	A little more than average	None	Rusty stain on almost any rock.	Really iron rust and looks like it.
10. Garnet	Red, pink, brown	Glassy	+ knife	A little more than average	None	Common in sand; some igneous and metamorphic rocks.	Pink, glassy grains in sand and metamorphic rocks.
11. Olivine	Olive green	Glassy	+ knife	Average +	None	Grains in sand; certain igneous rocks.	Green, glassy grains in sands or lava flows.
12. Chlorite	Grey to dark green	Shiny to dull	— nail	Average —	Good (1)	Foliated or scaly masses.	Common in schists and as an alteration of dark minerals.
*1 + means harder than; —means softer than; knife is ordinary jackknife blade. *2 Good (2), means good cleavage in two directions.

Very few naturally occurring compounds in nature are without some impurities of foreign matter so that there is usually a narrow range of chemical and physical properties for any one mineral. Sphalerite or zinc sulphide, the mineral from which most commercial zinc is obtained, is nearly colourless or pale honey-yellow when pure. But in nature there is almost always a small quantity of iron with it. The result is that the colour is nearly always brown or even black. It is because of the range in characteristics like this that the definition is elastic.

How Minerals are Recognized

One can see from the definition, which says that minerals have distinctive chemical and physical properties, that one should be able to tell minerals apart on the basis of either chemical or physical tests. Chemical analyses of minerals are usually expensive and take a lot of time. Physical tests, on the other hand, can be made very quickly and easily in most cases. In fact, geologists and mineralogists can usually tell what a mineral is by its general appearance. This is not so unusual when we come to think of it, for we recognize our friends that way and we know which house is our own by its look, its shape, its size, its colour, where it is located and so on. With minerals we use colour, lustre, hardness, cleavage, crystal form and specific gravity. Not all these are familiar so they will be explained briefly one by one.

Colour

Minerals can be almost any colour you can think of. At St. Lawrence, in Newfoundland, where over 100,000 tons of the mineral fluorite (or fluorspar) were mined in 1955, you can see purple, yellow, green, pink and white fluorite. The fluorite has different colours because of tiny amounts of different impurities in it or very small structural differences. This variation in colour is found in lots of minerals. We can say, however, that certain colours are usual for a particular mineral, even though we know that sometimes that mineral might have other colours because of impurities.

Lustre

Lustre is the appearance of the surface of the mineral. Is it sparkly or is it dull? Is it greasy looking or hard and bright looking? Lustre is talked about in no special scientific words but in terms of things everyone knows about. Glassy lustre means that the mineral looks like glass. Pearly lustre means that it has the same sheen that pearls do. Salmon scales, for example, have pearly lustre. Dull lustre or bright lustre is another way of describing it so that you might talk of bright, pearly lustre or dull, greasy lustre. The lustre of diamonds is brilliant.

Hardness

Everyone knows that some things are hard and others are soft. Glass is quite hard but lead is quite soft. The scientific definition of this property of minerals and other substances says that hardness is the resistance to abrasion. This is just a way of saying that hardness is a measure of how difficult it is to scratch. It is very easy to scratch lead with your fingernail. But on glass your fingernail makes no impression at all. However, with a good quality razor blade or a file you can scratch the glass. This means that your fingernail is harder than the lead, the glass is harder than your fingernail, and the file or razor blade is the hardest of the lot.

A long time ago, a man by the name of Mohs picked ten minerals and arranged them so that each one farther along the row was harder than the one before it. He then said that mineral number one had a hardness of 1, mineral number two had a hardness of 2, and so on up to 10. This has been used ever since and is called Mob's scale. Lead is about 2, the fingernail is about 2-1/2, the glass is about 5-1/2 and the file is about 6 on Moh's scale. Now we can describe the hardness of a mineral by simply giving it a number.

Specific Gravity

Some things are heavier than others. We know that this is so sometimes because they are larger. But some things are heavier than others of the same size. This is what we mean by having a greater specific gravity. A cork float on the top of a net is certainly a lot lighter than a lead weight of the same size on the bottom. To make it easier to talk of this matter of how heavy things are, the scientist has decided that the weight of water makes a good thing to compare with the weight of other materials, and has given it a value of 1.

Now, if we take three portions of material of exactly the same size we can figure out how heavy they are in relation to one another. Suppose we take sand, sawdust, and lead shot. Now, if we measure out cupsful of each and weigh them we find that the sawdust weighs less than the same cupful of water, so it is said to have a specific gravity of less than 1. The sand will weigh about twice what the water weighed so it has a specific gravity of about 2, and the lead shot, correspondingly, will have a specific gravity of about 8, because it weighs eight times as much as the same volume of water.

Most rocks are made up of minerals which are about 2-1/2 times as heavy as the same volume of water so that most rocks have a specific gravity of about 2-1/2. Some of the minerals of such heavy metals as lead, zinc and copper are much heavier than that.

A hard resistant layer of limestone forms the lip of Snake Indian Falls in Jasper National Park. The turbulent waters have cut a considerable canyon below the present location of the falls where undermining of the limestone ledge is constantly going on. © D. M. Baird, 1963

Streak

When we scratch a coloured mineral and churn up a little of its powder we sometimes find that it is a different colour from the mineral itself. Perhaps you have already noticed that scratches on the rocks, made by a bulldozer blade scraping over them or by the nails in someone's boots, are nearly always white or grey and are, at any rate, much lighter coloured than the rock itself.

Although it is found that minerals are of different colours because of small amounts of impurities in them, like the St. Lawrence fluorite we mentioned before, their streaks or scratches always stay the same. The different colours of fluorite, for example—the red, purple, blue or white—all have the same white streak. Hematite, a common ore of iron, may be red, brown, black or of a shiny metallic colour, but its streak is always dark red. Streak, then, is a useful way of identifying minerals.

What is a Crystal?

Almost everything in nature, aside from living things and their remains, comes in crystals. If you look at the white sugar you use at mealtime you will see that each little grain is a rectangular block of clear material. Look at the salt you sprinkle on your food and the same is true. The salt used for icy streets is the same, except that the crystals are much larger, and you can see clearly the same rectangular blocks of salt, or pieces of rectangular blocks if they have been broken.

Minerals occur in crystals which have a great variety of shapes and sizes. Some minerals occur in nature with neat crystals while others are very rarely found in crystals big enough to see with the naked eye. Some minerals come in rectangular blocks like the salt, but others come in six-sided spikes, with six-sided points on top, and still others in more peculiar shapes. Thus it is that we can recognize certain minerals by looking at their crystal shapes. It has been recognized now that the hundreds of different crystal forms can be referred to only six crystal systems with a great variety of forms in each.

The best crystals are usually found in openings or cracks in rocks into which natural solutions have circulated. In these the crystals have grown out from the walls into open spaces, and were free to develop their own symmetrical forms. In all the natural world there is nothing more spectacularly beautiful than clusters of natural crystals.

Cleavage

If we break a piece of wood it usually breaks in a splintery way. If we smash a piece of glass it breaks into sharp, curved pieces. If we take a piece of coarse salt, however, and break it we will find that it will break into neat little rectangular blocks, very much like the crystal it comes from. A piece of mica, which is a commonly known mineral, can be split again and again along smooth flat surfaces. This odd property of minerals, of breaking along smooth surfaces in certain directions, is called cleavage. It is due to an orderly internal arrangement of the chemical constituents of the mineral. Some minerals have well developed cleavage and others do not. Some minerals have more than one direction of cleavage which meet at different angles from others. Thus it is that we can tell minerals apart by whether or not they have this breaking property and its nature if they do.

Chemical Composition

Chemists and physicists, men who study such things, tell us that all the materials on the earth are made up of only 92 naturally occurring, different substances called elements, and various combinations of these elements. The iron in a piece of pipe or a shovel is one of these elements in almost pure state. When it rusts, all that happens is that the iron combines with oxygen, an element in the air, to form a combination of iron and oxygen, which we know as rust.

Some rare minerals are elements, but most minerals are made of combinations of elements. So we talk of pyrite or fool's gold as being iron and sulphur in combination, or iron sulphide. The common minerals from which we obtain lead and zinc are lead and zinc sulphides and refineries for these metals are places where the combinations are broken apart and the pure metals extracted.

Some minerals such as those we have already mentioned are very simple combinations of two elements but others are very complicated both in the numbers of different elements in them and in the ways they are combined. A table which lists some of the properties of minerals you might encounter in your ordinary travels is included and also a little key or guide to help in their identification.

To identify minerals which you may find as constituents in rocks, in veins or as grains in sand start with the key, then compare what you have with the properties in the table. It lists only 12 minerals but these are the ones which make up most of the common rocks, fill almost all veins and form nearly all the grains in sands.

What colour is the mineral?

If it is	Try
White or glassy Green Pink or red Black Grey Brown or yellow	1, 2, 4, or 7 5, 11, or 12 2 or 10 3, 5, or 8 2, 4 or 7 6, 9 or 10