OGRe.net        Rocks and Minerals

The Basic "Stuff" of Minerals and Rocks

At the base of all minerals and rocks are the earth’s elements. Elements are fundamental forms of matter which cannot be broken down into simpler substances by ordinary chemical processes. That’s why elements are considered the building blocks of our earth. Only about ten elements are very abundant and these combine to form about 99 percent of the solid matter of the Earth.  The most abundant elements in the Earth’s crust, in order of weight percent are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen, titanium, and all other elements.

What's the Difference Between a Mineral and a Rock ?

What is the difference between a mineral and a rock? A mineral is a naturally occurring, inorganic solid with a definite chemical composition and crystal structure with specific colour and hardness. (It might be easy to remember that a diamond is a mineral in order to remember these specifics). A mineral may consist of only one of the abovementioned 10+ elements, for example, a diamond is composed only of carbon (one of the “other” elements); or a mineral may consist of several elements.

From this basis, we can define all rocks as composed of one or more minerals. A rock is composed of mineral(s) but is not a mineral. A rock, therefore, is an aggregate of one or more minerals which are all firmly locked together to form a hard solid. Instead of defining each rock as a specific individual, as we are able to do with minerals, rocks are classified into three basic rock families: igneous, sedimentary, and metamorphic rocks. The key differences are: igneous rocks are formed from the “ignis” or fire from the earth’s core; sedimentary rocks are formed by sediments accumulated over time; and metamorphic rocks are either of the first two classes of rock that have metamorphosed with high temperature and pressure over time into different rocks.

How are Minerals Identified ?

The structure of all things begins with the atom. The atom is the basic unit of an element, with its nucleus (protons and neutrons) and its electrons. When atoms become positive or negative (which is done easily because electrons move easily) they then become ions, which are attracted or bonded to other atoms which are also no longer neutral. These bonded atoms and ions are known as compounds. Most minerals are compounds. (Gold and silver are minerals but not compounds because they are single-element minerals).

When compounds are classified as minerals, it is because they have bonded together in a certain / specific chemical recipe or proportion. This is why minerals are expressed as chemical formulae.

Because minerals follow exacting chemical specifications and bonds, they form crystals. Crystal atoms are arranged in a set order, and a regular, periodically repeated pattern.  The crystalline structure of a mineral is, therefore, a crucial and defining mark. The variety of the shapes and sizes of crystals is almost overwhelming. Crystals have a regularity of form, or symmetry, because of their orderly arrangement of atoms and they can form many interesting geometric shapes of all sizes. The surface of any crystal consists of flat planes, and these planes are of various shapes and sizes too. Finally, there are six crystal systems which designate the lines of symmetry possible in all types of crystals. All minerals have a certain type of crystal structure with its inherent internal lines of symmetry.   Even so, some substances form differing crystal structures called polymorphs (graphite and diamonds are polymorphs of carbon). These are two minerals of identical chemical composition, but different internal crystal structures. Thus, while the chemistry needs to be exact it also needs to follow the pattern or fit a certain crystalline structure to finally identify a mineral.

Minerals can be only identified absolutely by x-ray analysis and chemical tests. X-ray analysis determines the structure of the mineral whereas chemical tests determine the composition of the mineral. These are expensive tests which may destroy the mineral specimen. Fortunately, both structure and composition affect certain physical properties which make it possible to do preliminary and fairly accurate “field” tests of minerals.  Fortunately, there are also many physical properties of minerals which we can use for identification purposes. Most minerals can be identified by inspecting or testing their physical properties of:

§         Crystal form (the fact that elemental atoms bond in the shape of crystals to form certain minerals)

§         Cleavage (the tendency that some minerals have to break along definite parallel planes)                     (The mineral mica has “perfect” cleavage in one direction, which means it breaks or peels in thin strips. Feldspar minerals break into two planes at right angles to each other, and create a prism-shape.          The halite mineral (table salt) breaks or shatters into three planes at right angles to each other, and this creates tiny cubes. Interestingly, the mineral calcite also has three planes of cleavage or cleavage planes and yet because they do not meet at right angles, the resulting mineral shape is rhombus / rhombic.)

§         Fracture (the way a mineral breaks if NOT along planes of cleavage; resulting in irregular, rough, splintered, jagged breaks OR smooth curved "conchoidal" breaks)

§         Hardness (how well the mineral resists scratches; related to the strength of a mineral’s chemical bonds)

§         Colour (the obvious and observable exterior colour of the mineral, not reliable alone)

§         Streak (the colour of the powder “streak” a mineral leaves behind when it is scratched / rubbed)

§         Luster (the way the surface of the mineral reflects light: metallic or nonmetallic for example)

§         Specific Gravity or Density (the density of a mineral compared with the density of water) Or, the weight of a mineral relative to the weight of an equal volume of water. If a mineral weighs 3.5 times as much as an equal volume of water, its SG (or density) is 3.5 The Ontario Curriculum asks for DENSITY which is the same as SG. The density of water is one gram per cubic centimetere.

An ideal physical property is one that will give a unique result for each mineral and will always give the same result, again and again, for any and every specimen of that mineral. This is, of course, idealized. The physical properties alone usually don't, so we must catalog all the results of several known physical property tests and to find enough positive results out of these to identify an unknown mineral.

Why are Rocks and Minerals Important To the Average Person ?

Why are the minerals formed by these elements, and the rocks that the minerals form, so important? Minerals are the building blocks upon which life and our modern societies depend. Our Earth produces vast amounts of renewable resources – wind, water and soil components, for example. However, sometimes these resources aren’t enough and “if it can’t be grown, it’s got to be mined”. In other words, we need to tap into the non-renewable riches of the earth – its minerals and the rocks which contain them. Minerals are valued for everything from their beauty, rarity and hardiness as precious gemstones to their useful practicality in the pharmaceutical, manufacturing, construction, petroleum, and high-tech industries. Rocks house these minerals and also provide for many uses: as the foundation from which soil is produced; as the foundations of naturally occurring mountains; as building blocks for most of the great monuments of human history; and as the decorative stones of current architecture and design. Earth abounds with very important resources and humanity has had relatively short time to learn how to explore them. Our roles as future citizens of planet Earth are to explore for and conserve Earth's resources in a responsible manner. Look around and see how many things you use in your everyday life that came from minerals and rocks !